DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

�

NONRESIDENTTRAININGCOURSE

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July 1997

Fire Controlman

Volume 6—Digital Communications

NAVEDTRA 14103�

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

Although the words “he,” “him,” and“his” are used sparingly in this course toenhance communication, they are notintended to be gender driven or to affront ordiscriminate against anyone.

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PREFACE

By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy.Remember, however, this self-study course is only one part of the total Navy training program. Practicalexperience, schools, selected reading, and your desire to succeed are also necessary to successfully roundout a fully meaningful training program.

COURSE OVERVIEW: In completing this nonresident training course, you will demonstrate aknowledge of the subject matter by correctly answering questions on the following subjects:

• types of communications systems;• the decibel system of power measurement;• synchronous and asynchronous communications as used in data communications systems;• methods of data modulation and demodulation used in various types of data networks;• the operation of modems used in data communications;• methods of multiplexing data in communications networks;• equipment associated with and the operation of the Link-11 data communications system;• equipment associated with and the operation of the Link-4A data communications system;• equipment associated with and the basic operation of local area networks.

THE COURSE: This self-study course is organized into subject matter areas, each containing learningobjectives to help you determine what you should learn along with text and illustrations to help youunderstand the information. The subject matter reflects day-to-day requirements and experiences ofpersonnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers(ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational ornaval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classificationsand Occupational Standards, NAVPERS 18068.

THE QUESTIONS: The questions that appear in this course are designed to help you understand thematerial in the text.

VALUE: In completing this course, you will improve your military and professional knowledge.Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you arestudying and discover a reference in the text to another publication for further information, look it up.

1997 Edition Prepared byDSCS(SW/AW) Robert M. Maynard

FCCS(SW) Edwin L. Rodriguez

Published byNAVAL EDUCATION AND TRAINING

PROFESSIONAL DEVELOPMENTAND TECHNOLOGY CENTER

NAVSUP Logistics Tracking Number0504-LP-026-7710

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Sailor’s Creed

“I am a United States Sailor.

I will support and defend theConstitution of the United States ofAmerica and I will obey the ordersof those appointed over me.

I represent the fighting spirit of theNavy and those who have gonebefore me to defend freedom anddemocracy around the world.

I proudly serve my country’s Navycombat team with honor, courageand commitment.

I am committed to excellence andthe fair treatment of all.”

TABLE OF CONTENTS

CHAPTER

1

2

3

4

5

6

APPENDIX

I

II

PAGE

Fundamentals of Data Communications . . . . . . . . . . . . . . . . 1-1

The LINK-11 System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

LINK-11 Fault Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

LINK-4A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

New Technology in Data Communications . . . . . . . . . . . . . 1-1

Local-Area Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AI-1

References Used to Develop the TRAMAN . . . . . . . . . . . . AII-1

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .INDEX-l

NONRESIDENT TRAINING COURSE follows the index

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INSTRUCTIONS FOR TAKING THE COURSE

ASSIGNMENTS

The text pages that you are to study are listed atthe beginning of each assignment. Study thesepages carefully before attempting to answer thequestions. Pay close attention to tables andillustrations and read the learning objectives.The learning objectives state what you should beable to do after studying the material. Answeringthe questions correctly helps you accomplish theobjectives.

SELECTING YOUR ANSWERS

Read each question carefully, then select theBEST answer. You may refer freely to the text.The answers must be the result of your ownwork and decisions. You are prohibited fromreferring to or copying the answers of others andfrom giving answers to anyone else taking thecourse.

SUBMITTING YOUR ASSIGNMENTS

To have your assignments graded, you must beenrolled in the course with the NonresidentTraining Course Administration Branch at theNaval Education and Training ProfessionalDevelopment and Technology Center(NETPDTC). Following enrollment, there aretwo ways of having your assignments graded:(1) use the Internet to submit your assignmentsas you complete them, or (2) send all theassignments at one time by mail to NETPDTC.

Grading on the Internet: Advantages toInternet grading are:

• you may submit your answers as soon asyou complete an assignment, and

• you get your results faster; usually by thenext working day (approximately 24 hours).

In addition to receiving grade results for eachassignment, you will receive course completionconfirmation once you have completed all the

assignments. To submit your assignmentanswers via the Internet, go to:

http://courses.cnet.navy.mil

Grading by Mail: When you submit answersheets by mail, send all of your assignments atone time. Do NOT submit individual answersheets for grading. Mail all of your assignmentsin an envelope, which you either provideyourself or obtain from your nearest EducationalServices Officer (ESO). Submit answer sheetsto:

COMMANDING OFFICERNETPDTC N3316490 SAUFLEY FIELD ROADPENSACOLA FL 32559-5000

Answer Sheets: All courses include one“scannable” answer sheet for each assignment.These answer sheets are preprinted with yourSSN, name, assignment number, and coursenumber. Explanations for completing the answersheets are on the answer sheet.

Do not use answer sheet reproductions: Useonly the original answer sheets that weprovide—reproductions will not work with ourscanning equipment and cannot be processed.

Follow the instructions for marking youranswers on the answer sheet. Be sure that blocks1, 2, and 3 are filled in correctly. Thisinformation is necessary for your course to beproperly processed and for you to receive creditfor your work.

COMPLETION TIME

Courses must be completed within 12 monthsfrom the date of enrollment. This includes timerequired to resubmit failed assignments.

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PASS/FAIL ASSIGNMENT PROCEDURES

If your overall course score is 3.2 or higher, youwill pass the course and will not be required toresubmit assignments. Once your assignmentshave been graded you will receive coursecompletion confirmation.

If you receive less than a 3.2 on any assignmentand your overall course score is below 3.2, youwill be given the opportunity to resubmit failedassignments. You may resubmit failedassignments only once. Internet students willreceive notification when they have failed anassignment--they may then resubmit failedassignments on the web site. Internet studentsmay view and print results for failedassignments from the web site. Students whosubmit by mail will receive a failing result letterand a new answer sheet for resubmission of eachfailed assignment.

COMPLETION CONFIRMATION

After successfully completing this course, youwill receive a letter of completion.

ERRATA

Errata are used to correct minor errors or deleteobsolete information in a course. Errata mayalso be used to provide instructions to thestudent. If a course has an errata, it will beincluded as the first page(s) after the front cover.Errata for all courses can be accessed andviewed/downloaded at:

http://www.advancement.cnet.navy.mil

STUDENT FEEDBACK QUESTIONS

We value your suggestions, questions, andcriticisms on our courses. If you would like tocommunicate with us regarding this course, weencourage you, if possible, to use e-mail. If youwrite or fax, please use a copy of the StudentComment form that follows this page.

For subject matter questions:

E-mail: n311.products@cnet.navy.milPhone: Comm: (850) 452-1503

DSN: 922-1503FAX: (850) 452-1370(Do not fax answer sheets.)

Address: COMMANDING OFFICERNETPDTC N3116490 SAUFLEY FIELD ROADPENSACOLA FL 32509-5237

For enrollment, shipping, grading, orcompletion letter questions

E-mail: fleetservices@cnet.navy.milPhone: Toll Free: 877-264-8583

Comm: (850) 452-1511/1181/1859DSN: 922-1511/1181/1859FAX: (850) 452-1370(Do not fax answer sheets.)

Address: COMMANDING OFFICERNETPDTC N3316490 SAUFLEY FIELD ROADPENSACOLA FL 32559-5000

NAVAL RESERVE RETIREMENT CREDIT

If you are a member of the Naval Reserve, youmay earn retirement points for successfullycompleting this course, if authorized undercurrent directives governing retirement of NavalReserve personnel. For Naval Reserve retire-ment, this course is evaluated at 6 points. (Referto Administrative Procedures for NavalReservists on Inactive Duty, BUPERSINST1001.39, for more information about retirementpoints.)

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Student Comments

Course Title: Fire Controlman, Volume 6—Digital Communications

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NETPDTC 1550/41 (Rev 4-00

CHAPTER 1

FUNDAMENTALS OF DATA COMMUNICATIONS

INTRODUCTION

Although you, as a Fire Controlman, may not be directly involved in datacommunications, you definitely need to be aware of how data communicationsaffects your ship’s mission. This training manual introduces and explains thebasics of data communications. Computer data frequently must be transmitted fromone point to another. The distance involved maybe a few feet, or it may be hundredsof miles. Data transited over long distances often must be converted to a formcompatible with either landline or radio wave transmission and reception. Thischapter explains how such conversion occurs and techniques used in the conversionand transmission procedures.

After completing this chapter, you should be able to:

State the types of communications systems.

Describe the decibel system of power measurement.

Explain asynchronous and synchronous communications as used in datacommunications systems.

Describe the methods of data modulation and demodulation used invarious types of data networks.

Describe the operation of modems used in data communicationsnetworks.

Describe the methods of multiplexing data in communications networks.

COMMUNICATIONS SYSTEMS

The devices used to transfer digital data makeupwhat is known as a communications system. In itsmost basic form, a communications system consists ofthe three components shown in figure 1-1. They are

Figure 1-1.—Communications system.

the transmitter, the receiver, and a communicationschannel that connects the two units.

The transmitting equipment converts the data ofthe sending system into a form that can be sent overthe communications channel, accepted by thereceiving equipment, and converted back into usabledata by the receiving system. Data sent over acommunications system is in one of the following twoforms: analog or digital.

An analog signal used in data communicationsvaries continuously between a minimum and amaximum value. As the signal varies, it assumes aninfinite number of specific values between the two

1-1

digital data pulses into a form acceptable to thevarious types of communications channels. Theequipment most often transmits digital data over adistance by varying a continuous analog signal inamplitude, frequency, or phase.

Communications channels that can pass data intwo directions (transmit and receive) are known asduplex channels. Single-direction channels aresimplex channels. Duplex channels may operate inone of the following two modes: half-duplex orfill-duplex. Half-duplex channels transmit data inone direction, pause, and then receive data comingfrom the opposite direction. Full-duplex channels, onthe other hand, can transmit and receive datasimultaneously.

TYPES OF COMMUNICATIONS CHANNELS(TRANSMISSION MEDIA)

In the fleet and at shore activities, you willencounter several forms of communications channels.The most common channels are landlines and radiocommunications.

Landlines

Landlines are physical lines or cables that connectthe digital equipment. Originally, landlines referredto telephone lines and were limited to carrying analogaudio frequencies (voice frequencies). For digitalinformation to be carried over these lines, the

limits. The signal can be varied in amplitude(amplitude modulation), frequency (frequencymodulation), or phase (phase modulation) to conveythe data. We will discuss each type of modulationlater in this chapter.

A digital signal has a limited set of values (1 or 0,true or false, etc.). A limited number of discretepulses can be transmitted in a fixed period. Theunique sequence of the bits represents the data.

Digital equipments (computers and peripherals)within a system normally communicate with eachother in pure digital pulses (serial and parallel).Transmitting digital information over a distancerequires the use of special equipment to convert

characteristics of one or more tones or carriers in theaudio-frequency range had to be modified inamplitude, frequency, or phase.

Today, telephone lines are commonly used inmany network applications. Bulletin boards, such asBUPERS ACCESS, use existing telephone lines; butmany landline-based systems use dedicated lines.Dedicated lines are common in local area networks(LANs). In a LAN system, several computers arejoined together to share information with all the userson the system. System connections are made usingcoaxial, dual-coaxial, fiber-optic, or twisted-paircable. The type of cable depends on several factors,such as the number of users on the LAN and themaximum distance between workstations.

The device used to convert the digital data into aform usable by the communications channel and backto digital data is known as a modem.

Modem is an acronym for MOdula torDemodulator. The modulator function converts thedata of the transmitting system into discretemodifications of the tone or carrier signals. The de-modulator converts the data-carrying tone or carriersignal into digital data for the receiving system.

Radio

Radio waves have been used for teletype andvoice communications for many decades. Theadvantages of radio-based systems are that they aremore mobile and can communicate over barriers suchas large bodies of water. Tactical information links,like those we will cover in chapter 2, are almostexclusively radio-based.

Radio communications are based on frequencyranges or radio-frequency bands. The frequencyrange of the carrier frequency determines theoperational characteristics of the system. Table 1-1illustrates the international frequency bands and theiruses. The tactical digital information systems used bythe Navy generally use portions of the hf and uhfbands.

1-2

Table 1-1.—Frequency Bands and Their Applications

In the radio transmitter, the data signals (discreteor tones) are modulated (impressed) on to the carrierfrequency and transmitted into space when thetransmitter is keyed. A receiver tuned to the carrierfrequency picks up the signal and demodulates thedata-carrying signals from the carrier. The datasignals can then be converted to digital data by theappropriate devices. For more information on radiooperations, refer to Navy Electricity and ElectronicsTraining Series (NEETS), Module17— Radio-Frequency Communications Principles.

THE DECIBEL MEASUREMENT SYSTEM

Technicians who deal with communicationsequipment often speak of the gain of an amplifier ora system in units called decibels (dB). Decibels areused as an indication of equipment performance;therefore, you need a basic understanding of thedecibel system of measurement.

As the actual calculation of decibel measurementis seldom required in practical applications, theexplanation presented in this text is somewhatsimplified. Most modern test equipment is designedto measure and indicate decibels directly. This design

eliminates the need for complicated mathematicalcalculations. Nevertheless, because many data linksystem alignment procedures center around dBreadings and references, you need to understand thesignificance of an equipment gain rating as expressedin decibels.

The equipment used in communications systemsconsists of several components, such as amplifiers,communications lines, antennas, couplers, andswitches. Each component in the system will affectthe signal by introducing a signal loss or gain. Theselosses and gains can be described by a ratio of thepower input and output by the equipment or cable.The ratio can be calculatedformula:

Output power =Input power

by using the following

Power ratio

If a communications system has four components,the gain or loss at each component must be calculatedand these ratios multiplied. The following is anexample of the gain/loss calculation of afour-component system:

1-3

In this system, the output of the signal is twice asstrong as the input to the system.

As you can see, this constant multiplication of theratios can be wearisome, and the products can beextremely small or large. Therefore, the discoverythat adding the logarithms of the numbers would yieldthe same result as this calculation led early scientiststo develop the unit of measure called the bel.

The bel, named in honor of Alexander GrahamBell, expresses the logarithmic ratio between the inputand output of any given component, circuit, orsystem. The bel maybe expressed in voltage, current,sound levels, or power. The formula is as follows:

The gain of an amplifier can be expressed in bels(N) by dividing the output (P1) by the input (P2) andtaking the base 10 logarithm (log 10) of the resultingquotient. Thus, if an amplifier doubles the power, thequotient will be 2. When you consult a logarithmtable, you will find that the base 10 logarithm of 2 is0.3; so the power gain of the amplifier is 0.3 bel.

Experience has shown that the bel is a rather largeunit that is difficult to apply. A more practical, easierunit to apply is the decibel (1/10 bel). Any figureexpressed in bels can be converted to decibels bymultiplying the value by 10. Thus the ratio of 0.3 belis equal to 3 decibels.

The reason the decibel system is used to expresssignal strength is shown in table 1-2. For example,saying that a reference signal has increased 50 dB ismuch easier than saying that the output has increased100,000 times.

The basis of the decibel measuring system is theamount of increase or decrease from a reference level.Whether the input power is increased from 1 watt to

1-4

100 watts or from 1,000 watts to 100,000 watts, theamount of increase, or gain, is still 100 times or20 dB. Examine table 1-2 again, taking particularnote of the power ratios for source levels 3 dB and 6dB. As the table illustrates, an increase of 3 dBrepresents a doubling of power. The reverse is alsotrue. If a signal decreases by 3 dB, half of the poweris lost. For example, a 1,000-watt signal decreased by3 dB will equal 500 watts, while a 1,000-watt signalincreased by 3 dB will equal 2,000 watts.

Table 1-2.—Decibel Power Ratios

When you speak of the dB level of a signal, youare actually speaking of the logarithmic comparisonbetween the input and output signals. The inputsignal is normally used as the reference signal. Insome instances, a standard reference signal must beused in place of the input signal. The most widelyused reference level is a 1-milliwatt signal (600-ohmload). When the 1-milliwatt reference is used, thestandard decibel abbreviation of dB is changed todBm; dBms are used as an indication of power, whiledBs are used to indicate the ratio between the inputand output.

A signal level of +3 dBm is 3 dB above 1milliwatt, and a signal level of –3 dBm is 3 dB below1 milliwatt. Whether you are using dB or dBm, a plussign (+) or no sign indicates that the output level is

Figure 1-2.—Asynchronous character code.

greater than the reference (power gain), while a minussign (–) indicates that the power level is less than thereference (power loss). The value 0 dBm indicatesthat the output power is equal to the 1-milliwattreference. It is also used to express a definite amountof power (1 milliwatt). The value 0 dBm equates to1 milliwatt.

DIGITAL DATA COMMUNICATIONSTECHNIQUES

Data signals transmitted over communicationschannels need to follow specific protocols to ensurethey are synchronized. In normal I/O data exchanges,this process is accomplished by the system of requestsand acknowledges. In addition, the data signals haveto be properly formatted for the receiving computer todecode them properly.

ASYNCHRONOUS AND SYNCHRONOUSCOMMUNICATIONS

Two major data-formatting methods are used tomake sure the transmitting computer and the receivingcomputer(s) are synchronized: asynchronous(character framed) and synchronous (messageframed). Both methods are used to identifyintelligence transmitted in the form of serial bitstreams.

Asynchronous Transmission

Asynchronous transmission of data is commonlyfound in landline communications systems and someforms of teletype communications. Generally,asynchronous, or character-framed, transmission isused to transmit seven- or eight-bit data, usually inASCII character format. Each character has a specificstart and end sequence—usually one start bit and oneor two end (stop) bits. Figure 1-2 illustrates thetransmission format of an asynchronous data stream.A parity bit (even or odd) maybe included to ensurethe accuracy of the transmitted data. Asynchronouscharacters may be transmitted one at a time or as astring of characters; however, each charactertransmitted will have start and end bits. When datasignals are transmitted in this format, synchronizationoccurs on a character-by-character basis between thetransmitting and receiving devices and provides someallowance for timing inaccuracies. Any inaccuracy intiming is corrected with the arrival of the nextcharacter.

Synchronous Transmission

Most tactical digital information linkscommunicate using synchronous messages.Synchronous transmission is a more sophisticatedmethod of data transmission. It sends data in longuninterrupted streams, with a predefine start and stopsequence. The start sequence is generally referred to

Figure 1-3.—Synchronous message format.

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as the preamble. The principal function of thepreamble is to alert the receiver of incoming data andprovide a reference to synchronize the receiver withthe transmitted signal. Following the preamble is astart code that informs the receiving equipment of thebeginning of the message data. The basic format ofthe synchronous data message is shown in figure 1-3.The incoming bit stream is then used to synchronizethe receiver or demodulator timing. A stop codefollows the message data to indicate the end oftransmission.

MODULATION/DEMODULATION

Modulation modifies a signal so it can carry dataover the communications channel. The demodulatorremoves the data from the carrier. For most datacommunications applications, the carrier is acontinuous sinusoidal waveform (sine wave). Thefrequency of the carrier varies, depending on theapplication. Landline transmission generally uses theaudio-frequency bandwidth signals (300 to 3,000 Hz).Radio channels use audio-frequency tones as datacarriers modulated to a radio-frequency signal, or theymodulate the radio-frequency signal itself to conveydata.

The three basic modes of modulation areamplitude modulation, frequency modulation, andphase modulation. Each of these modes modifies thecarrier signal in some manner to convey data.

Amplitude Modulation

When amplitude modulation is used for digitaltransmissions, the amplitude of the carrier signalrepresents the two discrete data states (1 or 0). Thesignal represents a logic 1 when the amplitude(peak-to-peak), at the same frequency, is greater at adifferent time, as shown in figure 1-4. The decreasein signal amplitude, below a predetermined threshold,indicates a change from a logic 1 to a logic 0.

Frequency Modulation

The frequency of the carrier signal or audio tonesmodulated to the carrier signal can be modified toindicate the two discrete states. As shown in figure

Figure 1-4.—Amplitude modulation.

1-5, a selected frequency can be used to indicate the1 state of a bit, and another selected frequency can beused to indicate the 0 state. The change in frequency,or frequency shift, indicates the same relationship asthe change in amplitude did in amplitude modulation.

Figure 1-5.—Frequency modulation.

Shifting the frequency of the carrier signal iscalled frequency-shift keying (FSK) or binaryfrequency-shift keying (BFSK). FSK usuallyinvolves shifts to frequencies above or below aselected center frequency. Transmission of thefrequency above the center frequency indicates abinary 1; the frequency below the center frequencyindicates a binary 0. The center frequency is nottransmitted. FSK is used in systems such as link 4A.

Another method of using frequency shifts involvesaudio-frequency tones. Two discrete audio tones maybe modulated to a constant frequency carrier signal.One of the tones is used to indicate a mark, or binary1, the other a space, or binary 0. This method offrequency modulation is called audio-frequency toneshift (AFTS).

Phase Modulation

Phase modulation ismodulation. It is based

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a more complex mode ofon the relationship of the

360-degree carrier sine wave to the baseline of thesine wave. The carrier signal starts on the baseline, asillustrated in figure 1-6, and continues to form a curvecalled the sine wave. When the sine wave reaches itsmaximum positive amplitude, it is at the 90-degreepoint. When it returns to the baseline, it is at 180degrees. When it reaches its maximum negativeamplitude, it is at 270 degrees; and when it returns tothe baseline, it is at 360 degrees or the 0-degree pointfor the start of the next cycle. This process occursover a period, with the number of full cycles persecond (Hz) being the frequency of the signal. A fullcycle is the transition from the 0-degree point to the360-degree point.

Figure 1-6.—Carrier sine wave,

For a particular frequency this process continueswithout interruption. Phase modulation involvesinterrupting the cycle at one or more degree pointsand instantaneously changing the direction oramplitude of the sine wave. Figure 1-7 shows how a180-degree phase shift is used to indicate two discretestates. The third cycle of the carrier is interrupted atthe 180-degree point. Instead of continuing in thenegative direction, the sine starts at the 0-degree pointagain. The resultant signal has the same frequencyand amplitude as the original signal but is 180 degreesout of phase. This phase shift can be directly relatedto a digital input at a modulator in which one

particular phase represents the 0 bit and the otherphase represents the 1 bit.

Multibit Modulation

While the 180-degree phase shift can be used toindicate two discrete states, many points on the sinewave can be defined to represent different bitconfigurations. Individual phase changes of 0degrees, 90 degrees, 180 degrees, and 270 degreesfrom a reference phase can each represent twoseparate data bits. For example, a 0-degree phaseshift or no phase shift could indicate a binary 00; a90-degree phase shift, a binary 01; a 180-degree phaseshift, a binary 10; and a 270-degree phase shift, abinary 11. This type of modulation is known as amultibit, or quadrature (four-state) phase-shiftmodulation, as shown in figure 1-8. Keep in mindthat only one continuous frequency and amplitudesignal is being phase-modulated to transmit two bitsof data for each phase shift.

Figure 1-8.—Multibit phase modulation.

A modification of the quadrature phase-shiftmodulation, called differential quadraturephase-shift keying, uses the difference between aphase-shifted signal and its preceding sine wave torepresent data. When a phase shift is detected, thecurrent signal is compared with the previouslytransmitted phase signal. The difference between thetwo signals is computed to determine the amount ofphase shift. The previously transmitted signal is usedas the reference phase for demodulating the data bits.Two binary digits are represented by phase changes of-45, -135, -225, and -315 degrees. The -45 degreeshift indicates a binary 11; the -135 degree shift, abinary 01; the -225 degree shift, a binary 00; and the-316 degree shift, a binary 10.

Figure 1-7.—Phase modulation.

1-7

Figure 1-9.—Full-duplex modem.

MODEMSTransmitter Section

Modems come in a variety of configurations.Their design dependsincluding the following:

Asynchronoustransmissions

on a number of factors,

or synchronous data

Simplex, half-duplex, or full-duplexcommunications

Type of communications channel

Type of modulation/demodulation used

Modems may be stand-alone devices with theirown power supplies and indicators. They may also beintegrated into the design of larger equipments inwhich the modulations or demodulations are only oneof the functions performed by the device.

A functional block diagram of a modem is shownin figure 1-9. A full-duplex modem consists of twosections: the transmitter and the receiver sections.These two sections are functionally separate fromeach other.

The transmitter section consists of a data encoder,the modulator, the band-pass filter, and the transmitcontrol logic. The data encoder takes the digital datasignal to be transmitted, and when necessary, convertsit into the bit pattern acceptable to the modulatorcircuit. The modulator converts the data into thecarrier signal. The most popular forms of modulationare frequency-shift keying (FSK), phase-shift keying(PSK), and quadrature phase-shift keying. After thedata signals are modulated, they are fed to theband-pass filter circuitry. The band-pass filter thenallows only the desired frequency to pass through thecommunications channel. The transmit control logicprovides the timing signals necessary for thetransmission of data to take place.

Receiver Section

The receiver section consists of a band-pass filter,a demodulator, a data decoder, and the receivercontrol circuit. The band-pass filter allows only thedesired carrier signal to be received from thecommunications channel. The demodulator removesthe data from the carrier signal and feeds the data to

1-8

Figure 1-10.—A time-division multiplexer (TDM) system.

the decoder. The decoder reassembles the data into a be designated in a single modulation change. If twoform compatible with the receiving system. In thereceiver section, the incoming signal is often fed tothe receiver timing logic to control the receiver timingcircuitry.

MULTIPLEXING

One requirement of a data communications systemis for it to transmit as many intelligent signals aspossible in a fixed period using a single-communications channel. The rate of datatransmission is measured in the number of bits persecond (bps) transmitted. The bps rate is oftenconfused with the baud rate. Baud refers to the rate atwhich a modulated signal between two deviceschanges in 1 second. For example, if the signalbetween two modems changes frequency or phase ata rate of 2,400 times per second, the baud is 2,400. Ifyou are using a modulation method in which a singlemodulation change carries one bit, the 2,400 baud isalso 2,400 bits per second. Using more sophisticatedmodulation methods, several bits of information can

bits of data are transmitted with each modulationchange, the data transfer rate is 4,800 bits per secondat 2,400 baud.

The data signals being transmitted are normallymultiplexed to increase the transmission rate of dataover the communications channel or to increase theefficiency of the channel by allowing multiple usersof the same channel. The two methods commonlyused to multiplex communications channels aretime-division multiplexing and frequency-divisionmultiplexing.

Time-Division Multiplexing

Time-division multiplexing (TDM) grants eachuser full channel capacity, but assigns time slots toeach user. Each user is connected to a time-divisionmultiplexer. Data signals from the user are fed to thetime-division multiplexer buffer, and the time slotsare rotated among the users and scanned for data.Figure 1-10 illustrates the typical construction of a

1-9

time-division multiplexer system. The data from eachuser can be in the form of bits, bytes, or blocks. Thedata signals from all users are compiled into framesf o r t r a n s m i s s i o n o n a single, high-speedcommunications channel.

Transmit and receive frames are used forhalf-duplex communications. Transmit frames aresent and a receive time slot is enabled for returninformation. In this manner, a single carrierfrequency and modem may be used to transmit andreceive information at a fairly high rate of speed.

Since time slots are preset and assigned, if a userhas no data to transmit, the time slot is wasted.Advantages of a TDM system include the following:its ability to handle devices with varying speeds, itseffectiveness when used with devices that transmitdata almost continuously, and its simpleimplementation.

Frequency-Division Multiplexing

Frequency-division multiplexing (FDM) dividesa band of frequencies into several distinct channels ortones. Each tone carries a portion of the data beingtransmitted. FDM devices can be complex because aseparate modulator/demodulator circuit is required foreach tone used. The composite tones are thenmodulated to a single carrier frequency for radiotransmission.

FDM allows for the parallel transmission of dataover a single communications channel. For example,the Link-11 communications system uses 15 audiotones to transmit 30 bits of parallel data. Each tonetransmits two bits of differential quadraturephase-shift keyed data.

SUMMARY—FUNDAMENTALS OF DATACOMMUNICATIONS

This chapter introduced you to the building blocksof a data communications system. The followinginformation summarizes the important points youshould have learned.

COMMUNICATIONS SYSTEMS— Digitaldata devices that exchange data over distances areknown as communications systems. A basiccommunications system consists of the followingthree components: a transmitter, a receiver, and acommunications channel. The transmitter convertsdigital data into a form (digital or analog) useable bythe communications channel. The receiver acceptsdata from the communications channel and convertsthe data back to its pure digital form.Communications systems that can transmit andreceive data are known as duplex systems, whilecommunications systems that are limited to transmitonly or receive only are simplex systems. Duplexsystems that transmit data, pause, and then receivedata are half-duplex systems. Full-duplex systemscan transmit and receive data simultaneously.

COMMUNICATION CHANNELS— Severaltypes of communications channels are in use today.The most common are landlines and radiocommunications. Landlines are physical cables thatconnect computers; they are common in local areanetworks. Radio communications use theradio-frequency bands to exchange information. Themost common bands used in the Navy are the HF andUHF bands.

DECIBEL MEASUREMENT SYSTEM— Thedecibel measurement system is used to measure thegain or loss of amplifiers, antennas, communicationslines, and other types of communications equipment.A gain of +3 decibels (dB) indicates that the outputpower of the circuit, compared to the input power, hasdoubled. Each +3 dB gain indicates a doubling ofpower. For example, a signal that has a gain of 6 dBis twice as strong as a signal that has a gain of 3 dB.

A S Y N C H R O N O U S T R A N S -MISSION— Asynchronous transmission refers todata sent without the use of timing pulses. Datasignals are sent a byte at a time, with start, stop, andparity bits added to each byte.

S Y N C H R O N O U S T R A N S -MISSION— Synchronous transmission refers to thesending of long, uninterrupted streams of data with apredefined start and stop sequence.

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MODULATION/DEMODULATION— Modul-ation is the modifying of a signal to carry intelligentdata over the communications channel. Several typesof modulation are available, depending on the systemrequirement and equipment. The most frequentlyused types of modulation are amplitude modulation,frequency modulation, and phase modulation.Demodulation is the act of returning modulated datasignals to their original form.

AMPLITUDE MODULATION— Amplitudemodulation refers to modifying the amplitude of asine wave to store data.

FREQUENCY MODULATION— Frequencymodulation refers to changing the frequency of asignal to indicate a logic 1 or a logic 0. Onefrequency indicates a logic 1, and the other frequencyindicates a logic 0.

PHASE MODULATION— Phase modulation ismore complex than amplitude modulation orfrequency modulation. Phase modulation uses asignal frequency sine wave and performs phase shifts

of the sine wave to store data. A modification ofphase modulation involves the use of several discretephase shifts to indicate the state of two or more databits.

M O D E M S — A modem is a device thatMOdulates and Demodulates data in a digitalcommunications system. Modems are available in avariety of types, with various speeds and capabilities.A modem consists of two functionally separateareas—the transmitter section and the receiversection. The transmitter section prepares, ormodulates, the data for transmission. The receiversection demodulates, or returns, incoming data to itsoriginal form.

MULTIPLEXING— Multiplexing refers toprocesses used in digital communications systems tomake the most efficient use of system time.Multiplexing can involve time-sharing of thecommunications channel by several users or assigningseveral frequencies for the parallel transmission ofdata.

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CHAPTER 2

THE LINK-11 SYSTEM

INTRODUCTION

Tactical data links are usually limited to a specific area of operation and are usedfor command and control of specific forces. Link-11 is the U.S. Navy shipboardversion of NATO’s Tactical Data Information Link “A” (TADIL A). The Link-11system is used to provide high-speed, computer-to-computer exchange of digitaltactical information among ships, aircraft, and shore installations, as shown in figure2-1.

Figure 2-1.—Tactical digital information links.

Link-11 data communications can operate with either high-frequency (HF) orultra-high-frequency (UHF) radios. In the HF band, Link-11 provides gaplessomnidirectional coverage of up to 300 nautical miles from the transmitting site. Inthe UHF band, the Link-11 system is capable of line-of-sight omnidirectionalcoverage, approximately 25 nautical miles between ships and 150 nautical miles forship-to-air links.

To understand the operation of the Link-11 system fully, you must be able toidentify the hardware components that compose it and the functions they perform.Keep in mind that although the specific equipment used on board your ship maydiffer from the examples used in this chapter, the purpose of your Link-11 setup isstill the same, that is, to pass tactical data to other units.

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After completing this chapter you should be able to:

Describe the composition of a typical Link-11 system.

Describe the operation of the Link-11 transmission and receive cycles.

Describe the six operating modes of the Link-11 system.

Describe the function of the Link-11 encryption (security) device.

Describe the audio tones generated by the Link-11 Data Terminal Set.

Describe the word formats used to transmit Link-11 tactical data.

Describe the message formats used in the various Link-11 operatingmodes.

Describe the operation of the Link-11 Data Terminal Set.

LINK-11 FUNDAMENTALS

To monitor the operation of and performmaintenance on the Link-11 system, you mustunderstand how the different pieces of equipmentinteract with each other. Let’s take a look at a basicLink-11 system.

LINK-11 SYSTEM OVERVIEW

A typical shipboard Link-11 communicationssystem (figure 2-2) consists of the followingcomponents: the CDS digital computer, acryptographic device, the Link-11 data terminal set,the communications switchboard, and the HF or UHF

radio set transceivers (transmitter/receiver), anantenna coupler, and an antenna. The data terminalset is the center of the Link-11 system and is coveredin detail later in this chapter. The communicationsswitchboard is used to select the desired HF or UHFtransceiver. An external frequency standard is alsopart of many Link-11 systems. Additionally, theShipboard Gridlock System (SGS) is installed onmany ships. On SGS-equipped ships, an AN/UYK-20is placed in the line between the CDS computer andthe crypto device.

Figure 2-2.—The Link-11 communications system.

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CDS Computer

The central processor of the Combat DirectionSystem is the CDS computer. Keeping a data base oftracks is among the many functions of the operationalprogram. The information about these tracks can betransmitted to other units over the Link-11 net. Thecomputer sends data to the data terminal set using 24-bit data words. The computer also receivesinformation about remote tracks from other units inthe net and displays these tracks through the displaysystem.

Shipboard Gridlock System

Gridlock is the matching of track positions heldby other ships with the tracks held by your own ship.Gridlock is a procedure for determining dataregistration correction by comparing remote tracksreceived from a designated reference unit to localdata. Ideally, tracks received from remote units thatare also displayed by onboard sensors should betransparent, that is, in the exact same position on theCRT. If the gridlock system does not providecorrelation between local and remote tracks, theremote tracks may be painted twice and overlap eachother, as shown in figure 2-3.

Figure 2-3.—Tracks out of gridlock.

Failure to maintain gridlock maybe the result ofinaccurate positioning data from a ship's sensor, fromthe Ship's Inertial Navigation Systems (SINS), or

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from the ship’s gyro. Failure to maintain gridlockmay also be the result of an inaccurate operator entry.

The SGS computer performs continuousautomatic gridlock calculations. In the event of anSGS computer failure, the flow of Link-11 data to theCDS computer is interrupted. To restore Link-11 dataflow, all SGS installations have switches installed thatallow the technician to bypass the SGS computer untilthe fault is corrected.

Link-11 Security Device

A standard model security device, such as theTSEC/KG-40, commonly referred to as the KG-40, isused with the Link-11 system. When the DTS istransmitting data, the KG-40 receives parallel datafrom the CDS computer, encrypts the data, and sendsit to the DTS. When the participating unit (PU) isreceiving data, the TSEC/KG-40 receives encrypteddata from the DTS, decrypts, and sends to the CDScomputer.

Because of the specialized training and securityrequirements of cryptographic equipment, we will notcover the internal operation and controls of thesecurity device.

Data Terminal Set (DTS)

The data terminal set (DTS) is the heart of theLink-11 system. The DTS is the systemmodulator/demodulator (MODEM). The CDScomputer sends 24 bits of data to the DTS via theSGS computer and the encryption device. The DTSadds six bits of data for error detection and correction.These six bits are called hamming bits. The 30 bitsof data are phase shift modulated into 15 audio tones.These 15 data tones and a Doppler connection tone arecombined into a composite audio signal which is sentto either the UHF or HF radio for transmission.

The DTS receives the composite audio signalfrom the radio and separates the 15 data tones and theDoppler correction tone. The 15 data tones aredemodulated into 30 data bits. The six hamming bitsare checked for errors and the 24 data bits are sent to

the CDS computer via the encryption device and theSGS computer.

Link-11 Communications Switchboard

The communications switchboard provides systemflexibility and casualty recovery capabilities byallowing manual switching of the data terminal setand individual HF and UHF radios. A typicalswitchboard will provideinterconnections:

The Link-11 data terminalHF radio sets to provideLink-11 capability

the following

set to one or morethe standard HF

A Link-11 data terminal set to one or moreUHF radios sets to provide UHF Link-11capability

The same communications switchboard may alsobe used for connecting a Link-4A data terminal set toone or more UHF radios to provide standard UHFLink-4A (TADIL C) capability. Link-4A is coveredin detail later in this book.

Radios

The Link-11 system can operate with either an HFradio or a UHF radio. Long-range communicationsare achieved by the use of the HF system. UHFcommunications are limited to line of sight. “Line ofsight” means the radio wave will not bend over thehorizon; therefore, the use of an antenna mountedhigh on the mast will increase the range of UHFcommunications.

Antenna Couplers

Antenna couplers are used to connect a specificradio set to a specific antenna. The coupler providesfor the correct impedance matching of the antenna andthe radio set. For many of the multi-couplers to workproperly, it is extremely important that the correctfrequency spacing be observed. A general rule is toensure a frequency spacing of 15 percent.Frequencies that are too close together can causeinterference and distortion, increasing the signal tonoise ratio and causing bit errors in the data.

Antennas

In oversimplifying the theory of antennaoperation, an antenna is just a piece of wire thatradiates electromagnetic energy from the radio intothe atmosphere and converts atmosphericelectromagnetic radiation into RF current to beprocessed by the radio. As electromagnetic energyfrom the atmosphere passes through this wire, itinduces a current in the wire. This current is fed tothe radio receiver. If the receiver is tuned to the samefrequency as the received signal, the signal can beprocessed. The same wire will radiate anelectromagnetic field if current is flowing through it.

The frequency at which a radio operatesdetermines what size antenna is most suitable fortransmitting and receiving. The higher the frequency,the smaller the antenna will be. Lower frequenciesrequire larger antennas. For example, the full-wavelength of an antenna designed to operate at 4 MHz isabout 250 feet. Since this is too long for shipboardapplication, antennas are designed in submultiplelengths. These include half-wave and quarter-waveantennas.

An antenna can be tuned by introducing acapacitive or inductive load. This loading effectivelychanges the electrical length of the antenna and can beused to extend the frequency range of the antenna.For more information on antenna design andoperation, refer to the Navy Electricity andElectronics Training Series, Module 10, IntroductionTo Wave Propagation, Transmission Lines, andAntennas, NAVEDTRA B72-10-00-93.

Transmission Cycle

The data flow for the Link-11 transmission cycleis shown in figure 2-4. The CDS computer receivesdata from the various ship’s sensors, navigationsystems, and operator entries, and stores this data in adata base. When a Link-11 transmission is required,the computer outputs parallel digital data through theSGS computer to the cryptographic device. Thecryptographic device encrypts the data and sends theencrypted data to the data terminal set (DTS). TheDTS converts the digital data to analog audio tones,

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Figure 2-4.—Link-11 data flow for the transmit cycle.

keys the transmitter using the radio set keyline, andpasses the audio tones, via the communicationsswitchboard, to the transmitter for modulation to theRF carrier signal. The radio set keyline is a signalthat switches the radio to the transmit mode. Whenthe signal is stopped, the radio reverts to the receivemode.

When you are using the HF band, the radiofrequency signal modulation uses amplitudemodulation independent sideband; that is, the uppersideband (USB) and lower sideband (LSB) aretransmitted independently in an effort to overcomepropagation-caused signal losses. The UHF radiouses frequency modulation; therefore, only the USBis used.

Receive Cycle

When a transmitted signal is received, the receiverdemodulates the audio tones from the RF carrier andpasses them via the communications switchboard tothe DTS. The DTS demodulates and demultiplexesthe audio tones into digital data. The digital data issent to the cryptographic device where it is decryptedand sent to the CDS computer for processing.

LINK-11 NET OPERATING MODES

Before we look into the actual operation of thedata terminal set, you need to have some knowledgeof the Link-11 modes of operation and how themessages are formed. Link-11 employs networked(net) communications techniques for exchangingdigital information among airborne, land-based, andshipboard systems. As you have seen, the amount ofhardware required to support Link-11 operations isrelatively small; however, establishing andmaintaining a successful link can be very complex.

Establishing a Link-11 Net

The establishment of a successful link involvesthe interaction and teamwork of the operators andtechnicians of several units working towards thecommon goal. If one unit is having trouble with thelink radio, data terminal set, or other equipment, it canmake the entire link unreliable.

When a task force is about to deploy, the taskforce commander will issue a message that has thenecessary information required to establish Link-11communications. The information in this messageincludes a list of primary and secondary frequencies,designation of the initial net control station, an initialgridlock reference unit (GRU) designation, PUidentification and addresses, an initial data linkreference point (DLRP), and required operatingprocedures. Voice communications are required fornet control and coordination during initialization.

When the task force is formed, the picket stationsinform the net control station (NCS) of their readinessto establish link operations. Upon establishingcommunication with all units, NCS transmits NetSynchronization (Net Sync). If the NCS is usingcorrected timing (normal mode), the Net Sync verifiesthe communications path between NCS and all picketunits. If a picket unit cannot receive Net Sync, itcannot participate in the net. Net Test should followNet Sync. Net Test is used to confirm connectivitybetween the Link-11 units. Units having difficulty inreceiving Net Sync or Net Test should report to NCSthat they are not able to participate in the net and thenbegin corrective action.

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When Net Test is completed, all picket stationsreport their status to NCS. Then NCS directs all PUSto switch to the Roll Call mode and initiate linkoperations. Net Synchronization and Net Test areused in the initialization of the net. The normal modeof operation is Roll Call.

The above scenario has introduced you to severalnew terms and modes of operation. These areexplained in detail in the following paragraphs.

The following are the six modes of Link-11operation:

Net Synchronization

Net Test

Roll Call

Broadcast

Short Broadcast

Radio Silence

Net Synchronization

The Net Sync mode of operation is used toestablish a uniform time base from which all net datacommunications normally initiate. The Net Syncmode is usually initiated when establishing a link netafter all operator entries have been properlycompleted. The Net Sync transmission is manuallystarted by the operator on the NCS platform andcontinuously transmits the Link-11 preamble untilstopped by the operator.

The preamble consist of two tones–the 605-Hztone and the 2,915-Hz tone. During the transmissionof Net Sync, the 2,915-Hz tone is periodically phasedshifted 180 degrees. The time between these shifts isdetermined by the selected data rate and is called aframe.

Each PU is equipped with a very accurate timebase in the form of a frequency standard (internal orexternal). When the NCS transmits Net Sync, each

unit receiving the transmission synchronizes itsindividual time base with the Net Sync signal. If thepicket station is operating in the corrected sync mode,as is normally the case, the picket will check to seethat it can recognize the Net Sync signal as a means ofverifying that a good radio link has been established.If a picket is going to operate in the stored sync mode,it will align its stored frame timing to the timing ofthe NCS, using the received Net Sync signal. Sincestored sync timing locks the picket to the time base ofthe NCS, data from other pickets may be lost.Therefore, this mode should only be used duringtimes of poor radio propagation or signal jamming.After the completion of Net Sync, the next operationperformed in establishing a link is usually Net Test.

Net Test

Net Test provides an overall evaluation of the netand equipment performance. When you are operatingin this mode, NCS will broadcast canned test data toall pickets within the net. The data terminal setcontains a code generator that generates twenty-one30-bit data words. Once all the words in the wordtable have been generated, the process automaticallystarts over and keeps running until stopped by theoperator.

Net Test will test the connectivity between allunits and the operation of the DTS. Since it is a localtest, Net Test does not check the interface between theCDS computer and the DTS. Net Testis also helpfulto the technician for setting the audio input and outputlevels of the DTS or radio set.

Roll Call

Roll Call is the normal mode of operation. In thismode, the operator on the NCS platform entersownship’s address and an assigned address (PUnumber) for each PU in the proper switch position.When the link is initiated, each PU is polled for data.Polling consists of sending a call-up message. If thePU fails to respond, the call-up is repeated. If the PUstill does not respond, it is skipped and the next PU ispolled. When a PU recognizes its own address, thePU will transmit its data to all the participants in thelink. When the NCS recognizes the end of the PU

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reply, it automatically switches to the transmit modeand calls up the next PU address. After all the units inthe net have been polled, the NCS transmits its owndata and the process is continuously repeated. TheRoll Call mode provides all PUS with continuous,near real-time exchange of tactical information.

Broadcast

When the Broadcast mode is used, one PU willcontinuously send a series of data transmissions to allthe members of the net. Once manually initiated, thetransmission will continue to be sent automaticallyuntil the operator manually stops it. Through the useof the broadcast mode, other picket stations canreceive real-time tactical information withoutbreaking radio silence.

Short Broadcast

In the Short Broadcast mode, a picket station orthe NCS sends a data transmission to the othermembers of the net. The transmission is initiated bythe operator depressing the TRANSMIT STARTbutton on the DTS control panel and is terminatedautomatically when the computer has finished sendingthe DTS data. This mode is used only when no otherunit is transmitting.

Radio Silence

In the Radio Silence mode, the radio set key lineand the data terminal set audio output are disabled.The receive capability of the DTS is not affected. TheRadio Silence mode is manually initiated andterminated.

BUILDING A LINK-11 MESSAGE

Information transmitted from the DTS originatesfrom two sources. Tactical data always originatesfrom the CDS computer. Other information,including the preamble, phase reference, start andstop codes, and address frames, originates within thedata terminal set. These additional special-purposeframes are added to the data frames to form the propermessages.

For the DTS to control the net properly, strictadherence to the correct message format and netprotocol are required. Every Link-11 message has aspecific format and function. Each Link-11 messagegenerated by the DTS begins with a header consistingof the preamble (five frames) and the phasereference frame (one frame). Control codes, such asthe start code, the picket stop code, and the controlstop code, are also required.

Preamble

The preamble, as previously covered, consists ofa two-tone signal. The two tones are the 605-HzDoppler tone and the 2,915-Hz sync tone. Thepreamble is five frames long and is transmitted at fourtimes the normal power, as shown in figure 2-5. Amore detailed explanation of the preamble tone isprovided later in this chapter.

Figure 2-5.—The Link-11 preamble power levels andframe count.

Phase Reference Frame

The phase reference frame follows the preambleand is shown in figure 2-6. This frame is composedof the normal 16-tone composite signal with the datatones transmitted at 0 dB and the Doppler tonetransmitted at +6 dB. The phase reference frame

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provides the reference for the first frame of data.Each succeeding frame becomes the phase referencefor the following frame.

Figure 2-6.—The phase reference frame added to thepreamble with normal data tone levels.

Information Segment

The information segment of the Link-11 messageis composed of control code frames and message dataframes. Control code frames consist of a start code,a stop code, and an address code. Each control codeis two frames in length and performs a specificfunction. Control codes are not sent to the CDScomputer.

START CODE.— The start code is a two-framecode that follows the phase reference frame, as shown

Figure 2-7.—The start code added to the Link-11transmission.

in figure 2-7. When sensed by the DTS, the start codecauses the DTS to send a prepare-to-receive datainterrupt to the CDS computer.

MESSAGE DATA FRAMES.— Message dataframes contain the tactical data being disseminatedand follow the start code, as shown in figure 2-8. Thenumber of message data frames depends on theamount of tactical information the unit transmits. The24 bits of data contained in each frame is sent to theCDS computer.

Figure 2-8.—The message data frames added to theLink-11 transmission.

STOP CODE.— The stop code is a two-framecode that follows the data message in a Link-11transmission and is shown in figure 2-9. There aretwo types of stop codes: the control stop code and thepicket stop code. The control stop code is used inmessages originated by NCS (NCS report) andindicates that a picket address code follows the stopcode. The picket stop code indicates to the NCS thatthe picket unit has completed its messagetransmission. Both the control stop code and picketstop code cause the receiving DTS to send the End-of-Receive interrupt to the CDS computer.

LINK-11 MESSAGE FORMATS

The formats of the messages transmitted by theLink-11 system vary with the mode of operation.

Roll Call Mode Messages

In the Roll Call mode, the unit designated as thenet control station sends out two types of messages.These are the NCS call-up message (interrogation)and the NCS report (message with interrogation). Athird message, the picket reply message, is sent bypicket unit in response to interrogation messages.

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Figure 2-9.—The stop codes added to the Link-11transmission.

C A L L - U P ( I N T E R R O G A T I O N )MESSAGE.— This message shown in figure 2-10consists of the five-frame preamble, the phasereference frame, and the two address frames. Thecall-up message does not use start and stop codes.

Figure 2-10.—The NCS call-up message.

NCS REPORT AND CALL-UPMESSAGE.— This message shown in figure 2-11consists of the five-frame preamble, the phasereference frame, the two-frame start code, the dataframes containing the NCS report, the two-framecontrol stop code, and two frames containing theaddress code for the next PU.

Figure 2-11.—The NCS report message.

PICKET REPLY MESSAGE.— The picketreply message shown in figure 2-12 consists of the

two-frame start code, the data frames, and the two-frame picket stop code.

Figure 2-12.—The picket reply message.

Short Broadcast Messages

The Short Broadcast is a single data transmissionto all members of a net by a station that may be actingas either picket or NCS. It is the same format as thepicket reply message shown in figure 2-12. The ShortBroadcast message is manually initiated by theoperator at the DTS.

Broadcast Mode Messages

The Broadcast mode messages consist of acontinuous series of short broadcast messages,separated by two frames of dead time, as shown infigure 2-13. The message format is the same as apicket reply message. In the Broadcast mode, onlyone unit will transmit.

Net Test Mode

The Net Test message consists of the five-framepreamble, the phase reference frame, and the Net Testwords generated by the DTS. When all the Net Testwords in the library have been transmitted, thesequence starts over until the operator stops the NetTest.

LINK-11 DATA TERMINAL SET (DTS)

As you have seen, the data terminal set is the heartof the Link-11 system. The DTS performs themodulation, demodulation, and control functionsrequired for proper Link-11 operation. It accepts datafrom the CDS computer in the form of 24-bit datawords, adds six bits of error detection and correction(EDAC) data, and converts all 30 bits into an audio

five-frame preamble, the phase reference frame, the tone package that is sent to the transmitter portion of

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Figure 2-13.—Broadcast mode message format.

the radio set. The key-line signals necessary tocontrol the transmit and receive states of the radio setare also generated by the DTS. Data received fromthe upper sideband (USB) and lower sideband (LSB)p o r t i o n s o f t h e radio set receiver, inthe form of audio tones, is converted into parallelbinary data and sent to the CDS computer.

Currently several design generations of Link-11data terminal sets are used in the fleet. These includethe AN/USQ-59 and 59A, the AN/USQ-63, and theAN/USQ-74. Originally introduced in the early1960s, each successive generation of the Link-11 dataterminal set reflects additional knowledge gainedfrom fleet use and advances in technology. Althoughthe technology used in the different models of theLink-11 DTS may be vastly different, all of themperform the same function.

Normally, the DTS operates in the half-duplex

mode, meaning it can either receive or transmit data,but it cannot do both at the same time. An exceptionis during system test when the DTS operates in full-duplex mode and can simultaneously send andreceive data.

DATA TERMINAL SET FUNCTIONS

The DTS also performs the following functions:

Error detection and correction

Audio signal generation

Link-11 protocol and interface control

Error Detection and Correction (EDAC)

The DTS receives data from the CDS computer inthe form of 24-bit binary data words. The 24-data bits

Table 2-1.—DTS Parity Bit Status Codes

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are expanded to 30 bits by adding six bits for errordetection and correction (EDAC). These six bits arealso called hamming bits. The value of these bits isbased on parity checks of specific combinations of the24-bit data word.

During the receive cycle, the six EDAC, orhamming bits, are examined for errors. There isenough redundancy in the EDAC to allow forcorrection of a single bit error. The operator cancontrol the selection of the error correction mode. Ifthe data word is not a control word, the word isexamined to determine if it is error-free, contains acorrectable error, or contains uncorrectable errors. Ifthe DTS is in the error detection and label mode, adetected error is identified and labeled before the dataword is sent to the CDS computer. In the errordetection and correct mode, the DTS attempts tocorrect an error before sending the data word to theCDS computer. In both modes, the six EDAC bits aredeleted and replaced with two parity error status bits.These status bits are defined in table 2-1.

Audio Tone Generation and Characteristics

The DTS converts the 24-bit data word, alongwith the six EDAC bits, into a composite audio signalconsisting of 16 tones. This composite 16-tone signalis the data frame. The tones range in frequency from605 Hz to 2,915 Hz and are the odd harmonics of 55Hz. The specific frequencies of the tones are shownin table 2-2. The 605-Hz tone is used for Dopplercorrection, and the 2,915-Hz tone is used for data andsynchronization. Each of the data subcarrier tones(tones 2 through 16 in table 2-2) represents two binarybits of differential quadrature phase-shift modulateddata.

The Doppler tone (605 Hz) is not phasemodulated. It is used to correct for Doppler shifts inthe received tones caused by the relative motionbetween the transmitter and the receiver. It is alsoused to correct for the Doppler shift that may occurbecause of differences between the transmitter andreceiver frequency standards.

The 2,915-Hz tone has two separate uses. Duringthe transmission of the preamble and Net Sync, the

2,915-Hz tone is used to identify frame timing. Thistone is phase shifted 180 degrees at the end of eachframe. When detected by the receiving DTS, thephase shift indicates the start of a new frame. Whenthe DTS is in corrected timing, this information isused to set the timing for the data frames that follow.When stored timing is used, the frame timing that wasset during Net Sync is used.

The Doppler and sync tones vary from each otherand the other data-carrying tones in amplitude. TheDoppler tone is 6 dB greater than the other tones.During the Net Sync and preamble frames, theDoppler tone is transmitted at 12 dB and the sync toneis transmitted at 6 dB. The Doppler tone istransmitted at 6 dB during the transmission of dataframes and the sync tone is used as a data tone. Datatones are transmitted at 0 dB.

The audio tones are divided into data frames toidentify the separate parallel groupings of 30 bits. Itis the phase angle shift of each of the 15 data tonesthat conveys the digital information contained in thetone. During each frame, each data tone frequencyhas a particular phase. At each frame boundary, thephase of each data tone is shifted with respect to theprevious frame. The amount of this phase change, orphase difference, determines the value of a two-bitnumber. Two data bits yield the following fourpossible combinations: 00, 01, 10, and 11. Eachcombination is associated with a phase difference ofone of four values: 45 degrees, 135 degrees, 225degrees, or 315 degrees from the previous position.

Each of these angles marks the center of aquadrant, as shown in figure 2-14. Each 90-degreequadrant is assigned a two-bit binary value. Anyphase difference falling within that quadrantrepresents that binary value. This system of dataencoding can tolerate an error in the prescribed phaseshift of up to ±44 degrees before a single bit error willoccur. An error in phase shift that is greater than 45degrees, but less than 135 degrees, will cause thephase angle to fall into an adjacent quadrant. Noticethat the values are assigned to each quadrant in sucha way that if a phase shift error occurs, only one biterror will be introduced as long as the quadrant intowhich it falls is adjacent to the target quadrant.

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Table 2-2.—Tone Library

Figure 2-14.—Link-11 data phase shift encoding.

Link Protocol and Interface Control

In addition to encoding data from the CDScomputer, the DTS generates and recognizes protocoldata that controls the type and number of linktransmissions. These protocol words include codesindicating the start of transmission, the end oftransmission, and the address of the next unit totransmit.

The interface with the CDS computer is under thecontrol of the DTS. The DTS signals the CDScomputer when it has input data or when it wantsoutput data through the use of external interrupts.These interrupts include the prepare-to-transmit,prepare-to-receive, and end-of-receive interrupts.

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Figure 2-15.—The AN/USQ-59 Mode Control panel.

DTS CONTROLS AND INDICATORS

Many parameters that affect the operation of theDTS are under the operator’s control, whether thestation is operating as a picket or as the net controlstation. Both the operator and the technician must befamiliar with the various controls and indicatorsassociated with the DTS. The AN/USQ-59 usesseveral control panels that are usually mounted nextto the operator’s display console. These panels enablethe operator and the technician to control and monitorthe net operation.

The control panels include a Mode Control panel,a TADIL A Control panel, and an Address SelectionIndicator panel. Although the AN/USQ-59 controlpanels are used here to show the controls andindicators of a Link-11 DTS, other data terminal setshave similar controls.

DTS Mode Control Panel

The DTS mode control panel controls andindicators are shown in figure 2-15. The following isa summary of how the controls affect the operation ofthe link and what the indicators mean.

TRANSMIT MODE INDICATOR— Lightswhen the DTS is in the transmit mode.

RECEIVE MODE INDICATOR— Lights whenthe DTS is in the receive mode.

SUMMARY FAULT INDICATOR— Lightswhen a fault in the DTS is detected while the DTS isin the OPERATE mode. There are 27 performancemonitor fault-sensing circuits in the data converter(modem) of the DTS. During the OPERATE mode,14 of these sensors can cause a summary fault. Thefault-sensing circuits monitor areas such as variouspower supplies, signal quality, preamble presence,timing, and audio signal quality. When the DTS is inSELF TEST, the summary fault lamp is lighted whena fault is isolated to a function defined by switchpositions on the fault isolation control and built-intests routines.

LAMP TEST BUTTON— Causes all indicatorson the mode control panel, the TADIL A controlpanel, and the address control unit to light.

FAULT MONITOR/RESET SWITCH— In theMONITOR position, this switch allows the fault-sensing function of the DTS to operate normally andprovide a fault summary signal to the DTS control.When the switch in placed in the RESET position, thefault-sensing circuits of the DTS are reset. TheSUMMARY FAULT lamp is turned off when thefault-sensing circuits are reset.

INTERNAL 100 KHZ/EXTERNAL SWITCH-Allows for the selection of the internal or external100-kHz frequency standard.

DOPPLER CORR ON/CORR OFF SWITCH-Enables the DTS Doppler correction when placed inthe CORR ON position.

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F U L L - D U P L E X / H A L F - D U P L E XSWITCH— In the FULL-DUPLEX position, thisswitch enables full-duplex operation of the dataconverter and the computer I/O adapter. It alsoenables loop back processing of the transmit sidetonedata for input to the computer. In the HALF-DUPLEX position, the DTS operates in the half-duplex mode and the transmit sidetone is disabledfrom being processed and input to the computer.Link-11 uses the half-duplex mode.

SIDEBAND SELECT SWITCH— When theSIDEBAND SELECT switch is placed in the LSB orUSB position, the DTS processes only the lowersideband or upper sideband of the received signal.When the switch is in the DIV position, the DTScombines both the upper sideband and the lowersideband signals to create frequency diversity data forinput to the computer. When the switch is in theAUTO position, the DTS selects the signal with thebest receive quality for processing. The AUTOposition is the normal position of this switch.

DATA RATE SWITCH— Selects the data ratethat the data converter uses. When the switch is in theDUAL 1200 position, the data converter can transmitand receive two unrelated streams of data at 1200 bps.When the switch is in either the 1200 or 2400position, the data converter transmits and receives asingle data stream at 1200 or 2400 bps, respectively.When the switch is in the TADIL A position, the datarate is controlled by the DATA RATE switch on theTADIL A control panel. The TADIL A position isthe normal position for Link-11.

SYNC MODE SWITCH— The SYNC MODEswitch selects the mode of synchronization used bythe DTS receive circuitry and is used in conjunctionwith the TIMING STORED/CORRECTED switch onthe TADIL A control panel. The normal operatingposition for the SYNC MODE switch is in theFAST/CONT position.

When the switch is in the FAST/CONT position,both the fast and continuous synchronization circuitsof the DTS are selected. Synchronization is initiallyobtained during the five-frame preamble andmaintained continuously during the data portion of the

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transmission. The TIMING switch on the TADIL Acontrol panel must be in the CORRECTED position.

When the FAST pos i t ion i s se lec ted ,synchronization is only during the five-framepreamble. If the CONT position of this switch isselected, only the continuous synchronization circuitsare selected. Synchronization is obtained only duringthe data portion of the transmission. The TIMINGswitch on the TADIL A control panel must be in theCORRECTED position for both of these modes.

The INHIBIT position of this switch disables boththe fast and continuous synchronization circuits of theDTS. The DTS will maintain the time base that wasstored when the switch was turned to INHIBIT. Forsynchronization to be held, the unit with its syncmode inhibited must maintain its original geographicrelationship to all other units in the net. This positionis used when the received signal contains interferencethat could cause loss of synchronization.

OPERATE/SELF TEST SWITCH— Thisswitch must be in OPERATE for normal on-lineoperations. When the switch is placed in the SELFTEST mode, the DTS is placed in an off-line modeand the fault isolation circuitry is enabled.

CONTROL ON/OFF SWITCH— When theCONTROL switch is placed to the ON position,+28Vdc is applied to the fault isolation control panel,the mode control panel, the TADIL A control panel,and the address control panel.

TADIL A Control Panel

The TADIL A control panel provides the controlswitches and indicators required to control andmonitor Link-11 operations. Figure 2-16 shows theAN/USQ-59 TADIL A control panel.

XMT DATA ERROR INDICATOR— Thisindicator is lighted when the DTS detects an errorwhile transmitting data in the TADIL A, or Link-11,mode.

RCV DATA ERROR INDICATOR— Thisindicator is lighted when the DTS detects an error inreceived data being sent to the CDS computer.

CODE ERROR INDICATOR— The CODEERROR indicator is lighted when the DTS detects anerror in the received or sidetone (transmit) controlcodes during TADIL A operations.

NET BUSY INDICATOR— The NET BUSYindicator is lighted when the DTS detects that thecommunications net is busy. It is activated when asignal called signal presence is generated by theDTS.

SYNC COMPT INDICATOR— The SYNCCOMPT indicator is lighted continuously, or flashes,when the DTS has achieved synchronization with theNCS data terminal.

TIMING STORED/CORRECTED SWITCH-The TIMING STORED/CORRECTED switchdetermines how the DTS is synchronized. When theswitch is in the CORRECTED position, the fastsynchronization and/or the continuoussynchronization circuitry in the DTS is used. Theposition of the sync mode switch on the mode controlpanel determines whether the fast, continuous, or bothcircuits are used to maintain synchronization. Whenthe switch is in the STORED position, the DTS usesthe time base stored during Net Sync. During normaloperations, this switch should be in the CORRECTEDposition.

OPERATE/RADIO SILENCE SWITCH— TheOPERATE/RADIO SILENCE switch is a two-position toggle switch that allows the DTS to inhibitradio transmissions. When the switch is in theOPERATE position, the DTS operates normally.When the switch is switched to the RADIO SILENCEposition, the radio keyline and transmit audio circuitsare immediately disabled.

NET CONTROL/PICKET SWITCH— TheNET CONTROL/PICKET switch configures the DTSto operate as the net control station or a picket stationin Roll Call mode.

ERROR CORRECT/LABEL SWITCH.— TheERROR CORRECT/LABEL switch determines howthe DTS processes detected errors. When the switchis in the CORRECTED position, the DTS attempts tocorrect detected errors. If a single bit error isdetected, the location of the erroneous bit is detectedand corrected. If an even number of bit errors occurs,the correction circuitry is inhibited. If an odd numberof bit errors occurs, the correction circuitry attemptsto correct the data; however, if an odd number ofmultiple bit errors occurs, an erroneous correction ismade. When the switch is in the LABEL position, theDTS does not attempt to correct detected errors.Instead, the data word sent to the computer is labeledto indicate that errors were detected in the data word.

Figure 2-16.—The AN/USQ-59 TADIL A control panel.

TRANSMIT RESET SWITCH— TheTRANSMIT RESET switch is a momentary contactpushbutton switch. When depressed, this switchcauses any transmission in progress to be terminated.The DTS stops the transmission by inhibiting thegeneration of the output data request, causing a stop

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code to be transmitted. The DTS also resets theaddress control address sequence logic.

NET BUSY INDICATOR— The NET BUSYindicator is lighted when the DTS detects that thecommunications net is busy.

TRANSMIT INITIATE SWITCH— TheTRANSMIT INITIATE switch is a momentarycontact pushbutton switch that causes the DTS toinitiate data transmission when the DATA RATEswitch is in the TADIL A position. The TRANSMITINITIATE switch must be depressed to initiate allDTS transmissions except when the DTS isconfigured as a picket and is in the Roll Call mode.When the net is in the Roll Call mode, only the netcontrol station is required to initiate transmission bydepressing the TRANSMIT INITIATE switch.

MISS CALL INDICATOR— The MISS CALLindicator is lighted when the net control station hasdetected no response from a picket station after twosuccessive interrogations. Once lit, it will remain lituntil a picket responds or the TRANSMIT RESETswitch is depressed.

ADDRESS COMPUTER/CONTROLSWITCH- The ADDRESS COMPUTER/CONTROLswitch determines the source of the address used bythe DTS. When the switch is in the CONTROLposition, addresses are obtained from the addresscontrol unit. When the switch is in the COMPUTERposition, addresses are obtained from the CDScomputer, provided the computer is configured forexternal function operations. The normal position forthis switch is depends on the configuration of thesystem on your ship.

NET MODE SWITCH— The NET MODEswitch determines the mode of operation of the DTS.The modes are BC or broadcast, SHORT BC, ROLLCALL, NET SYNC, and NET TEST.

DATA RATE SWITCH.— The DATA RATEswitch determines the speed and frame timingoperation of the DTS. When the switch is in the1364/9.09 position, the DTS transmits and receives

interval is approximately 9.09 milliseconds. Whenthe switch is in the 2250 position, the DTS transmitsand receives data at a rate of 2250 bps and a frameinterval of 9.09 milliseconds. When the switch is inthe 1364/18.18 position, the data rate is 1364 bps, butthe frame phase shift interval is increased to 18.18milliseconds.

OWN STATION ADDRESS SWITCH— TheOWN STATION ADDRESS switch consists of twothumb wheel switches in which an address is enteredto identify the address the DTS will respond to as itsown. In the Roll Call mode and with the DTSconfigured as a picket station, the DTS will transmitits tactical data when the interrogation messageaddress matches the address entered into the OWNSTATION ADDRESS switches.

RANGE IN MILES SWITCH— The RANGEIN MILES switch also consists of two thumb wheelswitches. These switches are used to select theapproximate distance between the net control stationand the picket station. The range entered into theseswitches causes the DTS to alter the frame timing tocompensate for the signal propagation delay betweenthe picket station and the NCS. The range in milessetting for the NCS is always zero miles.

Address Control Indicator

The address control indicator is used to set theaddress of the picket stations to be interrogated whena unit is configured to operate as the NCS. Theaddress control indicator is shown in figure 2-17. Theaddress control indicator consists of 20 identicaladdress selection modules, which are used to addressup to 20 stations. More than one address controlindicator may be installed in a system to provide theability to interrogate more than 20 stations.

Each address selector module has two thumbwheel switches in which one of 64 octal addressesmay be entered (address 00 and 77 octal are invalid).Also, each address selector module has a power on/offswitch, a power on indicator lamp, and a callindicator, as shown in figure 2-18.

data at 1364 bps. The data frame phase identification

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When a unit is configured as the NCS, theoperator enters all the assigned addresses of the netparticipating units into the address selector modules,and turns on each module with a valid address. Oncethe Roll Call mode is initiated, the DTS will checkeach module sequentially. If the power of the moduleis on and a valid address is entered, the address is sentto the DTS for use in an interrogation message. If thepower switch is in the OFF position, that module isskipped, even if it contains a valid address. Whenenabled by the DTS, the address selector modulesends the address entered in the thumb wheels to theDTS and the call indicator light. The call indicatorwill remain lit until the DTS sequences to the nextaddress module.

CDS INPUT/OUTPUT CONTROL

The data terminal set controls the exchange ofdata with the CDS computer. As describe earlier,input/output communications p r o t o c o l i saccomplished through the use of external interrupts.The prepare-to-transmit data interrupt, the prepare-to-receive data interrupt, and the end-of-receive datainterrupts control the DTS to the computer interface.

CDS Computer Input (Receive) Data Cycle

The input data cycle is initiated by the DTS.When the DTS recognizes the second frame of thestart code, it sets the prepare-to-receive data interrupton the input data lines and sets the external interruptline. The computer acknowledges the receipt of theinterrupt by sending an input data acknowledge (IDA)to the DTS.

Upon receipt of the first message frame, the DTSdemodulates the 24-bit word and places it on the inputdata lines, along with the two error detection andcorrection bits. Once the data is placed on the inputdata lines, the DTS sets the input data request (IDR)line. The computer will sample the data and send anIDA. This process repeats for all frames of themessage. The first frame of the stop code is alsotreated as a message frame and sent to the CDScomputer. When the DTS recognizes the secondframe of the stop code, it will place the end-of-receiveinterrupt on the input data lines and set the interrupt

Figure 2-17.—The Address Control Indicator C9062/U.

line. The interrupt is then processed by the CDScomputer and the input buffer is closed.

If the received stop code is a picket stop code, theDTS simply resets itself. If the stop code is a controlstation stop code, the DTS will compare the next twoframes received with its own station address code.

CDS Computer Output (Transmit) Data Cycle

The output data cycle is initiated when the DTSdetects its own station address, either in an

Figure 2-18.—An address selector module.

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interrogation message or at the end of an NCS reportand interrogation message. When the DTS recognizesits own station address, it starts to transmit thepreamble. During the first frame of the preamble, theDTS sets the prepare-to-transmit interrupt on theinput data lines. The computer samples the interruptand sends an IDA to acknowledge receipt of theinterrupt.

The DTS finishes sending the preamble and phasereference frames. During the second frame of thestart code, the DTS sets the output data request (ODR)active, requesting the first word of the tactical data.The CDS ‘computer responds by placing 24 bits ofdata on the lines and then setting the output dataacknowledge (ODA). The DTS samples the data andclears the ODR. The first frame of data is processedfor transmission and the ODR line is then set torequest the next data word.

This procedure is repeated until all the data wordshave been transmitted. Once the CDS computer hascompleted sending all the data words, it will notacknowledge the ODR from the DTS. If the CDScomputer has not acknowledged an ODR from theDTS in a preset amount of time, the DTS will clearthe ODR line and generate a stop code. Upontransmission of the two-frame stop code, the DTS willreturn to the receive mode.

Net Control Station (NCS) I/O Operations

The station acting as NCS follows the sameprotocols when communicating with the CDScomputer. Some differences exist in the generation ofthe control codes. The net control station isresponsible for interrogating each station. Uponreceipt of a picket stop code, the DTS checks the nextstation address and sends an interrogation message.After the interrogation message is transmitted, theDTS waits to receive a start code from theinterrogated station. If a start code is not recognizedafter 15 frame intervals, the station will bereinterrogated. If a start code is not received afteranother 15 frame intervals, the address control unitwill advance to the next active picket address andrepeat the interrogation process.

The other major difference is when the net controlstation has completed its own tactical datatransmission, a control stop code, followed by thenext station address, is transmitted. Again, if a startcode is not received within 15 frame intervals, asecond interrogation is sent. This secondinterrogation is a normal interrogation messageconsisting of the preamble, phase reference frame, andaddress code.

Modulator/Demodulator

The modulator/demodulator function of the DTSprovides the digital to analog and analog to digitalconversion. During data transmission, the 24-bitbinary data word is expanded to 30 bits by adding thesix bits for error detection and correction. The 30 bitsare then examined in pairs to determine the requiredphase angle shift for each of the 15 data-carryingtones in the audio package.

Figure 2-19.—Link-11 frame boundary phase shifts.

At the frame boundary, the phase of each datatone is shifted with respect to the previous frame.

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Figure 2-19 shows the four possible phase shifts. Asixteenth tone, the 605-Hz Doppler correction tone, isadded to the tone package. The Doppler tone is notphase modulated and is used to correct for Dopplershifts caused by the relative motion between thetransmitting station and the receiving station. The 16tones are combined into a composite audio signal andsent to the radio set. The radio set transmits thecomposite tone package on the carrier frequency inindependent sideband form.

During receive operations, the tone package isreceived from the radio set. The 30 bits of data areextracted from the tone package by determining thephase shift of each data tone with respect to theprevious frame. The 30 bits, which contain 24-databits and six-EDAC bits, are examined for errors. Thesix-EDAC bits allow for the detection of errors andprovide enough redundancy to allow for correcting asingle bit error.

The operator can select whether or not the DTSattempts to correct detected errors, as explained earlierin this chapter. In the error detect (label) mode, adetected error is identified and labeled before it is sentto the CDS computer. In the error correction (correct)mode, the DTS attempts to correct a detected error,labels the error, and sends the data word to the CDScomputer.

The DTS is capable of receiving and processingboth the upper sideband and the lower sideband whenusing a HF radio, depending on the position of thesideband select switch. When you are using a UHFradio, only the upper sideband is received andprocessed.

If the sideband select switch is in the USB or theLSB position, only the designated sideband isprocessed. In the diversity (DIV) mode, the 30-bitword is generated by adding the relative phase anglesof the USB and the LSB.

Because of propagation anomalies, noise, andinterference, the AUTO mode can be used to selectthe sideband (USB, LSB or DIV) that yields the mostcorrect data automatically. In the AUTO mode theDTS processes a word from each sideband and the

diversity combination. The decoded words areexamined for errors in the following order or priority:DIV, USB, and LSB. A search of the three words ismade to find a data word with no error. If one isfound, it is selected for input to the CDS computer. Ifnone is found, the RCV DATA ERR indicator is litand the diversity combination data word is sent to theCDS computer.

Radio Set Interface

The DTS generates the following outputs to theradio set: upper sideband composite audio, lowersideband composite audio, and key line. It receivesupper sideband composite audio and/or lowersideband composite audio. UHF radio sets use onlythe upper sideband signal and the key-line signal.

The key-line signal controls the transmit andreceive state of the radio set. The key line is set totransmit Link-11 data. When the key-line is cleared,the radio set returns to the receive mode.

SUMMARY—THE LINK-11 SYSTEM

This chapter has introduced you to the Link-11, orTADIL A, system. The following informationsummarizes important points you should havelearned.

LINK-11 SYSTEM— Link-11 is used to transmitREAL-TIME tactical information between CDS-equipped ships and similarly equipped ships, aircraft,and shore stations. The typical shipboardconfiguration of Link-11 hardware consists of thefollowing:

CDS computer

Shipboard Gridlock System

Cryptographic security device

Data terminal set

Communications switchboard

HF or UHF radio set

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Antenna coupler and antenna

LINK-11 NET OPERATING MODES— Thesix modes of Link-11 operation are as follows:

Net Synchronization

Net Test

Roll Call

Broadcast

Short Broadcast

Radio Silence

The Net Synchronization mode establishes theinitial time base between the NCS and allparticipating units. The Net Test mode tests theconnectivity of all units in the net and the operation ofthe DTS. The Roll Call mode is the normal mode ofLink-11 operations. The Broadcast mode allows asingle unit to transmit tactical data to all other unitsrepeatedly. The Short Broadcast mode allows a unitto broadcast its tactical data once every time theoperator depresses the transmit start switch. RadioSilence allows a unit to receive Link-11 data, but thatunit will not transmit data.

BUILDING A LINK-11 MESSAGE— ALink-11 message consists of the preamble, the phasereference frame, and the information segment. Thepreamble is five frames long and contains the 605-HzDoppler tone and the 2,915-Hz sync tone. The phasereference frame is one frame and provides a startingreference for the information segment. Theinformation segment of the Link-11 message containscontrol codes and tactical data. Control codes are thestart codes, the stop codes, and the address codes.

LINK-11 MESSAGE FORMATS— The formatof a Link-11 message is depends on the mode ofoperation. The Roll Call mode consists of thefollowing three different messages: the call-up(interrogation) message, the NCS report and call-upmessage, and the picket reply message. The call-upmessage consists of the preamble, the phase reference

frame, and the address code of the unit beinginterrogated. The NCS report is made up of thepreamble, the phase reference frame, the start code,the tactical information, the control stop code, and theaddress of the next unit. The picket reply message iscomprised of the preamble, the phase reference frame,the start code, the tactical data, and the picket stopcode.

The Broadcast and Short Broadcast messages arethe same format as a picket reply message. In theBroadcast mode, one unit repeatedly broadcasts itstactical data and all other units in the net monitor thisdata. In the Short Broadcast mode, the operator mustinitiate each transmission of data.

In the Net Synchronization mode, the message isa continuously broadcast preamble. The Net Testmode message consists of the preamble, the phasereference frame, and a preset series of data words thatare repeated until the test is stopped by the operator.

LINK-11 DATA TERMINAL SET (DTS)— TheLink-11 data terminal set is the heart of the Link-11system. The data terminal set performs themodulation, demodulation, and control functionsrequired for Link-11 operations.

DTS ERROR DETECTION ANDCORRECTION (EDAC)— The DTS is capable ofdetecting and correcting single bit errors in receiveddata. It accomplishes this correction by decoding thesix hamming bits that are added to the 24-bit dataword by the transmitting DTS. When a correction ismade, if there are multiple errors or the DTS is in theerror detect and label mode, the error is designated bytwo parity bits added to the 24-bit data word beforethe data is sent to the CDS computer.

DTS AUDIO TONE GENERATION ANDCHARACTERISTICS— The DTS generates a16-tone composite audio signal. It converts(modulates) the 24 data bits into 12 audio tones. Eachtone contains two data bits. Added to these 12 tonesare the three tones containing the six EDAC hammingbits. A sixteenth tone is used for Doppler correction.The audio tones are the odd harmonics of 55 Hz. The

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data and hamming tones are quadrature phase shiftedwith respect to the previous frame.

LINK-11 PROTOCOL AND INTERFACECONTROL— The DTS controls the proper protocolsand interface with the CDS computer. The protocolsare controlled by the generation of the control codes.Interface with the CDS computer is controlled by theDTS through the use of external interrupts.

DTS CONTROLS AND INDICATORS— TheDTS control panels provide the operator the means forcontrolling the operation of the DTS. The physicaldesign of the control panels of the various dataterminal sets varies, but the panels all perform thesame functions. Important controls include thesideband select switch, the data rate switch, the syncmode switch, and the net control/picket switch.

Indicators are provided to indicate several types oferrors.

ADDRESS CONTROL INDICATOR— Theaddress control indicator is used to set the addressesof the participating units in the net. The addresscontrol indicator is only used when the DTS is in thenet control station mode.

RADIO SET INTERFACE— The radio setinterface of the DTS provides for the transfer to andreception from the radio set of the composite audiotone package. The DTS receives the composite audioon the upper and lower sidebands when using HFfrequencies and the upper sideband only when usingUHF frequencies. The DTS provides a key-line signalto put the radio set in the transmit mode during thetransmission cycle.

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CHAPTER 3

LINK-11 FAULT ISOLATION

INTRODUCTION

A communications network, such as the Link-11 system, can be very complexwhen the goal is to maintain high-quality communications with all units in the net.Distance, atmospheric anomalies, corrosion, and even the time of day can affect thequality of Link-11 communications. The Link-11 technician has many tools toenable him to pinpoint problems. However, oftentimes the technician maymisunderstand such tools, forget them, or not have the knowledge to use themeffectively.

Problems occurring with Link-11 communications are best approached by meansof the team concept. A typical link team is usually composed of a team leader, anET, a FC, an OS, and an RM. The team leader is usually a senior ET and could bethe electronics material officer (EMO) or combat systems maintenance officer.

After completing this chapter, you should be able to:

Describe the procedures required for running the single stationProgrammed Operational and Functional Analysis (POFA) on the DTS.

State the circuits verified by the successful completion of single stationPOFA.

Describe the procedures for running multi-station Link-11 POFA.

Describe the components of the LMS-11.

Describe the information presented in each of the LMS-11 displaymodes.

Recognize common Link-11 problems as displayed on the LMS-11.

LINK-11 MYTHS AND FACTS

When a Link-11 problem occurs, usually the link troubleshooting team is calledto the combat direction center. Here they can meet with the operator, talk to otherships in the link, and analyze the displays on the LMS-11. Through these initialsteps, the team can determine several things, such as whether the problem is local orif the entire net is experiencing problems. Because of the complexity of linkequipment, a variety of methods was used over the years to solve link problems. Ifa particular action worked once, it was often assumed that it would work in allinstances. Over the years, this led to a type of folklore or mythology on howtechnicians were to troubleshoot the link. Senior link techs would pass these myths

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on to junior link techs and the mythology developed a life of its own. In thefollowing paragraphs, we examine some of these myths and seek to clarify the realproblems that led to the evolution of the myths.

Myth: Changing the NCS Will Always Solve Net Problems!

Changing the NCS may solve net problems, but only if the current NCS iscausing the problem. What is the problem? If data is not being received from a unitbecause the current NCS has entered the PU number incorrectly, shifting NCSfunctions to a station with the PU data entered correctly will solve the problem.However, it would be easier if the current NCS were simply to enter the correct PUnumbers.

When the current NCS is using a radio set with poor receiver sensitivity and ispolling on top of picket responses, effectively jamming the entire net, changing NCSis imperative. Also, if several units are not recognizing their interrogations becausethe NCS is out of range or in an RF propagation shadow, changing to a unit in abetter location should improve net communications.

Myth: Changing Frequency Always Solves Net Problems!

Here again is a myth that has some basis of fact. Changing frequency is a time-consuming process. When all the procedures are not carefully followed, changingthe frequency induces additional problems into the net. This myth developedbecause improperly set switch positions and patch panel configurations were oftenset to the proper position during the frequency changing process. When the problemis connectivity on the current frequency, the proper action is to find a betterfrequency.

Myth: More Power Improves Link Performance!

This is a myth. On the transmit side, the idea behind the myth is that keeping thelink HF transmitter tuned to maximum output power will result in maximum areacoverage. In fact, constantly outputting maximum power can lead to seriousRFI/EMI problems (on the ship doing so) and will not significantly increase thesignal propagation range.

The idea behind the myth on the receive side is that by keeping the HF receiveraudio output control maximized, receive quality improves. In fact, maximizing theaudio output saturates most data terminal sets. Saturation generally occurs in theDTS at around 3 dBm. Signal inputs above this level actually increase receive dataerrors.

Myth: Dummy PUS Improve Link Quality!

A dummy PU is an address insert into the polling sequence by the NCS for whichthere is no live unit. Dummy PUS cause the net cycle time to increase and netefficiency to decrease. The idea that the NCS must use dummy PUS for the link to

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operate properly is not generally true. It may be true only in infrequent, isolatedcases. Studies have shown that in the old NTDS system (CP-642 computer and theAN/USQ-36 DTS), a dummy PU entered between a live PU and own address wasrequired for NCS data to be output at each NCS report opportunity. Since theCP-642 computer and the AN/USQ-36 DTS have virtually disappeared, dummy PUSshould not be used.

Myth: Radio Silence Reduces Net Cycle Time!

The effect Radio Silence has on net cycle time depends on a number of factors.As you saw in the last chapter, if a PU does not respond to a call up in 15 frames, itis interrogated again. After another 15 frames, if the PU still does not respond, theNCS polls the next PU. If the PU that goes to Radio Silence was sending reports thatexceeded 38 frames, net cycle time will be reduced by the PU going to radio silence.Effective net management would be to eliminate the PU number of the unit that hasto go into Radio Silence until that unit is able to reenter the net.

As you can see, there are several misconceptions on the proper way to manageand troubleshoot the Link-11 system. In this chapter, we concentrate on the toolsavailable to the technician to aid in the isolation of link problems.

LINK-11 PROGRAMMED FUNCTIONALAND OPERATIONAL ANALYSIS (POFAs)

Two types of POFAs are used in the Link-11system. These are the single station POFA, used tocheck components of the Link-11 on board a singlestation, and the multi-station POFA, used to check theconnectivity of several units.

SINGLE STATION POFA

The single station POFA is an end-around test thattransfers canned data from the computer through thecrypto device and the data terminal. The singlestation POFA can also be run through the radio set tocheck out part of the audio communications pathfurther.

POFA Setup

The POFA is a special program that is loaded intothe computer. It is very important that you follow theinstruction manual when attempting to run the POFA.The POFA is designed to run in full-duplex mode.Normal link operations use the half-duplex mode.“Full duplex” means the system is configured to

transmit and receive data at the same time. In theDTS, this is accomplished by the transmit audio beingfed directly into the receive input. Also, if the DTS isoperating in full-duplex mode, the rest of the system,especially the crypto device, must be in fill duplex.On the KG-40, full duplex is accomplished when thefront panel switch is turned to the POFA TESTposition.

Analyzing Single Station POFA

When a single station POFA is completed, aprintout of the results is produced. To analyze thisprintout properly, the technician must understand

Figure 3-1.—Single station POFA configurations.

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what equipment is being tested. The configuration inwhich the POFA is run determines some of theequipment being tested. The POFA can be run in twoconfigurations, as shown in figure 3-1.

In the full configuration, the single station POFAwill test the following areas:

CDS computer I/O channel interruptrecognition and acceptance

Security device I/O path

Data terminal transmit and receive registers,multiplex and demultiplex, and transmit andreceive sequence operations

Switchboard integrity

DTS-to-radio and radio-to-DTS audio path

Capability of the HF radio set to develop andaccept sidebands (both transmit and receive).

By studying the above list, you can see that mostnormal link operations are tested during a singlestation POFA. Certain functions, however, are notchecked by running a single station POFA. The DTSuses the transmit timing as the reference for the entiretest; therefore, the receive timing circuitry is notchecked. Also, certain other functions, such asDoppler correction, are not checked.

The printout generated at the end of a singlestation POFA lists interrupt status, illegal interrupts,parity, and bit-by-bit word errors. A single stationPOFA should always produce a totally error-freeprintout. However, when a printout with errors isreceived, the technician needs to be able to analyzethe error package effectively.

The interrupts, for example, must occur in thefollowing sequence:

Prepare to transmit

Prepare to receive

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End of receive.

If you receive interrupts in any other order, suchas two consecutive prepare to transmit interrupts or anend of receive before the prepare to receive, an errorcondition exists.

The parity should always equal zero. As youlearned in the previous chapter, the parity, or errordetection status bits, indicates an error has beendetected in the received data. When errors aredetected, they are listed in the bit-by-bit section of theprintout.

Even if the printout indicates a few random biterrors, this condition should not be ignored. Randombit errors can be caused by several areas in the system,including the CDS computer, the data switchboard, orthe DTS. You can narrow down to the exact areacausing the problem by running the POFA in severalconfigurations. Changing computers and cryptodevices can aid you in determining the malfunction.

Because of the unique function of the cryptodevice, a single broken line in the switchboard couldcause all the bits to be picked up randomly ordropped. When the broken wire is on the encryptedside of the switchboard, the crypto device reads thestate of that line during the decryption cycle and theentire decryption cycle is changed.

MULTI-STATION POFA

The multi-station POFA is a test of the Link-11system that involves more than one platform.Because this POFA most closely represents normallink operations, more equipment is tested. The multi-station POFA is run in the Roll Call mode using a setof known data words. Figure 3-2 shows the data flowfor a multi-station POFA. A designated unit transmitsa block of 230 data words that are received by theother platforms involved in the multi-station POFA.The receiving computer(s) compare(s) the data againstthe known pattern, count(s) the words in error, andsend(s) this count back to the original ship. Thistransmission is known as the error status report.Ideally, the multi-station POFA should run error-free.

Figure 3-2.—Link-11 multi-station POFA data flow.

Multi-Station POFA Procedures

The procedures for running a multi-station POFArequire coordination of all participating units. For thisto be a good test, all units must be positioned within25 miles of each other. This is usually coordinated bythe Link-11 manager in conjunction with the battlegroup commander.

Just before the time the multi-station POFA is tobe conducted, NCS should end the operational linkand direct all stations to run a single station POFA.The picket station reports back to NCS when thesingle station POFA has been completed. The picketstation will also report the status of the single stationerror printout. Any errors noted during single stationPOFA should be corrected before the multi-stationPOFA, or the station experiencing errors should notbe included in the multi-station POFA.

The multi-station POFA should be run using thesame frequency as the current operational frequency.After running the single station POFA, NCS shoulddirect all participants to go to Radio Silence. Duringthis time, all stations should monitor the assignedfrequency for noise. The frequency can be monitoredthrough the headphones or by using a frequencyanalyzer. A noisy frequency can cause errors in themulti-station POFA. If the frequency is too noisy,consider using an alternate frequency.

Once the frequency has been checked, NCS willtell all participants to prepare to receive POFA. Afterall stations report that they are ready, NCS initiates

the POFA. All stations monitor the POFA, and checkthe control panel of the DTS for errors. After aminimum of 5 minutes, NCS terminates the POFA.

When the POFA is terminated, a printout isgenerated. The final step in running a multi-stationPOFA is the analysis of the printout.

Analyzing Multi-Station POFA Results

Running a multi-station POFA closelyapproximates actual link operating conditions. Toanalyze the printout fully, the technician needs to beaware of some of the factors that can affect linkoperations.

When the printout is completed, the analysis iseasier to complete if the technician records thefollowing information on the printout:

Which station is NCS

Distance and relative bearing of allparticipating units

Frequency used

Frequency quality

Equipment used (radio, trunk line, computer,crypto, etc.)

Start and stop time of the POFA

The printout will contain a summary of theactivity that includes the time, in minutes andseconds, that the station was on the air, the totalnumber of words transmitted, the total number ofwords received, and the total number of words witherrors. This information can be used to calculate thelink quality factor. To calculate the link qualityfactor, divide the number of words received by thenumber of words transmitted. When the quotient isgreater than 95 percent but less than 100 percent,consider the POFA successful.

Next, compute the receive error factor. Ideally,the POFA should run with zero errors. Since the

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multi-station POFA is transmitted, atmosphericinterference, ship’s position, antenna location, andEMI are just a few of the things that can induce errorsin the radio signal. Determine the receive error factorby dividing the number of words with errors by thenumber of words received. When the receive errorfactor is less than 1 percent, consider the POFAsuccessful.

When the printout indicates that data was receivedfrom an unrecognized station (UNREC STA), thetechnician should check the number of wordsreceived. The multi-station POFA transmit bufferconsists of 230 words. One buffer of 230 words froman unrecognized station is acceptable and generallydoes not indicate a problem. More than one buffermay indicate a problem, but multiple buffers from anUNREC STA can also be caused by interference onthe frequency.

The printout will also indicate the parity status ofthe words received in error. During the POFA, sincethe computer knows the contents of the received datablock, it performs a parity check on all receivedwords. These parity checks are compared with theparity status received from the DTS. The printoutindicates these parity checks. The heading PARITYSTATUS OF ERROR WORDS lists the number oferror words detected by the DTS and the parity (1, 2,or 3). The heading PARITY STATUS OFCORRECT WORDS indicates the computer paritycheck of words received as correct from the DTS.When an error is detected, the number of words inerror for each of the three parity status conditions arelisted here. The final part of the printout indicates theremote station reports. These reports are sent by otherstations as part of the data transferred during thePOFA.

Since a multi-station POFA is subject to varioustypes of interference, both natural and man-made,several attempts may be required for you to achieveacceptable results. Shifting NCS and repositioningthe ships are just two of the actions that couldcontribute to achieving a successful multi-stationPOFA.

THE LINK-11 MONITORINGSYSTEM (LMS-11)

“The link is down” is a statement that can strikefear into even the most seasoned technician. As wehave seen, the operation and maintenance of a high-quality link can be affected by many factors. Foryears, operators and technicians commonly blamedeach other for poor link operations. Some typicalLink-11 problems areas follows:

Participating units (PUS) not responding tocall-ups

Garbled data

The link goes completely dead, normaloperation ceases

Inability to establish a net

Excessive net cycle time

When such a problem occurred, the Link-11technician would run a single station POFA anddeclare that the DTS was sound and it must be theother ship, a poor frequency, or an operator error. Theoperator would blame the frequency or the NCS.Other units would say the problem was anotherplatform jamming the entire net. Typical strategiesused to solve link problems usually began with arecommendation to change frequency. When thisstrategy failed to solve the problem, the next step wasto change the NCS. If the problem still existed, theNCS would eliminate PUs from the net, one at a timeuntil the problem unit was identified. All of theseactions took time and were hit-and-miss techniques.This tendency of trial-and-error troubleshooting andpointing fingers defined the need for a reliable visualsystem of monitoring the Link-11 network.

This need was filled with the development of theLink Monitoring System, AN/TSQ-162(V)1,commonly called the LMS-11. The LMS-11 providesan operator or a technician with a real-time visualdisplay of the Link-11 network while it is operating.

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The LMS-11 is capable of measuring anddisplaying link signal data for the network as a whole,as well as for individual units. It can be used forperiodic equipment checks or for continuousmonitoring to determine the condition of all membersof the net.

LMS-11 SYSTEM CONFIGURATION

The LMS-11 consists of three groups ofequipment: a data processing group (DPG), acontrol/display group (CDG), and an accessory group(AG). The LMS-11 is shown in figure 3-3.

The LMS-11 is designed to be portable, and theequipment is installed in three carrying cases. Theequipment cases that house the electronic units of theDPG and CDG provide isolation from shock andvibration. The CDG is designed to be mounted on thetop of the DPG cases. Four latches fasten the twounits together and provide a desk height, self-contained workstation. The system printer, which ispart of the accessory group, is mounted on the top ofthe CDG equipment case. When the LMS-11 isinstalled, the accessory group case provides storagefor the DPG and CDG equipment case covers. TheLMS-11 is normally located near the data terminalset, but it may be installed anywhere near a 600-ohmLink-11 audio signal.

Data Processing Group

The equipment required for the LMS-11 toreceive, sample, and process Link-11 audio signals iscontained in the data processing group. The DPG alsoprovides power control and distribution to the CDGand accessories. The DPG consists of the followingequipment:

The control processing unit

The audio interface unit

The dual 3.5-inch floppy disk drive unit

The power control unit

Figure 3-3.—The LMS-11.

CONTROL PROCESSING UNIT.— Thecontrol processing unit consists of the HP9920Ucomputer with an additional 2 MB of ram andassociated circuit card assemblies (CCA). Thesecircuit cards include the following:

Color output CCA

Composite Video CCA

Data communications interface

HP interface bus (HP-IB)

Analog-to-digital converter assembly

Fast Fourier Transform (FFT) processor

The color output CCA and the composite video CCAprovide the necessary signals to drive the colormonitor. The data communications interface providesan RS-232C asynchronous serial interface for thecolor printer. The HPIB is used to interface thesystem keyboard and the dual disk drives to thecomputer.

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Figure 3-4.—The LMS-11 keyboard.

The analog-to-digital converter converts theLink-11 audio signal into a digital signal for use bythe LMS-11. This digital signal is then transferred tothe computer, where the FFT converts it to afrequency domain. The Fast Fourier Transformconsists of a complex mathematical formula used todetermine the phase shift of a signal.

AUDIO INTERFACE UNIT.— The audiointerface unit connects the upper sideband (USB) andlower sideband (LSB) audio signals from an HF radioor the USB from a UHF radio to the LMS-11. Theaudio signals are input to the analog-to-digitalconverter of the control processing unit. The audiointerface unit does not add a load to the audio signal.

DUAL 3.5-INCH FLOPPY DISK DRIVEUNIT.— The dual 3.5-inch floppy drive unit is usedto load the LMS-11 programs and to record Link-11data. The disk drives use 788 Kbyte, double-sided,double-density disks.

POWER CONTROL UNIT.— The powercontrol unit provides the control, distribution, and

conditioning of the 115 VAC input power.

Control/Display Group (CDG)

The CDG consists of a color graphics monitor anda keyboard. The monitor displays operator-entereddata and system operation. The keyboard provides theoperator interface with the LMS-11.

COLOR DISPLAY MONITOR.— The colordisplay monitor is capable of displaying bothcomposite and RGB video. The computer generatescomposite video during the start-up and testing of theLMS-11. The RGB input with an external sync isused for displaying graphics during normal LMS-11operations. The monitor is also equipped with aspeaker and audio input to provide the operator withthe capability of monitoring the Link-11 audio signal.

KEYBOARD.— The keyboard is mounted on atray under the monitor. Under the tray, there is astorage slot for the LMS-11 technical manual. Thefunctional keys on the keyboard are color-coded to

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facilitate operator selections and entries. The LMS-11keyboard is shown in figure 3-4.

Many of the keys on the LMS-11 keyboard are notused and the software does not recognize these keys.The actual functions of the keys are covered later inthis chapter.

Accessory Group (AG)

The accessory group contains a color graphicsprinter and spare parts and supplies for the LMS-11.The shipping container is also used to store the DPGand CDG container covers when the printer isremoved and mounted on the LMS-11. The colorgraphics printer is used to provide hard-copy printoutsof the display screen on plain paper or cleartransparency material.

LMS-11 OPERATION AND DISPLAYS

The LMS-11 provides real-time monitoring ofLink-11 operations. Problems with the net can beeasily detected in real time and you can determine thecause of the problems by evaluating the differentdisplays. When the cause is determined, the operatoror technician can take corrective action.

System Initialization

When the LMS-11 is turned on following thecorrect power-up sequence, the computer runs a groupof self-tests and then boots the disk in drive 0. Whenthe booting is complete, the LMS-11 monitor displaysthe following message: “BOOTING COMPLETE,SWITCH TO RGB.” At this time, the operatorshould depress the RGB button on the monitor. TheInitialization display is the first screen displayed afterthe software is loaded. The operator can also recallthe Initialization display by pressing the INIT buttonof the keyboard. The Initialization display screen isshown in figure 3-5.

PRINTER. Selects which printer, if any, isbeing used with the LMS-11.

NET-MODE. Selects the Link-11 mode: NetSync, Net Test, Roll Call, Broadcast, or ShortBroadcast. The default is Roll Call.

DATA RATE. Selects whether the link isoperating in the fast or slow data rate.

FREQ-CORR. Enables or disables Dopplercorrection.

CALL-TIMEOUT. Allows the operator tospecify the number of frames for the missedcall timeout. Normal link operations is 15frames but is increased to 127 frames forsatellite link operations.

When all the required data is entered, the operatorshould select the desired mode of operation for theLMS-11. The five on-line modes are as follows:LINK MONITOR, NET, PU, SPECTRUM, andCARRIER SUPPRESSION.

Each mode has a unique display screen. Alldisplay screens consist of the following three parts:the header, the link signal or information area, and thestatus display. The header is at the top of the screenand indicates the mode being displayed. Theinformation area is the middle section of the display,and the status display is at the bottom of the screen.The status display is the same for all on-line modes.

Link Monitor Mode

The link monitor mode display reflects linkactivity in real time. This display allows the operatoror technician to monitor link operations and detectproblems as they occur. To select the link monitormode, the operator presses the function key labeledLM. The link monitor mode display is shown infigure 3-6.

During the initialization process, the operator isrequired to enter the following Link-11 operatingparameters:

DATE and TIME.

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Figure 3-5.—The LMS-11 Initialization display screen.

The top lines of the link monitor display screen report ends with the interrogation of the next PU incontain the header information. The LMS-11 mode isin the top center. The link mode is centered just underthe LMS-11 mode. In figure 3-6, this is RC FAST.This means the link is in the Roll Call mode, fast datarate. The right side of the header displays the dateand time. The left side of the header informationallows the operator to enter the NCS address and thesideband to monitor. The LMS-11 uses the address77 as a default for the NCS. However, recall fromchapter 2, that 77 is an illegal address and would notbe used in an active link. Since the NCS never sendsan interrogation to itself, the LMS-11 uses thisaddress to designate the NCS.

the polling sequence. -

The display sweeps from left to right and from topto bottom. The display is color-coded and uses astair-step pattern that is easy to understand. Thedisplay of a single NCS report and the meaning of thecolors and levels is shown in figure 3-7. Figure 3-8shows how the different messages appear on theLMS-11 link monitor screen. Note that the NCS

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Figure 3-6.—The link monitor display screen.

Study figure 3-6 again and follow the pollingsequence of the four units in the net. The last reporton the top line is an NCS report and call to PU 04.This is followed by PU 04’s response on the left sideof line two. Next, PU 56 is called and responds witha picket reply. Upon completion of PU 56’s reply, PU64 is called. After 15 frame times without a response,PU 64 is called again. PU 64 appears to haveresponded to the second call, but the LMS-11 onlyrecognized the five preamble frames.

By using the link monitor display, the operator ortechnician can make sure the connectivity has beenestablished and that all the correct PUS are beingpolled and are responding.

Figure 3-6 also shows several problems thatcommonly occur during Link-11 operations. Noticethat PU 64 sometimes responds to the first call-up,sometimes to the second call-up, and sometimes PU64 does not respond at all. PU 56 responds all thetime except for the call-up at the end of line two andbeginning of line three. On line six there is a doubleresponse, or echo, from PU 04. If you were toexamine this particular sequence using the frame-by-frame analysis, you would find the PU 04 was calledagain. This indicates the NCS did not receive thereport from PU 04 and repeated the call-up during themiddle of the response.

Status Display

As shown in figure 3-9, the status display is at the

Figure 3-7.—The link monitor display pattern.

Figure 3-8.—Link-11 messages as displayed by theLMS-11 link monitor mode.

bottom of each of the LMS-11 display screens. Thestatus display consists of the status box and two linesof information just above the status box. The top line,with the heading “XMT-ADDRS:” displays theaddresses of all PUS in the order they are being polled.The operator can monitor the polling in real time.The displayed addresses change colors to indicatetheir status. If the address is yellow, it is currentlybeing interrogated. The yellow address turns greenwhen the start code is received. The yellow addressturns red when the PU has been interrogated twicewith no response.

The line under the “XMT-ADDRS:” is used todisplay system messages and LMS-11 alerts. Alertsare displayed on the left side of this line. Systemmessages are displayed on the right side of the line.

The status box provides the operator withinformation about signal processing, link activity, andraw recording of link data. Just below each of theframe types, a small green box, or light, appears toindicate the type of frame being processed. Thesesignal processing status indicators are not displayed inreal time. They are updated approximately every 50milliseconds. The signal processing indicators are asfollows:

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Figure 3-9.—The LMS-11 Status display.

LMS— Should always be green.

LSN— Indicates that the LMS-11 is listeningfor the link audio.

PRE— Indicates that a preamble has beendetected.

PHA— Indicates that a phase reference framehas been detected.

CC1— Indicates the first frame of a controlcode.

CC2— Indicates the second frame of a controlcode.

EOT— Indicates that the LMS-11 hasdetected the end of transmission.

NOIS— Indicates that the received data framedid not pass the data quality test.

DATA— Indicates that the LMS-11 hasdetected a data frame that has passed thequality test. Note that the control codes andphase reference frames are also data frames.

REC— Shows the status of the raw recordfunction of the LMS-11. The indicator will begreen when the recording is turned on and redwhen the recording is stopped.

The last two fields of the status box indicate theperformance of the net. The “%DATA:” field will befollowed by a number representing the percentage ofnet cycle time that message data is transmitted withno errors. The “NCT:” displays the net cycle time inseconds. Net cycle time is the time required for one

complete polling of the net. It can be measured fromcontrol stop to control stop from the NCS, or theoperator can specify a PU to be the reference for netcycle time. The operator can also specify the numberof cycles to use to determine net cycle time. Theoperator makes these entries using the summarizeparameter in the NET DISPLAY mode.

Net Display

The Net Display mode is activated when theoperator presses the NET key on the keyboard. TheNet Display mode presents the following two separatetypes of information: a Net Summary (summarizemode) or a PU History (history mode). In the NetSummary mode, the Net Display presents a summaryof quantitative information about the performance ofup to 21 PUs. In the PU History mode, the LMS-11displays the most recent 21 transmissions for aselected PU.

The Net Display mode is only available when thelink is in the Roll Call mode. Figure 3-10 shows ascreen for the Net Display in the Summarize modeand figure 3-11 shows the screen for a PU Historymode.

After the operator enters the Net Display mode,there are four operator entries that can affect theinformation and how it is displayed. These entries areNCS, PU, SIDEBAND, and SUMMARIZE. All ofthe entries are displayed as part of the header of theNet Display screen. The NCS, PU, and SIDEBANDfields are on the left side of the screen, and theSUMMARIZE field is on the right side of the screenjust below the date and time fields.

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Figure 3-10.—The LMS-11 Net Display in Summarize Mode.

NCS.— The NCS field allows the operator todesignate the PU number of the NCS. When anumber is not entered in this field, the default addressof 77 is used. It will also be used as the PU numberin the polling display of the status area and in the PUfield of the Net Display.

PU.— The PU field is used by the operator todesignate the PU whose recurring transmission is usedto define a cycle. The PU field works with theSUMMARIZE field.

SIDEBAND.— The SIDEBAND field allows theoperator to designate which sideband (USB, LSB, orDIV) is used for the information displayed.

SUMMARIZE.— The SUMMARIZE fieldenables the operator to designate the number of cyclesover which the summary is computed. A “cycle” isdefined as the recurring transmission from thedesignated PU. The data is tabulated after thespecified number of transmissions are received fromthe designated PU or after 200 transmissions are

received by any station, whichever occurs first. TheSUMMARIZE field is also used to enable the PUHistory mode. The PU History mode is entered whenthe operator enters a zero in the summary field. Whenthe PU History mode is enabled by the operator, theword HISTORY is added to the Net Display title.The PU History mode display updates one line of dataimmediately after the specified PU has completed itstransmission.

The information displayed by the Net Displaymode is described in the following paragraphs.

PU.— The PU number. The first number listed isthe NCS, which has a default number of 77, or theaddress entered in the NCS field. The rest of the PUsare listed in numerical order.

SIG PWR.— The total signal strength of the 16tones, measured in dBm. A value of -51 indicates thatno signal was received.

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Figure 3-11.—The LMS-11 Net Display in PU History Mode.

SNR.— The signal-to-noise ratio, measured in dB.The SNR is calculated as the average power in thedata tones divided by the average power in the noisetones. The LMS-11 can measure a SNR of near 34dB. A number preface by the greater than symbol “>”indicates that the average power in the noise toneswas below the measurable threshold. In this case, thenumber represents the data tone signal strength only.An SNR value of 30 or higher is considered excellent.An SNR value of less than 10 is unusable.

FRAME CNT.— A count of all data framesreceived over the specified number of cycles. Dataframes include the phase reference frame and controlcode frames in each message. A value that isfollowed by a “?” and color-coded yellow is displayedif the frame count of a picket station average is lessthan or equal to six frames. The two start codeframes, the phase reference frame, the crypto frame,and the two stop code frames account for the sixframes. Therefore, if a picket unit transmits six orless frames, no actual message data is being received.This may indicate a problem with either the computeror the DTS of the unit. A yellow color-coded value

followed by the “?” is added for an NCS when thenumber of frames is equal to or less than eight. Thetwo additional frames account for the next stationaddress at the end of an NCS report.

%THRU.— This number is the percentage ofmessage data that is received error-free. Thepercentage is found by comparing the number oferror-free message data frames with the total numberof message data frames received.

CF.— This is a percentage of control code failures.A PU with strong signals that never misses a call willhave a 0 % code failure. A PU that never answers,such as a dummy PU, will have a 100 % code failure.Values between 1 and 100 could be due to noise orweak equipment or an equipment malfunction.

BER.— This is the bit error rate measured as thenumber of bit errors per 1,000. Bit errors increase asthe signal-to-noise ratio decreases. A bit error ratethat exceeds a theoretical value for a given SNR isindicated by displaying both the BER and SNR inyellow.

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Figure 3-12.—The LMS-11 PU Display mode.

REL 605.— This column indicates the relativepower of the 605-Hz tone with respect to the averagepower of the 15 data tones, measured in dB. It shouldbe +6 dB.

VAR DATA.— This is the variation of power inthe data tones in dB. The relative power of each ofthe data tones, with respect to the average power ofthe data tones, is determined. The variation is thedifference between the maximum and the minimum.Under ideal conditions, the variation is zero. TheTADIL A specification for maximum variation is 1.5dB l

PHASE ERR M.— This is the mean, or average,phase error of the data tones. The intelligence isstored in the data tones by use of the phase differencesthat are odd multiples of 45 degrees. If the phasedifference of a data frame is 50 degrees when theexpected difference is 45 degrees, the error is5 degrees. The phase errors for each tone are addedup, and after the specified number of cycles, the sumfor each tone is divided by the number of frames toobtain the mean phase error for each tone. The meanphase error for all 15 tones is then summed and

divided by 15 to obtain the value displayed.PHASE ERROR SD.— This is the standard

deviation of the phase error in all 15 tones.

RFE/DS.— This is the radio frequency error, orDoppler shift, measured in Hertz. If the Dopplercorrection was enabled during the LMS-11initialization, the value is color-coded green. If theDoppler correction is turned off, this value is color-coded cyan.

NCT.— This is the net cycle time, as measuredfrom phase reference frame to phase reference frame,of the reporting unit. Note that this measure of netcycle time is different from that used in other NCTcalculations.

PU Display

The PU display shows detailed information aboutthe signal received from the specified PU. The PUdisplay can operate in Broadcast, Short Broadcast, andRoll Call modes. In Broadcast and Short Broadcast,the display is updated after every transmission. In theRoll Call mode, the display is updated after thespecified number of net cycles or 200 transmissions,

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whichever occurs first. When the net cycles are set tozero, the display updates immediately after thedesignated PU has transmitted. The PU display isshown in figure 3-12.

The PU display is activated when the operatorpresses the PU function key on the keyboard. Theinformation in the PU display is presented in two bargraphs with additional amplifying information justunder the bar graphs. In the PU display header, theoperator enters the address of NCS (or 77), theaddress of the unit to be evaluated, the sideband to beevaluated (USB, LSB, or DIV), and the number ofcycles to summarize for the display. The followingparagraphs describe the information presented in thePU display.

RELATIVE POWER (dB).— This bar graphdisplays the relative power in each of the Link-11tones. The relative power is calculated with respect tothe average of the data tones. The expected valuesshould be +6 dB for the 605-Hz tone (tone 5) and 0dB for the data tones. The TADIL A specificationsallow for a difference of 1.5 dB between themaximum and minimum power levels of the datatones. A noisy signal may cause the power levels ofthe data tones to deviate considerably from thestandard. The bar graph for relative power is alsocolor coded. When the relative power of a data toneis ±1 dB, the bar is green. If the power level is in therange of +1 to +2 dB or -1 to -2 dB, the bar will beyellow. The bar is red if the power level is greaterthan +2 dB or less than -2 dB. The length of the barsplotted on the graph is rounded off to the nearest 1/2dB .

PHASE ERROR (DEGREES).— The phaseerror (degrees) bar graph shows the mean and thestandard deviation of the Link-11 tones. The standarddeviation of a tone is plotted by a color bar on thegraph. The size of the color bars is plotted to thenearest whole degree of deviation. The meandeviation of the tone is indicated by a small whiteline, usually in the center of the standard deviationcolor bar. The mean phase error should fall between+45 degrees and -45 degrees. If the data is bad, themean phase error is set to -45 degrees and thestandard deviation is set to 90 degrees. This causes

the bar to be drawn across both quadrants of thegraph.

The standard deviation is represented by a color-coded bar for each tone. A green bar is displayed ifthe standard deviation is within 10 degrees.Deviations between 10 degrees and 20 degrees arerepresented by a yellow bar, and deviations greaterthan 20 degrees are red. The standard deviation mustbe a positive value that is less than 45 degrees. If thestandard deviation is out of range for a given tone, thedata is bad. This condition is indicated by theLMS-11 by setting the mean deviation to 45 degreesand the standard deviation to 90 degrees. As with themean deviation phase error, this causes the bar to bepainted in both quadrants of the graph.

Some causes of phase errors are noise,simultaneous transmissions, poor framing, and errorsin Doppler correction due to noise on the preamble.For example, a picket unit transmitting Net Syncduring Roll Call will cause an error condition. Theexpected value of the mean deviation is 0 degreeswith a standard deviation of ±5 degrees. If only onetone has a mean value that is greatly different fromthe other tones, it maybe an indication of a frequencyerror on that tone.

SIGNAL POWER.— The signal power is part ofthe amplifying information under the two bar graphs.The signal power is the total signal strength in the 16tones. It is measured in dBm. If no signal is received,the default value of- 51 dBm is listed.

SNR.— This is the signal-to-noise ratio. It ismeasured in dB and calculated as the ratio of theaverage power in the data tones to the average powerin the noise tones. If the SNR value is preceded bythe symbol “>,” it indicates that the average power inthe noise tones is below the measurable threshold andthe actual SNR is greater than the value indicated.The maximum value that the LMS-11 can measure isabout 34 dB. An SNR that is greater than 30 dB isexcellent. If the SNR is less than 10 dB, the data isunusable.

BER.— This is the bit error rate per thousand.The incidence of bit errors increases as the signal-to-noise ratio decreases.

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Figure 3-13.—The Spectrum Display screen of the LMS-11.

are the odd harmonics of 55 Hertz. The spectrumMISSED CODES %.— This is a percentage of

each type of code that is missed. The number ofcodes (start, stop, and address call-ups) missed andreceived is tabulated and the percentage of each typemissed is calculated.

FRAMES.— This is the total number of dataframes received, including the phase reference andcontrol code frames.

CS.— This field displays the carrier suppressionvalue of the upper and lower sidebands as a ratio ofthe power in the 605-Hz tone to the power of thecarrier frequency. The value display is measured indB .

RFE/DS.— The radio frequency error or Dopplershift of the received signal in Hertz. The display iscolor-coded cyan if frequency correction was disabledduring LMS-11 initialization.

Spectrum Display

The spectrum display graphically shows the powerlevels of all the Link-11 tones and the noise tones that

display screen is shown in figure 3-13. The x-axis ofthe bar graph is numbered from 1 to 30 to represent 30tones. Tone 05 is the 605-Hz Doppler tone. Tones 8through 21 and tone 26 are the data tones. Theremaining tones are not used by the Link-11 systembut are sampled and displayed to give the operator anindication of the noise level.

The y-axis of the bar graph displays the relativepower of each tone in dB. The highest value of thescale is 0 dB and decreases to -40 dB. The tone withthe greatest amount of power is set to 0 dB on thescale. This should be the 605-Hz tone. Theremaining tones are measured relative to the tone withthe greatest power. A single blue line is drawnhorizontally across the screen at the -6 dB level.Ideally, all data tones should extend up to this line.

The 605-Hz tone and the data tones are displayedby solid green vertical lines. If the power of a datatone is greater than -6 dB with respect to the 605-Hztone, the area above the -6 dB line is indicated by anopen yellow bar on top of the green bar. If the powerlevel of a data tone is below the -6 dB threshold, anopen yellow bar is used to fill in the remaining

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distance. This allows the operator to view the effectsof the noise. The power of the noise tones is alsoindicated by open yellow bars.

To enter the spectrum display, depress the SPECTkey on the keyboard. Several options are available tothe operator by entering data into the header fields ofthe spectrum display. The operator may designate theaddress of the NCS. The default address is 77. Theoperator can also select a particular sideband (USB,LSB, or DIV) for display. By using the RESTRICTfield, the operator can restrict the display to only dataframes or only preamble frames, or choose norestrictions. The PU field allows the operator todesignate a particular PU for display. If 00 is enteredinto the PU field, the data display is continuouslyupdated with samples from the entire net.

Carrier Suppression Display

The carrier suppression display measures howsuccessfully the carrier frequency is suppressed. Thecarrier suppression measurements can only be madeduring Net Sync. To measure the carrier suppression,the radio must be off-tuned by -500 Hz for the uppersideband and +500 Hz for the lower sideband. Thisoff-tuning allows the program to measure andcompare the relative power of the carrier frequencyand the 605-Hz tone of the preamble.

RECOGNIZING LINK-11 NET PROBLEMS

The LMS-11 is very useful in evaluating Link-11net quality. As you have seen, the various on-linemodes can help you determine various problems.These include a station that is consistently missingcall-ups, poor signal-to-noise ratio, and low powerfrom a unit. Some common Link-11 problems and theLMS-11 display are covered in the next fewparagraphs.

Figure 3-14 shows an example of how a PU notresponding to call-ups would appear on the LMS-11operating in the Link Monitor mode. When a PUdoes not respond to a call-up, the reason maybe thatthe incorrect PU number was entered at the NCS or atthe DTS of the unit. It can also be caused by a poorreceiver at the PU, causing the PU to not receive its

call-up. A third problem could be a weak transmitterat the PU, causing the NCS to not receive theresponse and therefore, repelling the PU.

Figure 3-14.—A PU not responding to NCS call-up.

Figure 3-15 shows the display that appears whena PU is responding to NCS call-ups, but the reportcontains no data. Causes of this problem could bethat the KG-40 has an alarm, the CDS program isdown, or the problem is in the CDS computer to DTSpatching.

Figure 3-15.—A PU responding with no data.

When the NCS fails to receive a stop code from aPU, a stoppage of the net occurs, as shown in figure3-16. If this condition occurs repeatedly and can betraced to a single PU, the NCS should delete the PUuntil the stop code problem in the DTS is corrected.

Figure 3-16.—A net stoppage caused by NCS not receivinga stop code.

Figure 3-17 shows several PUs not responding tocall-ups. Some of the causes for this condition could

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be the following: the NCS having an incorrect PUaddress entered in the DTS, low transmitter power outfrom the NCS, an excessively noisy frequency, orweak PU receivers.

Figure 3-17.—Several PUs not responding to NCS call-ups.

The LMS-11 also has several off-line modes thatallow you to save data onto a disk and analyze thedata in detail. The off-line modes include a frame-by-frame display to analyze each frame of a transmission.This allows you to analyze the data of a particular PUand shows the status of each bit position. Rememberthat when you are doing a frame-by-frame analysis,the data has not been decrypted.

More information on all modes of the LMS-11 canbe found the System Operation and MaintenanceInstructions, Organization Level, Link MonitorSystem AN/TSQ-162(V)1, EE-190-AB-OMI-010/TSQ-162(V)1.

SUMMARY—LINK-11 FAULT ISOLATION

This chapter introduced you to some of the toolsavailable to ensure the Link-11 system is operating atmaximum efficiency. The following informationsummarizes some of the important points you shouldhave learned.

LINK-11 MYTHS AND FACTS— Throughtime, several myths about troubleshooting Link-11have evolved. We explored some of the myths andtried to determine the facts. Some of the myths are asfollows:

Changing NCS solves net problems. This isonly true if the NCS is causing the netproblems.

Changing frequency solves net problems.Again, this is true only if the frequency is

noisy or is being jammed by anotherfrequency.

More power improves Link-11 performance.This is a myth. In fact, too much power canactually degrade the Link-11 net.

Dummy PUs improve link quality. Again,this is a myth. This myth evolved from thetime of the CP-642 family of computers andthe AN/USQ-36 DTS. Back then, a dummyPU would help improve link quality, perhapsby providing a time delay for the computer toprocess all received data.

Radio Silence reduces net cycle time. Thiscould be true if the unit that goes radio silentis transmitting less than 38 frames of data. Asa rule, if a unit goes radio silent, its PU shouldbe deleted from the polling sequence until theunit is ready to rejoin the net.

LINK-11 POFAS— The two POFAs used in theLink-11 system are the single-station POFA and themulti-station POFA.

SINGLE-STATION POFA— The single-stationPOFA is an end-around test that will test most of theDTS, the computer input and output circuits, and theaudio path if the radio is not removed from the testpath. A single-station POFA does not check thereceive timing circuits. When a single station POFAis completed, a printout is produced that lists theerrors detected during the test. To be consideredsuccessful, a single station POFA should always runwith zero errors.

MULTI-STATION POFA— A multi-stationPOFA is a test between two or more units. The multi-station POFA closely represents normal linkoperations. A multi-station POFA requirescoordination among all units participating. All unitsshould be within 25 miles of each other whenattempting a multi-station POFA. Since the multi-station POFA actually transmits data over the air, it issubject to many types of interference, and severalattempts may be required before acceptable results areobtained.

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THE LINK-11 MONITORING SYSTEM(LMS-11)— The LMS-11 provides the operator andtechnician with a means of monitoring the integrity ofthe Link-11 net. With the LMS-11, the technician hasa real-time visual display of the link while it isoperating. The LMS-11 consists of the followingthree groups of equipment: the data processing group,the control/display group, and an accessory group.The data processing group contains the centralprocessor and the interfaces required to process thelink audio. It also contains a dual 3.5-inch floppy diskdrive and the power control unit. The control/displaygroup consists of a color graphics monitor and akeyboard. The keyboard provides the operatorinterface with the LM-11. The functional keys onthe keyboard are color-coded for ease of operation.The accessory group consists of the color graphicsprinter and the spare parts and supplies for theLMS-11.

LMS-11 OPERATION AND DISPLAYS— TheLMS-11 has five on-line modes of operation. Eachmode has a unique display screen and allows thetechnician to evaluate various parts of the link audiosignal and the link digital data. The five modes are asfollows: LINK MONITOR, NET DISPLAY, PU,SPECTRUM, and CARRIER SUPPRESSION.

LINK MONITOR MODE— The link monitormode provides the technician with a real-time displayof link activity. This allows the technician to monitorand detect link problems as they occur.

NET DISPLAY— The net display contains thefollowing two distinct modes of operation: the NETSUMMARY and a PU HISTORY. The Net Summary

mode presents a summary of quantitative informationabout the performance of up to a maximum of 21 PUsin the net. The PU History mode displays the samequantitative data for a single PU. This data is updatedeach time the selected PU transmits. The most recent21 transmissions of the specified PU are displayed inthe PU History mode.

PU DISPLAY— The PU display presents detailedinformation about the audio signal received from aspecified PU. The information is presented in two bargraphs and shows the relative power and the phaseerror of the received signal.

SPECTRUM DISPLAY— The spectrum displayshows the relative power level of the Link-11 tonesand the noise tones that are the odd harmonics of 55Hertz. The 605-Hz tone is used as a reference and isset to 0 dB. The relative power of the data and noisetones is displayed with respect to this level. In thismode, the effects of noise can be easily viewed.

CARRIER SUPPRESSION— The carriersuppression can only be measured while in Net Sync.The LMS-11 measures the power of the carrierfrequency and compares it to the power of the 605-Hztone. The carrier suppression can be measuredaccurately when the radio is off-tuned by – 500 Hz forthe USB and +500 Hz for the LSB.

EVALUATING THE LINK-11 NET— TheLMS-11 can be a very useful tool in evaluatingLink-11 net operations. The best way to becomeproficient on reading the various screens of theLMS-11 is through practice.

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CHAPTER 4

LINK-4A

INTRODUCTION

The Link-4A system is a fully automatic, high-speed data transmission systemused for aircraft control. The system provides controlling information to the aircraft,using radio transmission between the controlling ship and the controlled aircraft. TheCarrier Aircraft Inertial Navigation System (CAINS) is also a part of the Link-4Asystem. The CAINS system is used to load alignment and way-point data into theaircraft on the flight deck or the hangar deck.

After completing this chapter, you should be able to:

Describe the functions of the Link-4A system.

Describe the operating modes of the Link-4A data terminal set.

Describe the types of messages used by the Link-4A system.

Describe the functional operation of the Link-4A data terminal set.

Describe the test messages used in the Link-4A system.

LINK-4A SYSTEM OVERVIEW

The two major components of the Link-4A systemare the Link-4A CDS system and the CAINS system.Both systems use serial time-division multiplexing totransmit control and reply messages over a frequency-shift keyed (FSK) UHF radio communicationschannel. The CAINS system can also transmit datavia hard-wired stations on the flight deck or hangardeck.

LINK-4A CDS SYSTEM

The Link-4A CDS system is used to provide one-way or two-way communications between thecontrolling station and up to 100 controlled aircraft.The controlling station transmits to the aircraft controlmessages containing vectoring information,commands, and data pertaining to the target ordestination of the aircraft. The aircraft transmits replymessages containing information concerning itsheading, altitude, airspeed, and tactical readiness. The

aircraft control messages are developed by the CDScomputer using radar-derived target data, reply datafrom the aircraft, and other tactical data.

A typical shipboard Link-4A system configurationis shown in figure 4-1. It consists of the CDScomputer, a data terminal set, a communicationsswitchboard, and a UHF radio transceiver.

The CDS computer outputs parallel digital data tothe Link-4A data terminal set. Currently, the dataterminal set most shipboard installations use is a typeof the AN/SSW-1 (U). It will be designated as theAN/SSW-1A/B/C/D/E(U). The data terminal setconverts the computer data into a serial time-divisionmultiplexed pulse train that is transferred to the radiotransceiver through the communications switchboard.The communications switchboard connects theselected UHF transceiver to the data terminal set. Theradio transceiver converts the pulse train into FSKvariations in the carrier signal frequency.

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Figure 4-1.—The shipboard Link-4A CDS system.

After the aircraft receives the transmitted data, itmay respond by transmitting data to the controllingstation. This is the reception cycle. The receiverremoves the carrier frequency and forms the serialdata pulse train. The pulse train is sent to the dataterminal set via the switchboard. The data terminalset converts the serial pulse trains into parallel dataand sends the data to the CDS computer.

In a typical aircraft carrier system, the four distinctmodes of operation in the Link-4A system areintercept vectoring, air traffic control, automaticcarrier landing system, and precision course direction.

Intercept Vectoring

Intercept vectoring enables the controlling ship toguide an aircraft to an intercept point. The two typesof data sent to the aircraft during intercept vectoringare command data and situation data. Command dataprovides direct steering and control information,whereas situation data provides the aircraft with anoverall picture of the tactical situation with respect toits target.

This data is used to guide the aircraft withinstriking range of its target at optimum position andaltitude for an attack. The messages also containinstructions to the pilot, such as target identity, breakengagement, and return to base.

Air Traffic Control

In the air traffic control mode, Link-4A is used tocontrol the aircraft in the carrier’s traffic pattern. Thecontrol station transmits data to the aircraft tomaintain safe flight patterns and assigns priority forlanding approach. As each aircraft enters the landingpattern, it is transferred to the automatic carrierlanding system for final approach and landing.

Automatic Carrier Landing System

The automatic carrier landing system selectsaircraft in the order of priority from the pattern andenters them into the final approach. During the finalapproach, a precision radar tracks the aircraft. Correctinformation pertaining to the approach is transmittedto the aircraft’s autopilot. When conditions areunfavorable for a landing, the wave-off control isinitiated and the aircraft is guided through a shortpattern and the landing approach is repeated.

Precision Course Direction

The precision course direction mode is used in theremote guidance of bomber and reconnaissanceaircraft, and drones. The guidance messages containpitch, bank, heading, altitude, and airspeed commandsto permit very precise control of the aircraft’s flightpath .

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CARRIER AIRCRAFT INERTIALNAVIGATIONAL SYSTEM (CAINS)

The CAINS system is used to load alignment andway-point data into aircraft on the flight deck or thehangar deck. Aircraft alignment data consists oflongitude, latitude, and ship’s velocity data from theship’s inertial navigation system. Way-point data is aset of predetermined geographical points loaded intothe aircraft’s navigation computer. Way pointsprovide the aircraft with destination or targetinformation.

When the CAINS system is used, data can beloaded into the aircraft by either a hard-wired systemor RF radio transmission. The hard-wired insertion ofdata is accomplished when the aircraft is connected toa deck edge outlet box (DEOB). The pulse amplifiersof the AN/SSW-1D/E can provide outputs for up to40 of these DEOBs. After the initial data is loaded,the aircraft is disconnected from the DEOB, but itcontinues to receive alignment data until the launch.Then the aircraft system reverts to its original tacticalcondition.

LINK-4A MESSAGE FORMATS

The following are the three types of messagesused in the Link-4A system: control messages, replymessages, and test messages. These messages usetwo basic formats. Control messages are transmittedfrom the controlling ship to the aircraft. Replymessages are transmitted from the aircraft to thecontrol station.

The timing for Link-4A communications isdetermined from the duration of the transmit andreceive cycles. The standard CDS control messagesare 14 msec in duration, while the receive cycle forreply messages is 18 msec in duration. The CAINSsystem does not use reply messages; therefore, a2-msec receive cycle is substituted to allow time forthe Link-4A data terminal set to initialize the nextmessage. Thus we have the following two timingcycles: 14/18 (control message 14 msec/receive cycle18 msec) and 14/2 (control message 14 msec/receivecycle 2 msec).

CONTROL MESSAGE FORMAT

Control messages are assembled and transmittedduring the 14-msec transmit frame. Figure 4-2shows the standard structure of a Link-4A controlmessage. During the transmit frame, the transmit keysignal and the control message pulse train are sent tothe radio set transmitter. The transmit frame isdivided into seventy 200-µsec time slots that containthe sync preamble, the data bits, and thetransmitter un-key signal.

Figure 4-2.—The Link-4A control message format.

Sync Preamble

The sync preamble is made up of the first 13 timeslots of the control message. The first eight time slotseach contain one cycle of a square wave, consisting of100 µsec in the “0” state and 100 µsec in the “ 1” state.These eight time slots are known as the sync burst.Following the sync burst are four time slots in the “0”state, called the guard interval. The guard intervalindicates the changeover to the 200-µsec data signals.Time slot 13 is the start bit and is always a “ 1.”

Data Bits

The Link-4A message data is contained in the 56time slots (slots 14 through 69) that follow the syncpreamble. Each time slot contains one data bit. Thefirst 13 bits of this data is a binary number thatindicates the address of the particular aircraft. Onlythe aircraft with this preassigned address willrecognize the message and act on the message data.Following the address is a five digit label thatdesignates the type of data contained in the message.The labels correspond to the modes of operation. The

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last digit designates whether the message is an A or Btype. In most modes, both an A and a B type ofmessage are required to transmit all the necessary datato the aircraft. The remaining data bit time slotscontain the various control commands.

Transmitter Un-key Signal

The last time slot (slot 70) is a 200-µsec periodallotted for transmitter turn-off time and does notcontain any data.

REPLY MESSAGE FORMAT

Reply messages are received during the 18-msecreceive cycle. The reply message contains a total of56 time slots and occupies a period of 11.2 msec.This 11.2-msec reply must be received during the18-msec receive cycle. This allows for a maximum of4.8 msec for transmission delay.

The reply message consists of a sync preamble,42-data bit time slots, and a guard interval, as shownin figure 4-3. The sync preamble is identical to thecontrol message sync preamble. The information inthe 42-data time slots is divided into groups of digitsthat identify the source and type of message, and themessage data. The last time slot is the guard intervaland allows for transmitter turn-off time.

TEST MESSAGES

During Link-4A operations the controlling stationsends test messages at periodic intervals to the data

Figure 4-3.—The Link-4A reply message format.

terminal set for testing the message processing anddisplay circuitry of the aircraft being controlled. Thetest messages also check the data terminal set and itsinterfaces. The two types of test messages areuniversal test message (UTM) and monitor controland reply messages (MCM/MRM).

Universal Test Messages

Universal test messages (UTMs) are Link-4Acontrol messages that are always addressed to aparticular universal address and contain fixed, specificinformation in each data field. The UTMs provide thecontrolled aircraft with a means to verify properoperation of the link.

Monitor Control and Reply Messages

Monitor control messages (MCMs) are Link-4Acontrol messages that are sent to the data terminal setfrom the CDS computer to initiate internal testing ofthe data terminal set. After the data terminal setcompletes its self-check, the MCM is transmitted withthe universal address. Depending on the equipmentconfiguration of the aircraft, the MCM will either berejected or processed as a UTM.

The monitor reply message (MRM) is sent to theCDS computer upon the successful processing of theMCM. The MRM is effectively a return of the MCMdata content which indicates that the internal andinterface tests were successful.

THE LINK-4A SYSTEM COMPONENTS

The Link-4A system consists of the CDScomputer, a data terminal set, a communicationsswitchboard, and a UHF radio set.

DATA TERMINAL SET AN/SSW-1D/E

The Link-4A data terminal set is theAN/SSW-1D/E. The data terminal set performs thefollowing functions:

Provides overall Link-4A system timing

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Converts parallel data from the CDS computerinto serial data for transmission to controlledaircraft

Converts serial data received from controlledaircraft into parallel data for input to the CDScomputer.

The current five versions of the AN/SSW-1() usedin shipboard Link-4A systems are the AN/SSW-1A,1B, 1C, 1D, and 1E. The AN/SSW-1A, 1B, and 1Care operationally and fictionally identical, as are theAN/SSW-1D and 1E. The major difference betweenthe two groupings of versions is the single-channelcapability of the AN/SSW-1A/B/C and the dual-channel capability of the AN/SSW-1D/E. Each of thedual channels is capable of the link operations of thesingle channel AN/SSW-1(). The dual-channelAN/SSW-1D/E is also capable of transmitting CAINSdata. For purposes of this lesson, we use theAN/SSW-1D/E.

The AN/SSW-1D/E, shown in figure 4-4, consistsof the following eight major subassemblies: onecoordinate data transfer control, two digital-to-digital converters, two monitor test panels, twopulse amplifier assemblies, and a power supplyassembly. There are two independent equipmentgroups in the AN/SSW-1D/E. Each group is capableof simultaneous operations with separate anddedicated computer input-output channels anddedicated UHF radio sets,

Figure 4-4.—The AN/SSW-1D/E data terminal set.

Figure 4-5.—The coordinate data transfer control assembly (AN/SSW-1D/E).

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Coordinate Data Transfer Control

The coordinate data transfer control assemblyenables the connection of each of the digital-to-digitalconverters (DDC) to one of two different computers.The control panel for the coordinate data transfercontrol assembly is shown in figure 4-5. TheCOMPUTER SELECT provides switching, such thatDDC A is connected to computer 1 and DDC B isconnected to computer 2 or vice versa. Either of thetwo DDCs maybe connected to its monitor test panelfor off-line testing. The DDC output options are theCDS (old NTDS) radio set, the CAINS system, or thetest mode.

Digital-to-Digital Converter

The digital-to-digital converter assembly providessystem timing, converts parallel data from the CDScomputer into serial data for transmission by the UHFradio set, and converts serial data received from theradio set into parallel data for input to the CDScomputer. The DDC is the heart of the data terminalset.

Monitor Test Panel

The monitor test panel provides the technicianwith a means to monitor Link-4A operations and off-line testing capabilities. There is one monitor testpanel for each DDC.

Pulse Amplifier

The pulse amplifiers provide level and signalconversion functions to allow the AN/SSW- 1D/E dataterminal set to drive the serial output for the UHFradio set and the deck edge outlet boxes for CAINS.

COMMUNICATIONS SWITCHBOARD

The communications switchboard interconnectsthe AN/SSW-1() to the UHF radio sets. Thecommunications switchboard is similar to the Link-11switchboard described in chapter 2 of this manual.

LINK MONITOR SYSTEM (LMS-4)

The LMS-4 provides stand-alone Link-4Amonitor and readiness check capabilities. Itsoperation is similar to that of the LMS-11 covered inthe previous chapter. The monitor function listenspassively to the Link-4A communications between thecontrol station and the controlled aircraft. Signalanalysis and test message validity are performed onthe data. The readiness check function tests thereadiness of the control station to conduct live two-way Link-4A operations. Control messagestransmitted by the control station are monitored andthe LMS-4 generates the reply messages required tomaintain two-way communications.

SUMMARY—LINK-4A

This chapter introduced you to the Link-4Acommunications system. The following informationhighlights some of the important points you shouldhave learned.

LINK-4A CDS SYSTEM— The Link-4A CDSsystem provides one-way or two-way communicationbetween the controlling station and up to 100 aircraft.Link-4A messages contain flight commands andtactical information for the aircraft’s pilot. The fourmodes of operation for the Link-4A CDS system areintercept vectoring, air traffic control, the automaticcarrier landing system, and precision course direction.

Intercept vectoring mode is used to guide anaircraft to an assigned target. Air traffic control modeis used to control the aircraft in a carrier’s landingtraffic pattern. The automatic carrier landing systemuses the carrier’s precision approach radars to land anaircraft on the flight deck automatically. Precisioncourse direction mode provides very accurate controlof an aircraft’s flight path and is used for the remoteguidance of bombers, reconnaissance aircraft, anddrones.

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C A R R I E R A I R C R A F T INERTIALNAVIGATIONAL SYSTEMS (CAINS)— TheCAINS system is used to load way-point andalignment data into the navigation computer of an

aircraft. Data is initially entered via a hard-wireddeck edge outlet box and updated by UHF radio untilthe aircraft is launched. At launch, the aircraft’s on-board computer reverts to tactical operation.

LINK-4A MESSAGE FORMATS— The threetypes of messages used in the Link-4A system arecontrol messages, reply messages, and test messages.

CONTROL MESSAGE FORMAT— Controlmessages consist of seventy 200-µsec time slots thatcontain a 13-time slot sync preamble, 56 time slots ofdata bits, and a one time slot transmitter un-keysignal. Control messages are sent from thecontrolling station to the aircraft and are transmittedduring the 14-msec transmit frame.

REPLY MESSAGE FORMAT— Replymessages consist of 56 time slots that contain the syncpreamble, 42 data bits, and the transmitter un-keysignal. Reply messages are sent from the aircraft tothe controlling station in response to a controlmessage. Reply messages are received by thecontrolling station during the 18-msec receive frame.The additional time required for reply messages is tocompensate for time delays encountered duringtransmission of the control and reply messages.

TEST MESSAGES— The two types of testmessages used in the Link-4A system are theuniversal test message and monitor control/reply testmessage. Test messages are generated periodicallyduring Link-4A operations to verify proper operation.

Universal test messages are sent to the controlledaircraft, using a universal address and verify properoperation of the controlled aircraft’s system.

Monitor control messages are sent from the CDScomputer to the data terminal set and cause the dataterminal set to initiate a self-test. When the dataterminal set successfully completes the self-test, themonitor control message is sent back to the CDScomputer as a monitor reply message.

DATA TERMINAL SET AN/SSW-1D/E— Themost common Link-4A data terminal set is somevariation of the AN/SSW-1(). The AN/SSW-1D andAN/SSW-1E provide dual channel operation. Thedata terminal set provides the overall Link-4A systemtiming, converts parallel data into serial data fortransmission, and converts received serial data intoparallel data for input to the CDS computer.

LINK MONITOR SYSTEM (LMS-4)— TheLMS-4 is a stand-alone system that allows thetechnician to monitor Link-4A operations and performreadiness checks on the Link-4A system.

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CHAPTER 5

NEW TECHNOLOGY IN DATA COMMUNICATIONS

INTRODUCTION

The current Link-11 and Link-4A systems are being updated with new equipments. The DataTerminal Set AN/USQ-125 is currently replacing the older Link-11 data terminals. In addition, newcommunications systems, such as the Command and Control Processor (C2P) and the JointTactical Information Distribution System (JTIDS), are quickly becoming commonplace onvarious platforms in the Navy. This chapter will introduce you to some of the changes taking placeand the basic features of some of the new systems.

After completing this chapter, you should be able to:

Describe the various components of the AN/USQ-125 Data Terminal Set.

Describe the operation of the AN/USQ-125 in a typical Link-11 system.

State the purpose of the Joint Tactical Information Data System (Link-16).

Describe the components of the Link-16 system.

State the function of the Command and Control Processor (C2P) system.

Describe the components of the C2P system.

AN/USQ-125 DATA TERMINAL SET

The AN/USQ-l25 data terminal set is the newestLink-11 data terminal set in the Navy. It is quicklyreplacing older DTSs, such as the AN/USQ-36 andthe AN/USQ-59. There are several configurations ofthe AN/USQ-125. The CP-2205(P)(V)/USQ-125 dataterminal with the MX-512P/RC Remote Control Unitis one configuration. The other configuration isCP-2205(P)(V)2/USQ-125 data terminal with apersonal computer (386 or better) running theMXPCR software. The personal computer serves thesame function as the remote control indicator in thisconfiguration. The standard interface configuration ofthe AN/USQ-125 is shown in figure 5-1. In thischapter, we examine the data terminal and thefunctions of the control indicators, either theMX-512P/RC or a personal computer.

Figure 5-1.—The AN/USQ-125 data terminal set standardinterface block diagram.

THE CP-2205(P)(V)/USQ-125 DATA TERMINAL

The CP-2205(P)(V)/USQ-125 data terminal is acompact, state-of-the-art data terminal that is mountedin a standard 19-inch equipment rack. The data

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Figure 5-2.—The CP-2205(P)(V)/USQ-125 data terminal block diagram.

terminal has the following three major components: aprocessor board, a CDS interface board, and thepower supply. Figure 5-2 is a block diagram of theCP-2205(P)(V)/USQ-125 data terminal. Theprocessor board performs modulation/demodulationand error detection and correction, and provides theinterface with the radio set. The CDS interface boardprovides the interface with the CDS computer. TheCP-2205(P)(V)/USQ-125 data terminal performsmany of the same fictions as previous Link-11 dataterminal sets. These functions include the following:

Data conversion

Data error detection and correction

Control code generation and detection

Synchronization

Encryption device data transfer

Computer and radio control signals for two-way Link-11 data transfers

In addition, the CP-2205(P)(V)/USQ-125 dataterminal provides the following new features:

Both multi-tone and single-tone waveformoperations

Enhanced Link Quality Analysis (ELQA)

Maximum useable frequency (MUF) option Multi-Frequency Link

On-line and Off-line System Test Options

Multi-Tone Waveform Link

Multi-tone link operations are basically the sameas in the previous Link-11 data terminal sets and arecalled conventional Link-11 waveforms. The dataterminal generates the 605-Hz Doppler tone and 15data tones. The frequencies of the data tones are thesame as described in chapter 2. Message formats andmodes are also the same.

Single-Tone Waveform Link

Single-tone waveform link updates the 1960’stechnology used in data communications. The single-ton waveform is a 1,800-Hz phase-modulatedwaveform containing the Link-11 data in a serial bitstream. The single-tone waveform is most commonlyused with the wire-line option of the USQ-125 dataterminal. The CP-2205(P)(V)/USQ-l25 data terminalwire-line option provides an interface port that can beused with a standard wire-line or a satellite modem.Using this option expands the means in which Link-11 data can be exchanged, overcoming the limitationsof the traditional UHF and HF radio links.

Enhanced Link Quality Analysis (ELQA)

The Enhanced Link Quality Analysis option of thedata terminal incorporates almost all of the functionsof the LMS-11. This allows the operator to monitorand evaluate the performance of the link net.Information that can be displayed includes the

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following: sideband power, error rate, and percentageof interrogations answered.

Maximum Useable Frequency (MUF) Option

The maximum useable frequency option is aroutine that calculates the optimum frequency forLink-11 operations. This routine calculates afrequency for each hour of the day based ongeographic location, the range of other participants inthe net, and sunspot activity.

Multi-Frequency Link

The multi-frequency link option improves currentlink operations by simultaneously using fourfrequencies. The normal configuration for multi-frequency link operations uses three HF frequenciesand one UHF frequency. To implement this option,three additional processor boards are installed in thedata terminal. Each data terminal board is connectedto a separate radio, as shown in figure 5-3.

Figure 5-3.—Block diagram of the AN/USQ-125 dataterminal configured for multi-frequency link operations.

During the Link-11 receive cycle, each processordemodulates the link signal and sends the data to themaster processor board. The master processorcompares the received data and selects the signalswith the fewest errors to send to the CDS computer.Although this mode is normally used with three HFfrequencies and one UHF frequency, there is no setlimitation of the radio configuration.

On-line and Off-line System Test Options

The data terminal provides several options forboth on-line and off-line testing. These include thefollowing: radio echo test, loopback tests 1,2,3, and4, and DTS fault isolation tests. The radio echo test,loopback test 1, and loopback test 4 are on-line tests,while loopback test 2, loopback test 3, and the DTSfault isolation tests are off-line tests.

RADIO ECHO TEST.— When this option isselected, the data terminal is placed in full-duplexmode. This option is selected when a single stationPOFA is run with the radio and checks the operationof the computer interface, the crypto device, the dataterminal, and the radio.

LOOPBACK TEST 1.— Loopback test 1 isselected when running a single station POFA withoutthe radio. When you select this test option, the audiolines are internally disconnected from the radio andthe audio outputs are connected to the audio inputs.Full-duplex operation is also enabled. This testchecks the operation of the computer interface, thecrypto device, and the data terminal.

LOOPBACK TEST 2.— Loopback test 2configures the data terminal for an off-line self-test.The audio lines are disconnected from the radio andthe audio output lines are internally jumpered to theaudio input lines. A test message is internallygenerated and sent through the audio circuits. Thereceiver output is monitored for data errors, parityerrors, control code errors, and preamble recognition.Any errors detected will cause the LOOPBACK FAILindicator to be displayed.

LOOPBACK TEST 3.— Loopback test 3 is adata terminal to radio test. Normal audio connectionsare maintained, while the computer interface isdisabled. A test message is internally generated andrepeatedly sent through the radio. As with loopbacktest 2, the receiver output is monitored for data errors,parity errors, control code errors, and preamblerecognition. Any errors detected will cause theLOOPBACK FAIL indicator to be displayed.

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LOOPBACK TEST 4.— Loopback test 4 is usedto check the operation of the computer interface, thecrypto device, and the data terminal interfacecircuitry. When this test is selected, the audio circuitsare disabled and the data from the computer is sent tothe memory in the data terminal. Upon receipt of theend of transmit signal, the data in memory is sentback to the computer for evaluation.

DTS FAULT ISOLATION TESTS.— The DTSfault isolation tests are built-in tests (BIT) designed totest and isolate a fault to a particular circuit board.

REMOTE CONTROL UNIT

The C-12428/USQ-125 remote control unit (CU)enables the operator to control the data terminal froma remote location. The control unit, used with thedata terminal, forms the data terminal set (DTS). Thecontrol unit is used by the operator to enter operatingparameters, to start and stop link operations, and tochange link modes. One model, shown in figure 5-4,consists of a 486DX2/66 MHz AT compatiblepersonal computer in a rugged chassis for shipboardoperation. The keyboard/trackball unit is in a specialdetachable enclosure that also serves as a front coverfor the CU. A 386 or better personal computer maybe substituted for the control unit when loaded withthe proper software and connected to the dataterminal.

JOINT TACTICAL INFORMATIONDISTRIBUTION SYSTEM (LINK-16)

The Joint Tactical Information DistributionSystem (Link-16) is a new tactical data link that wasintroduced to the fleet in 1994. Link-16 has beenreferred to by several names and acronyms. TacticalDigital Information Link (TADIL) is a term used bythe U. S. Joint Services. The TADIL designation forLink-16 is TADIL J. The Joint Tactical InformationDistribution System (JTIDS) refers to thecommunications component of Link-16. Thecommunications component includes the terminalsoftware, hardware, RF equipments, and thewaveforms they generate. The NATO term for JTIDSis the Multifunctional Information DistributionSystem (MIDS). For our purposes, we will use theterm Link-16 when referring to this system.

FEATURES OF LINK-16

Link-16 allows for the exchange of real-timetactical information between units of the Navy, theJoint Services, and the members of NATO. Althoughsome of the functions are identical to the functions ofLink-11 and Link-4A, Link-16 also provides dataexchange elements that the other link systems lack.These include the following:

Nodelessness

Jam resistance

Flexibility of communication operations

Separate transmission and data securityfeatures

Increased numbers of participants

Increased data capacity

Network navigation features

Secure voice capabilities.

Figure 5-4.—The C-12428/USQ-125 Control Unit.

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Transmission Protocols

Since Link-16 exchanges much of the same datathat is used in both Link-11 and Link-4A, a briefcomparison of the architectures, the capacities, andthe data rates of the three systems is useful.

During normal operation, Link-11 operates usingthe protocols of the Roll Call mode. In this mode,each participating unit is polled by the NCS totransmit data. On completion of data transmission,the unit returns to the receive mode and the next unitis polled until all units have been polled. This cycleis continuously repeated. Link-11 messages are calledM series messages.

Link-4A uses the time-division multiplexingprinciple with a command-and-response protocol toenable the operator to control multiple aircraftindependently on the same frequency. Link-4Amessages sent to the controlled aircraft are referred toas V series messages and messages received from thecontrolled aircraft are called R series messages.

Link-16 uses the Time-Division Multiple Access(TDMA) principle of data communications. Usingthis architecture with time interlacing provides thesystem with multiple and apparently simultaneouscommunications nets. Instead of assigning each unita PU number, Link-16 assigns each unit a JTIDSUnit number, or JU. The JU identifies the units anddetermines a preassigned set of time slots thatdesignate when the unit transmits and receives data.Each time slot is 1/128 of a second, or 7.8125milliseconds, in duration.

When a JU transmits data, the frequency that thedata is transmitted on is changed every 13microseconds (µsec), according to a predeterminedpseudo-random pattern. Link-16 uses 51 differentfrequencies for data exchange. This frequencyhopping adds to the security and integrity of thesystem by making it nearly impossible to jam.

Link-16 Nets

Link-16 has the capability to handle multiple nets.A Link-16 net is a group of participants sharing

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mutually beneficial tactical information. Forexample, using the Link-11 system, a net is formed bya group of participants. These participants operate onthe same frequency. A separate net is formed whenanother group of participants operates on a differentfrequency. The second net would be used byparticipants involved in a fleet exercise that wouldn’twant the exercise data to interfere with the normaltactical net. The controlling station and aircraft usingLink-4A is also a net.

Link-16 has the ability to form multiple nets. TheLink-16 system has 128 numbers used to designateparticular nets (00-127). Net number 127 is reservedto indicate a stacked net. A stacked net is formed bysetting up the time slots so that they have the sameset, initial slot number, and recurrence rate. Whenthe system is initialized, the use of net number 127indicates a stacked net is to be used and the operatorcan then specify locally which net to use foroperations. Figure 5-5 illustrates the concept of astacked net used for air control. Net 1 is a group ofaircraft controlled by the ship, while Net 3 is a groupof aircraft controlled by an E-2. If the E-2 requiresadditional aircraft, the ship can direct the aircraftunder its control to the E-2. As the aircraftapproaches the E-2, the pilot can switch to Net 3 andimmediately become an active participant in the newnet. Even though the operator has several netsavailable to monitor or use, a single terminal cantransmit or receive on only one of them for each timeslot. Stacked nets are possible because the frequency-hopping pattern is different for each net. Examples ofstacked nets are voice nets and control nets.

Figure 5-5.—Stacked nets using Link-16.

Link-16 Data Exchange

Link-16 transmits data serially using 70-bit datawords. During the transmit time slot, either three, six,or 12 data words can be transmitted. The number ofwords transmitted depends on whether the standard,packed-2, or packed-4 data packing structure is used.The number of words that compose a Link-16message is variable but is normally 1, 2, or 3 words.There are three types of messages: fixed format, freetext, and variable format. The fixed format messagesare called J-series messages and are used to exchangetactical information. Free text messages are used forvoice communications, while the variable formatmessages are user defined in length and content.Variable format messages are not used by the Navy.

JTIDS Architecture

There are several features of the JTIDSarchitecture that have resulted in improvedcommunications of the Link-16 system. Thesefeatures include the following:

Nodelessness

Security

Network participation groups

NODELESSNESS.— A node is a unit required tomaintain communications of a data link. In Link-11,the NCS is a node. If the NCS goes down, the entirenet is inoperative. Link-16 does not need a dedicatedstation. When the Link-16 net is established, a singleJU transmits a Network Time Reference (NTR).The time established by this unit is the networksystem time. All other units in the net use the NTRmessage to synchronize with the network. Once theNTR and the network have been established, thenetwork can continue to operate regardless of theparticipation of any particular unit.

SECURITY.— The security of the Link-16system is vastly improved over that of the Link-11system. In Link-16, both the data and thetransmissions are encrypted. Data is encrypted by adevice similar to Link-11, using a specified

cryptovariable for message security. The security ofthe data transmission is provided by the use of asecond cryptovariable that controls the transmittedwaveform. Frequency hopping to prevent jamming isone of the features of the security system. Thetransmission security also provides for theintroduction of jitter and a pseudo-random noise to beadded to the waveform. The addition of jitter andnoise, along with the frequency hopping, makes thetransmitted signal extremely difficult to detect andjam .

N E T W O R K P A R T I C I P A T I O NGROUPS.— The time slots of a Link-16 network canbe broken down into separate Network ParticipationGroups(NPGs). An NPG is defined by its functionand determines the types of messages that aretransmitted on it. Some of the NPGs used by theNavy are as follows:

Surveillance

Electronic Warfare

Mission Management

Weapons Coordination

Air Control

Fighter-to-Fighter

Secure Voice

Precise Participant Location and Identification(PPLI) and Status

By dividing the net into NPGs, each JU canparticipate on only the groups that support the missionof the unit. Most Navy Command and Control (C2)units, both ships and aircraft, operate on all thedefined NPGs except the Fighter-to-Fighter NPG.

Link-16 New Capabilities

The increased size of the Link-16 enables thereporting of up to three times as much tacticalinformation as was available under the Link-11

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system. AreasLink-16 system

that have been improved under theinclude the following:

Number of Participants

Track Numbers

Track Quality

Track Identification

Friendly Status

Granularity of Measurement

Relative Navigation

Electronic Warfare

Land Points and Tracks

NUMBER OF PARTICIPANTS.— The numberof units that can participate in a Link-16 net has beenincreased dramatically over that of Link-11. TheJTIDS Unit number, or JU, is a five-digit octalnumber from 00001 to 77777. This allows for amaximum of 32,766 possible JUs. Addresses 00001to 00177 are normally assigned to units that have theneed and capability to participate in both Link-16 andLink-11. When a unit participates in both Link-11and Link-16, it must use the same address on bothlinks. For example, Link-16 JU 00043 is the same asLink-11 PU 043.

TRACK NUMBERS.— Link-16 replaces the oldfour-digit (octal) Link-11 track numbers with a five-character alphanumeric track number. The tracknumber can be within the range 00001 to 77777(octal) or 0A000 through ZZ777. This allows for amaximum of 524,284 track numbers, compared withthe 4,092 available with Link-11. One reason for theneed for the additional track number is that Link-16cannot operate in the track number pool mode, inwhich a common pool of track numbers is shared byseveral PUs. Every JU must be assigned a uniqueblock of track numbers.

To maintain interoperability with Link-11,Link-16 track numbers 00200 through 07777designate the same tracks as Link-11 track numbers0200 through 7777.

TRACK QUALITY.— The Track Quality (TQ)value used by Link-16 relates to the accuracy of thereported position of the track. The TQ has a range of0 to 15. To achieve the highest track quality, the trackmust be within 50 feet of the reported position.Link-11 uses the update rate to determine trackquality. Using Link-11, a track that is reported by aPU at every interrogation is usually assigned a TQ of7 ✎

TRACK IDENTIFICATION.— The Link-16system greatly expands the information that isreported with Track Identification (ID). The new IDreports include fields for platform, activity, specifictype, and nationality of the track. Additionalprovisions have also been added to identify a track as“Neutral,” and the Unknown Assumed Enemy ID ischanged to “Suspect.”

FRIENDLY STATUS.— The Link-16 systemalso provides for more detailed status reports fromfriendly aircraft. The following fields are added toLink-16 friendly status reports: equipment status,ordnance inventory, radar and missile channels, fuelavailable for transfer, gun capability, and station ETAand ETD.

INCREASED GRANULARITY.— Granularityrefers to how precisely an item is reported in the linkmessage. Link-16 has made major improvements inthe granularity of reports concerning track position,air track speed, altitude, and lines of bearing.

LINES AND AREAS.— The Link-16 systemallows the reporting of multi-segment lines and areasof all sizes and descriptions. Link-11, forcomparison, only allows reports of areas that arelimited in size and are circles, ellipses, squares, orrectangles. Link-11 does not have the capability toreport lines.

GEODETIC POSITIONING.— The Link-16messages use the geodetic coordinate system to report

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positions. This system uses latitude, longitude, andaltitude to report positions anywhere in the world.Link-11 uses the Cartesian coordinate system, whichrequires the reporting unit to be within a certain rangewhen reporting positions.

RELATIVE NAVIGATION.— The RelativeNavigation (RELNAV) function of the Link-16system is automatically started by every Link-16participant and is constantly operating. TheRELNAV function determines the distance betweenreporting units by measuring the arrival times oftransmissions and correlating them with the reportedposition of the unit. This information is required byeach terminal in the network to maintainsynchronization. The RELNAV data can alsoimprove a unit’s positional accuracy. Also, if two ormore units have accurate geodetic positions,RELNAV can provide all other units with accurategeodetic positions.

ELECTRONIC WARFARE.— The Link-16system increases the types and amount of electronicwarfare information that is exchanged between units.

LAND POINTS AND TRACKS.— The Link-16system adds Land as a track category, and allows thereporting of land objects, such as buildings orvehicles.

EQUIPMENT CONFIGURATION

Currently, Link-16 will be installed onboardaircraft carriers, cruises, destroyers, and amphibiousassault ships. Two phases of shipboard installation,designated Model-4 and Model-5, are planned.

Model-4 is being installed on ACDS and AEGISplatforms in conjunction with the installation of theCommand and Control Processor (C2P). Model-4does not implement any of the expanded dataexchange capabilities of Link-16. Instead, it supportsexisting Link-11 and Link-4A with its jam-resistant,increased capacity waveform. Platforms with theModel-4 Link-16 system will retain their originalLink-11 and Link-4A systems, and can use these

systems by placing the C2P in bypass. Model-4 isbeing installed on very few ships, most of which willbe upgraded to Model-5; therefore, our discussion ofLink-16 equipment will concern the Model-5 system.

Link-16 Model-5

The major components of the Link-16 system arethe Tactical Data System (TDS), the C2P, and theJTIDS terminal, as shown in figure 5-6. The TDS andC2P provide the JTIDS terminal with tactical data tobe transmitted. The Link-16 Model-5 fullyimplements all the capabilities of Link-16. For thisimplementation to take place, major software changesmust be made to the TDS and C2P programs. Also,the OJ-663 console replaces the current displayconsole.

Figure 5-6.—The Link-16 Model-5 ACDS system blockdiagram.

Data flow to the Link-16 JTIDS terminal is fromthe ACDS computer, through the C2P computer, tothe Link-16 computer. Link data generated by theACDS computer is now normalized to be independentof any one particular link system. The C2P computerreformats the normalized data into the formatnecessary for transmission over Link-16. The C2Pcomputer can also format the normalized data fortransmission over Link-11 and Link-4A. If necessary,all three link systems can be in operation at the sametime.

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The JTIDS Terminal

The JTIDS terminal used in Link-16 is theAN/URC-107(V)7. This is an advanced radio systemthat provides secure, jam-resistant, digital data andvoice communication among a large number of users.This radio system combines the functions performedby the Link-11 crypto device, data terminal set, andradio into one cabinet. Many other capabilities arealso incorporated in the radio. These addedcapabilities include the following:

Precise participant location and identification

Relative navigation

Synchronization

Secure voice

RelayFigure 5-7.—The AN/URC-107(V)7 JTIDS data terminal.

Built-in testfive drawer electronics cabinet, as shown in figure

Shipboard Terminal 5-7. The components of the JTIDS terminal includethe Digital Data Processor Group (DDPG), the

The AN/URC-107(V)7 JTIDS terminal is a single Receiver/Transmitter Group (R/T), the High-

Figure 5-8.—The JTIDS terminal functional block diagram.

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Power Amplifier Group (HPAG), and the PowerInterface Unit (PIU). The Secure Data Unit (SDU)is a separate assembly that is mounted to the DigitalData Processing Group. Figure 5-8 is the fictionalblock diagram of the JTIDS terminal.

DIGITAL DATA PROCESSORGROUP.— The third drawer of the terminal housesthe digital data processor group. The two majorcomponents are the interface unit (IU) and thedigital data processor (DDP). A battery assembly ismounted to the front of the DDPG drawer. Thisassembly consists of one nickel cadmium (NiCad)battery and two lithium sulphur dioxide cells. TheNiCad battery will provide power to criticalcomponents during short power failures. The lithiumsulphur dioxide cells supply power to thechronometer.

The Interface Unit controls the communicationsbetween the JTIDS terminal and the host computerand provides amount for the Secure Data Unit (SDU).On shipboard systems, the C2P is the host computer.The Subscriber Interface Computer Program(SICP) is a software program that controls the

communications with the host computer and providesthe data processing necessary to integrate the terminaland the host computer. The IU and SICP also providethe following functions: analog-to-digital and digital-to-analog conversion of voice signals, feed throughinterface between the DDP and the SDU, and primaryand backup power interface. The IU also provides theinterface for receiving and supplying the TACANblanking pulses. These blanking pulses prevent theTACAN and the JTIDS terminal from transmitting atthe same time.

The Digital Data Processor (DDP) controls thereceiver/transmitter and the high-power amplifiergroups. The DDP performs the processing requiredfor transmitting and receiving Link-16 messages.This processing includes the following:

Data encryption and decryption

Error detection and correction encoding anddecoding

Generation of the frequency-hopping pattern

Figure 5-9.—The Digital Data Processing Group functional block diagram.

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Selection of the carrier frequency

Measurement of time of arrival data forposition and synchronization calculations

Execution of the Built-in Tests (BIT) for faultisolation

Generation of alerts

The Network Interface Computer Program(NICP) is the software that runs in the DDP and isresponsible for the communications with the JTIDSRF network. The NICP controls messagetransmission and reception processing, coarse and fineterminal synchronization, relative navigationprocessing, and terminal and network monitoring.

Figure 5-9 is the block diagram for the DDPG.The global memory in the DDP is shared by all theprocessors in the terminal. Communications betweenthe processors is over an internal bus called the plaintext bus. All transactions on the plain text bus areeither read or write commands to the global memoryor port-to-port transfers. When the SICP, running inthe IU, needs to communicate with the NICP, it doesso by using the shared global memory in the DDP. Aport-to-port transfer is a transfer of data betweenports, such as when communicating with the hostexternal timer (see fig. 5-9).

SECURE DATA UNIT.— The SDU is aremovable assembly that is mounted to the IU. Itstores the cryptovariables that are loaded duringinitialization. The SDU provides for both messagesecurity and transmission security. Message securityis provided by the encryption of the data, whiletransmission security is provided by the pseudo-random frequency-hopping pattern and theintroduction of a pseudo-random pattern of noise andjitter on the RF signal.

RECEIVER/TRANSMITTER GROUP.— TheR/T is in the top drawer of the equipment cabinet andprocesses the radio frequency signals. The R/T alsogenerates a 75-MHz intermediate frequency signalused for internal communication between the R/T andDDPG. When a Link-16 message is received, the R/T

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converts the RF to the intermediate frequency andsends it to the DDPG for processing. When theterminal transmits a Link-16 message, the R/Treceives a Continuous Phase-Shift Modulation (CPS)IF signal from the DDPG. The R/T then converts it toa 200-watt RF signal that is sent to the high-poweramplifier group.

HIGH-POWER AMPLIFIER GROUP.— TheHPAG is in the second drawer of the equipmentcabinet and consists of a high-power amplifier and theantenna interface unit (AIU). The signal from the R/Tgroup is received by the HPAG and amplified from200 to 1,000 watts. The HPAG can also operate in alow-power mode, in which case the output signal isabout 200 watts. The AIU provides the interfacebetween the output of the HPAG and the antenna.

POWER INTERFACE UNIT.— There are twoPower Interface Units (PIUS) in the equipmentcabinet. The fourth drawer is the HPAG PIU and thebottom drawer is the PIU for the R/T and DDG. Thetwo PIUS are identical. The three-phase, 115-VAC,60-Hz input power is converted to two outputs: threephase, 115-VAC, 400-Hz, and one-phase, 115-VACat 400 Hz.

COMMAND AND CONTROL PROCESSOR

The Command and Control Processor (C2P) is amessage distribution system designed to control andmanage the interfaces between the three tactical datalinks (Link-4A, Link-11, and Link-16), the operator,and the hardware.

PURPOSE OF THE C2P

The C2P controls and manages the interfacesbetween the various data links on major surface andaircraft Command and Control (C2) platforms. Thesurface platforms that will have the initial installationsof the C2P system are aircraft carriers (CV, CVN) andAEGIS cruisers (CG), followed by installation onamphibious assault ships (LHA, LHD), and AEGISdestroyers (DDG). There are two configurations ofthe C2P, one tailored for ships with the AdvancedCombat Direction System (ACDS) Block 0configuration and one for ACDS Block 1

Figure 5-10.—The C2P system block diagram for ACDS Block 0 platforms.

configurations. On AEGIS ships, AEGIS Model 4 issimilar to ACDS Block 0, and AEGIS Model 5 issimilar to ACDS Block 1. The C2P system installedon an ACDS Block O platform is very similar to thesystem that is installed on an AEGIS Model 4platform. Figure 5-10 illustrates the system blockdiagram of the C2P for ACDS Block 0 platforms.

Link messages generated in the ACDS computerare sent to the C2P computer where they are formattedfor transmission on the proper link (Link-4A,Link-11, or Link-16). Depending on the mode ofoperation and operator entered parameters, somemessages may be sent over two or more data links.For example, it is not uncommon for Link-11messages to be transmitted over Link-11 and Link-16.The C2P computer stores the data in a central database, called the normalized data base, and thenformats the data in the proper message format for thelink system(s) being used.

Messages received by the various data links areprocessed for errors by the C2P computer and sent tothe proper destination. Received messages can alsobe reformatted for retransmission on a different link.A Link-11 or Link-4A message received by a C2Pplatform can be reformatted into a Link-16 messageand retransmitted on Link-16.

SYSTEM CONFIGURATION

The hardware block diagram of the equipmentused in the C2P system is shown in figure 5-11. TheAN/UYK-43(V) is a general-purpose, large scale,tactical computer used to store and execute the C2Psoftware. The C2P configuration of the AN/UYK-43consists of the following major modules:

Two central processor units

Two input/output controllers and adapters

Six expanded time volatile memory units

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Figure 5-11.—The C2P system hardware configuration

One embedded memory subsystem (EMS)with two embedded mass memory storagedevices (EMMSD)

A major change in the configuration of theAN/UYK-43A(V) is the EMS and its associatedEMMSDs. The EMS consists of two 383 megabytehard drives installed in the AN/UYK-43(V) cabinet.Even though these disk drives are internally installedin the computer, the software accesses them as if theywere external disk drives.

The AN/USQ-69(V) data terminal set is used toprovide the man-machine interface (MMI). It isinstalled next to the Track Supervisor in CIC.

Several equipments are shared between the ACDSsystem and the C2P system. These include themagnetic tape unit, a teleprinter, and a secondAN/USQ-69(V) data terminal set. The magnetic tapeunit is used for initial program loading (to EMS), dataextraction, and reading and writing JTIDSinformation to and from tape. It is also a backup loaddevice when the EMS is down. The teleprinter

provides hard copy printouts of C2Perror codes, and data dumps.AN/USQ-69(V) is used as a backup.

Shared equipments are switched

system status,The second

to the desiredsystems through the Combat Systems Switchboard.The switchboard also provides switches to connectLink-4A and Link-11 directly to the CDS computer,bypassing the C2P system.

SUMMARY—NEW TECHNOLOGY IN DATACOMMUNICATIONS

This chapter introduced you to some of the newchanges and systems concerning data communicationsin the Navy. The following information summarizesthe important points you should have learned.

THE AN/USQ-125 DATA TERMINALSET— The AN/USQ-125 data terminal set replacesmany of the older Link-11 data terminal sets in theNavy. It consists of the CP-2205(P)(V)/USQ-125data terminal and a remote control unit.

THE CP-2205(P)(V)/USQ-125 DATATERMINAL— The CP-2205(P)(V)/USQ-125 dataterminal performs the modulation/demodulation, errordetection and correction, CDS computer and radiointerface, signal analysis and built-in test functions.In addition to performing all of the standard multi-tone Link-11 operations, the data terminal has severalnew capabilities. These capabilities include thefollowing:

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Single-Tone Waveform Link— This optionof the Link-16 system is used to transmit dataover a standard telephone modem or a satellitemodem. It uses an 1,800-Hz phase-modulatedwaveform that has the Link-11 data embeddedin it as a serial bit stream.

Enhanced Link Quality Analysis— ELQAallows the operator to monitor and evaluatethe performance of the Link-11 net byproviding most of the functions of theLMS-11.

Maximum Useable Frequency— This optioncalculates the best frequency for use based ongeographic location, time of day, currentsunspot activity and other factors that canaffect HF radio-wave propagation.

M u l t i - F r e q u e n c y Link— T h emulti-frequency link enables the operation ofLink-11 on up to four frequencies. Normally,multi-frequency link uses three HFfrequencies and one UHF frequency.

REMOTE CONTROL UNIT— The remotecontrol unit is a personal computer based unit thatallows the operator to control the data terminal froma remote location.

THE JOINT TACTICAL INFORMATIONDISTRIBUTION SYSTEM (LINK-16)— Link-l6is a new data link that provides for the real-timeexchange of tactical data between Naval, JointService, and NATO units. It includes many of thefunctions of the current Link-11 and Link-4Asystems. Link-16 expands many of these functionsand includes several new features that the othersystems lack. These new features include thefollowing:

Nodelessness— Nodelessness means that noone unit is required to maintain linkoperations. Once the net is established, it cancontinue to operate regardless of theparticipation of any particular unit.

Jam resistance— Link-16 data is nottransmitted on any particular frequency, butchanges frequency every 13 microseconds.For additional security, a pseudo-randompattern of noise and jitter is added to thesignal. The combination of frequency hoppingand the noise and jitter makes the transmittedsignal hard to detect and jam.

Flexibility of communicationoperations— Link-16 provides both data andvoice communications circuits. In addition,different data packing methods allow for

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variation in the amount of data transmittedduring a given time slot.

Separate transmission and data securityfeatures— Link-16 transmissions areprotected by encrypting the data, frequencyhopping, and adding a pseudo-random noisepattern to the signal before transmitting

Network navigation features— The relativenavigation module is a software routine run bythe Link-16 terminal that compares the time ofreceipt of a transmission against the time itwas sent and calculates the distance of thesending unit from the receiving unit. Thisinformation is correlated against reportedgeographic position and used to maintain netsynchronization and improve positionalaccuracy.

Secure voice capabilities— The Link-16system secure voice communications have thesame transmission safeguards as datatransmissions.

Link-16 also provides the following new orenhanced capabilities:

Number of Participants— The number ofparticipants that can participate in a Link-16net has been increased dramatically. Using afive-digit octal number for addressing, apossible 32,766 addresses are available.

Track Numbers— Link-16 replaces the oldfour-digit (octal) track numbers used withLink-11 with a five-character alphanumerictrack number. This increases the tracknumber capacity from 4,092 to 524,284.

Track Quality— Link-16 track quality isimproved by comparing the reported positionof a track with the actual position of the track.To achieve the maximum track quality, a trackmust be within 50 feet of its reported position.

Track Identification— The informationreported when identifying a track has been

expanded due to the increased size of theLink-16 data word. The new reports includefields for track activity, specific type, and thenationality of the track. Tracks can also beidentified as “Neutral” and the unknownassumed enemy ID has been changed to“Suspect.”

Friendly Status— Friendly forces reportingtheir status can add additional information,such as equipment status, ordnance inventory,radar and missile channels, fuel status, guncapability, and station ETA/ETD.

Granularity of Measurement— Majorimprovements have been made in theprecision, or granularity, of reportingpositions.

Land Points and Tracks— Link-16 allowsfor the designationand tracks.

and tracking of land points

EQUIPMENT CONFIGURATION— TheAN/URC-l07(V)7 JTIDS terminal is the heart of theLink-16 system. This radio system provides, in asingle equipment cabinet, the secure, jam-resistant,digital data and voice communications required forLink-16 operations. The terminal consists of thefollowing four major groups: the digital dataprocessor group, the receiver/transmitter group, thehigh-power amplifier group, and the power interfaceunit. The secure data unit is a separate assembly thatis mounted on the front panel of the terminal.

THE COMMAND AND CONTROLPROCESSOR (C2P)— The C2P is a messagedistribution system that manages and controls theinterfaces of the three tactical data links and the CDScomputer. The C2P computer is placed in-line withthe CDS computer and the data links communicatedirectly with the C2P computer. The C2P computercan reformat data from one data link for transmissionon another data link. For example, a Link-11 messagereceived by the C2P can be reformatted andtransmitted as a Link-16 message. The C2P alsosends all data from the three data links to the CDScomputer.

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CHAPTER 6

LOCAL-AREA NETWORKS

INTRODUCTION

A local-area network (LAN) is a communications system designed to transmit and receive digitalinformation between computers. A LAN consists of nodes that are interconnected by links. Nodesare the hardware connected to the network, such as personal or microcomputers, printers, largecapacity hard drives, and so on. Links are the communications media, such as twisted-pair wire,coaxial, or fiber-optic cables that connect the nodes. In most applications, the LAN interconnectsa relatively small number of personal computers (PCs), data storage devices, printers, and otherperipherals. These nodes and links usually cover a relatively small geographical area, such as anoffice or a department. Through common usage, the term local-area network can also refer to muchlarger systems, such as the SNAP III system on a ship, which could have literally hundreds ofterminals and miles of cables. For our purposes, we will be using a small system in our discussionof LANs.

Any device connected to the network can send and receive data on the network. A majority ofdata exchanged over a network is text and graphics, which is assembled as structured data that canbe manipulated by computers. Unstructured data, such as pictures and facsimile messages, can bestored and retrieved efficiently, but cannot be manipulated easily by the computer.

After completing this chapter, you should be able to:

Describe the major components of a LAN.

State the types of cable used in a LAN.

State the function of the network interface card.

Describe the function of the various network servers required by a LAN.

Describe the function of the central mass storage area of a LAN.

Describe the Open System Interconnection (OSI) Reference Model used in the designand implementation of a LAN.

Describe the advantages and disadvantages of the different LAN topologies.

Describe the hardware systems used in LANs.

Describe the function of the software operating system of a LAN.

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LOCAL-AREA NETWORK HARDWARE

The basic hardware components of a LAN arecables, network interface cards, network servers,peripherals, and workstations. These components arecovered in the material that follows.

CABLES

Several types of cables can be used in LANapplications. The selection of the type of cabledepends on several factors, such as maximum lengthof a single cable run, security requirements, and thecapacity and speed of the system.

Twisted-Pair Cable

The twisted-pair cable is easy to install and costslittle on a per-foot basis. In some cases, existingtelephone cable may be used. Its disadvantagesinclude limitations in capacity and speed. It is alsosusceptible to electrical interference unless it isshielded.

Shielded Twisted-Pair Cable

The shielded twisted-pair cable is encased in anRFI shield. The stranded wire used as a conductor ismanufactured with greater precision and is capable ofgreater data transmission rates and longer cable runs.

Coaxial Cable

Coaxial cable networks have gained in popularitybecause of their use in cable television. Thequantities of cable and connectors produced for cabletelevision have greatly reduced the prices of thesecomponents for network users. Coaxial cable comesin various thicknesses and is designated by a number:RG-11, RG-58, RG-59, RG-62, and so forth. You canuse either baseband or broadband transmissionmethods with coaxial cable.

Baseband coaxial systems transmit digital signalsunchanged over a single channel and have severaladvantages. They are inexpensive, easy to install, andhave low maintenance. They also allow very high

data transmission rates. One disadvantage is that theyare limited to transmitting digital signals only.

In contrast, broadband coaxial systems requirethe digital signal to be converted to an analog signalbefore transmission and then back to digital bymodem at the receiving device. Broadband systemssupport data, voice, and video signals that may betransmitted simultaneously. Disadvantages ofbroadband systems are their higher installation costsand complex maintenance.

Fiber-Optic Cable

Fiber-optic cable is the best choice if a securenetwork is needed. Because the cable transmits light,the transmissions are immune to interference causedby electrical or electronic devices. Also, if yournetwork will run through an area of heavy industrialactivity or a work place with strong radio frequencyinterference, fiber-optic cable is the most appropriatechoice. Other advantages of the fiber-optic cable arethat it lasts longer than other types of cable and cancarry many more channels. Its disadvantages includeits high price, poor connectivity, and low flexibility.

NETWORK INTERFACE CARD

To attach personal computers to the LAN, youmust connect a network interface card (NIC) to eachPC and attach the network cable to the NIC. The NICis nothing more than a circuit board that normallyplugs directly into one of the expansion slots inside aPC. Sometimes, the NIC comes as a separate unit. Inthis case, you plug it into the back of the PC. MostNICs have their own built-in microprocessor(s)designed to take care of network communications.This relieves the PC’s main processor of thisresponsibility. The type of cable used on the networkis determined by the type of LAN to be installed.

NETWORK SERVERS

Your understanding the concept of a server isimportant to understanding how LANs work. Aserver is a combination of hardware and software thatis used to manage the shared resources of the network.The hardware may be a PC or a computer designed

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specifically to act as a server. In either case, thecomputer normally has a hard disk and the softwareneeded to run the network system. A network serveris able to control network traffic as well as the sharingof other resources, such as application programs, diskspace, data files, and printers. There are severaldifferent types of servers, and each has a particularfunction. In newer systems, some separate serverfunctions are combined into a central file server. Theservers we will look at are the disk server, the fileserver, and the print server.

Disk Server

The disk server was the first of the networkoperating systems. In the early days of PC networks,very few computers were equipped with a hard disk.When the hard disk became affordable, manufacturerswere asked to develop a system to allow several usersto share a single hard drive. The earliest disk serverswere multiplexer that polled each connectedcomputer for requests to write a file on the hard driveor to retrieve a file from the disk. The multiplexerthen responded accordingly. A major problem withthis process was that it did not allow for any type ofsecurity, data organization, or disk management.

As LAN technology evolved, the development ofthe disk server software in the early 1980s addressedsome of these issues. The disk server is a softwareroutine that was installed on each computer in thenetwork. The disk server software allowed each PCto access the shared hard drive as if it were a localdrive. In other words, the computer thought the drivewas installed in the computer, but in reality, the drivewas remotely located on the network.

The disk server also provided for someinformation sharing. One purpose of a network is toallow multiple users access to the same information.One problem encountered with early disk serversoccurred when two or more users updated the samefile at the same time. When the file was saved byboth users, the updates of one of the users was lost.

A method of preventing this information loss isfile locking. File locking means that when one useraccesses a file, all the other users are prevented from

accessing that file until the first user is finished withit. As you can see, this method severely limits thenumber of users able to access the information.

Another method used to prevent data loss is recordlocking. In a data-base environment, many userscould access the same data file, but when a record wasbeing modified by one user, the other users werelocked out of the record being modified. A data filecan be updated by several users without threateningthe integrity of the data by using this technique.

Although the disk server was used in most LANsdeveloped before 1985, a major problem still existedin maintaining data integrity. The two methodscovered in the previous paragraphs provided for datamanagement, but not for reliable disk management.A disk drive stores information on the next availableblock on the disk. When the disk server was used, itwas not uncommon for two users to try to write datato the same block at the same time. When thishappened, the second user would overwrite the datajust written by the first user, causing a loss of data.The development of the file server in 1983 solved allof the problems encountered with the disk server.

File Server

Currently, all local-area networks require sometype of file server. In most cases, the file server is adedicated PC or minicomputer. The file serverperforms the processing of the network controlsoftware and the central processing and storage pointof the application software and data files of thenetwork. The file server has a hard disk with a verylarge storage capacity.

The file server manages the hard disk and ensuresthat multiple requests for the same file do not conflictwith each other. In the disk server environment, eachPC workstation manages its I/O with the disk throughlow-level sector calls. In the file server environment,each workstation communicates with the central diskthrough the use of high-level calls to the file server.A high-level call can be a request to open a particularfile or to store a file, while a low-level call maybe towrite this file to sector xyz on the disk. The file serverconverts the high-level calls from the users to low-

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level disk commands, thus providing effective diskmanagement. The file server maintains the list ofprivileges and authorizations for each user. Thisprotects the data files from unauthorized access andprotects the data. An example of this is that one usermay be authorized total access to a data-base file toupdate the file, while another user may be authorizedread-only access to the information. Still a third usermay be denied access to the file altogether.

A network file server is a special-purpose unit thatcan reside in either a dedicated computer, or one ofthe workstations (a PC) that has a hard diskcontaining the software of the network. When thenetwork server is used solely for serving the networkand is NOT used as a workstation, it is referred to asa dedicated server. If the server can also be used asa workstation, it is referred to as a nondedicatedserver.

Some networks do NOT have a single dedicatedfile server. Instead, they use a distributed approachin which any of the nondedicated servers may makeavailable files that reside on their hard disks. Underthese circumstances, any workstation on the networkcan use or copy these files. Moving files back andforth on such a network establishes a temporaryrelationship, you might say, between the two PCsinvolved. One PC acts as the server, and the other PCacts as the receiving workstation.

Print Server

The print server is a software routine that allowsall the workstations on the LAN to use a singleprinter. When the laser printer was introduced to themarket, the extremely high-quality print and multiplefonts made it desirous for all correspondence.Unfortunately, the cost of a laser printer oftenexceeded that of an individual workstation and madeit impractical for each workstation to have a dedicatedprinter. The print server solved that problem byaccepting requests for print jobs from the networkusers and directing them to the printer. The printserver makes sure one job is completed before a newjob is started. Print server routines are included inalmost all network operating systems on the markettoday.

WORKSTATIONS

Workstations is another name for the PCs used ona network. The PCs can be of the same brand, such asZenith, or they can be a combination of differentbrands, such as IBM, Zenith, Compaq, along withother PC compatible computers (clones). Each PCcan be configured differently. Some might have theirown hard disk drives; others might have expandedmemory. Still others might NOT even have diskettedrives or printer ports of their own. Instead, these lessexpensive workstations use the storage and printingresources available through the network. Even thougha PC may be part of a LAN system, you can use itindependently as a stand-alone PC at any time or youcan use it as part of the LAN.

THE OPEN SYSTEM INTERCONNECTION(OSI) REFERENCE MODEL

Over the past few years, a number of networkstandards or protocols (rules to live by) have beendeveloped by the International Standards Organization(OSI) to provide some level of uniformity amongcomputer manufacturers and network vendors. OSI isone of several governing organizations in this fieldthat has developed such protocols. These seven layersof standards, shown in figure 6-1, define a generalizedarchitecture called the Reference Model of OpenSystems Interconnection. It is also known as theOSI reference model or OSI model. The primarypurpose of the OSI model is to provide a basis forcoordinating the development of standards that relateto the flexible interconnection of incompatiblesystems using data communications facilities.

The OSI model does NOT define any onevendor’s particular network software as such, nor doesit define detailed standards for any given software. Itsimply defines the broad categories of functions thateach of the seven layers should perform. The OSImodel can include different sets of standards at eachlayer that are appropriate for given situations. Forexample, in a very simple data communicationssystem, one that uses a simple point-to-point link, thesoftware at the higher level layers (say 5, 6, and 7)might be very simple or possibly nonexistent.However, in a very complex data communications

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Figure 6-1.—The OSI model showing the seven layers.

system, all seven software layers may beimplemented. Although there is no requirement forany hardware or software vendor to adhere to theprinciples set forth in the OSI model, there is aworldwide trend in the computer industry towardacceptance and conformance to these standards.

Ideally, if the hardware, network software,application software, and cabling were all supplied bythe same manufacturer, there would be relatively fewproblems for users to contend with when designingand implementing a network. Everything would worktogether rather smoothly. However, a computermanufacturer’s architecture can make it difficult tointerconnect hardware offered by other competingmanufacturers or vendors. The protocols used bycommunications devices are also highly complex andare often completely different from one manufacturerto another. Then there is the network software.Usually, the network software from one LAN vendorwill not work with that of a competitor; neither willthe application programs. Even the cabling must beselected for a specific local-area network.

HARDWARE LEVEL

The hardware level contains the first two layers ofthe OSI reference model. They are the physical layerand the data-link layer. These are concernedprimarily with the actual hardware used in a network.

Physical Layer

The physical layer is concerned with thetransmission of the unstructured raw bit stream overa physical medium. It describes the electrical,mechanical, and functional interfaces to the carrier.The physical layer carries the signals for all the higherlayers as follows:

Voltages and pulse encoding of bits

Media and media interface (cables,connectors, NIC, and so on)

Line discipline (full- or half-duplex)

Pin assignments

Data-Link Layer

The data-link layer provides error-freetransmission of information over the physicalmedium. This allows the next higher layer to assumevirtually error-free transmission over the link. Thedata-link layer is responsible for getting datapackaged and onto the network cable. It manages theflow of the data bit stream into and out of eachnetwork node as follows:

Creates and recognizes frame boundaries

Checks received messages for integrity

Manages channel access and flow control

Ensures correct sequence of transmitted data

The data-link layer detects and, when possible,corrects errors that occur in the physical layer withoutusing the functions of the upper layers. It alsoprovides flow-control techniques to ensure link-buffercapacity is not exceeded.

TRANSPORT LEVEL

The next three layers of the OSI reference modelmake up the transport level, also known as the subnet.The transport level defines the software protocols

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necessary to exchange data on the network. The threelayers of the transport level are the network layer, thetransport layer, and the session layer.

Network Layer

The network layer decides which physicalpathway the data should take based on networkconditions, priorities of service, and other factors.Software on the network interface card must build thedata packet, so the network layer can recognize androute the data to the correct destination address. Itrelieves the upper layers of the need to know anythingabout the data transmission and switchingtechnologies used to connect the systems. It isresponsible for establishing, maintaining, andterminating connections across the interveningcommunications facility as follows:

Addresses messages

Sets up the path between communicatingnodes on possibly different networks

Routes messages among networks

Is concerned with the sequence delivery ofdata packets

Controls congestion if too many packets areon the network

Translates logical addresses or names intophysical addresses

Has accounting functions to count packets orbits sent by users to produce billinginformation

Transport Layer

The transport layer makes sure data units aredelivered error-free, in sequence, without losses orduplications. It relieves higher layer protocols fromany concern with the transportation of data betweenthem as follows:

Message segmentation. Accepts data from thesession layer, splits it up into smaller units,and passes the units down to the network layer

Establishes and deletes host-to-hostconnections across the network

Multiplexes several message streams onto onechannel and keeps track of which messagebelongs to which connection

Provides reliable end-to-end delivery withacknowledgment

Provides end-to-end flow control and windowmanagement

Session Layer

The session layer allows users on differentmachines to establish sessions between one another.It performs the functions that enable two or moreapplications to communicate across the network,performing security, name recognition, logging,administration, and other similar functions. Unlikethe network layer, this layer deals with the programsin each machine to establish conversations betweenthem as follows:

Allows two applications processes ondifferent machines to establish, use, andterminate a connection (or session)

Performs synchronization between end-usertasks by placing checkpoints in the datastream so that if the network fails, only thedata after the last checkpoint has to beretransmitted

Provides dialogue control (who speaks, when,how long, and so on)

PRESENTATION LEVEL/LAYER

The presentation level consists of the presentationlayer. The presentation layer formats data to bepresented to the application layer. It can be viewed asthe translator for the network. This layer provides a

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common representation for data which can be usedbetween the application processes. The presentationlayer relieves the applications from being concernedwith data representation, providing syntaxindependence as follows:

Encodes data in a standard way (integers,floating point, ASCII, and so on)

Provides data compression to reduce thenumber of bits that have to be transmitted

Provides data encryption for privacy andauthentication

APPLICATION LEVEL/LAYER

The final level is the application level, whichconsists of the application layer. The applicationlayer serves as the window for the application processto access the OSI environment. This layer representsthe services that directly support users and applicationtasks. It contains a variety of commonly neededprotocols for the following items:

Network virtual terminals

File transfers

Remote file access

Electronic mail

Network management

USING THE OSI MODEL

A communications system that does not use alayered architecture can be designed. A specificallydesigned communications system is faster, moreefficient, requires less software code, and eliminatesredundant functions and activities. Why, then, is theOSI reference model considered the standard indesigning networks and writing software? It isconsidered the standard primarily because the use ofa layered architecture, such as the OSI referencemodel, provides the network with flexibility andmigration.

The greatest advantage of your using layerarchitecture in a network is hardware independence.As advances in technology continue, it is notnecessary to scrap a network completely because onecomponent has been superseded. For example, if youhave a network and need to upgrade the cable to atype that can handle increased data at a faster rate, thelayered architecture of the OSI model will allow youto make this replacement to the physical layer withoutchanging the other layers.

LAN TOPOLOGIES

The physical arrangement of the components of aLAN is called its configuration or topology. Thethree major types of configurations, or topologies, ofa LAN are the linear bus, the star, and the ring. Youcan also create hybrid topologies by combiningfeatures of these configurations. For example, severalbus networks can be joined together to form a ring ofbuses.

Each topology requires the components of a LANto be connected in a different arrangement. Thesecomponents are also referred to as nodes. A node isany point on a network where data can be sent(transmitted) or received—a workstation, a server,and so on.

LINEAR BUS NETWORK

The linear bus topology is like a data highway.That is, all components or nodes are connected to thesame cable, and the far ends of this cable never meet,as shown in figure 6-2. Linear bus LANs are bestsuited to applications involving relatively low usageof the bus coupled with the need to pass relativelyshort messages from one node to another. In manysuch networks, the workstations check whether amessage is coming down the cable before sendingtheir messages. Since all nodes share the bus, allmessages must pass through the other workstations onthe way to their destinations. Each node checks theaddress attached to the message to see if it matches itsown address. Bus topologies allow individual nodesto be out of service or to be moved to new locationswithout disrupting service to the remaining nodes.

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Because of the way linear bus cabling is laid out,this type of cabling is simple. The bus topology isvery reliable, because if any node on the bus networkfails, the bus itself is NOT affected, and theremaining nodes can continue to operate withoutinterruption. Many of the low cost LANs use a bustopology and twisted-pair wire cabling.

Figure 6-2.—A bus network topology.

A disadvantage of the bus topology is thatgenerally there must be a minimum distance betweenworkstations to avoid signal interference. Anotherdisadvantage is that the nodes must compete witheach other for the use of the bus. Simultaneoustransmissions by more than one node are NOTpermitted. This problem, however, can be solved byusing one of several types of systems designed tocontrol access to the bus. They are collisiondetection, collision avoidance, and token passing,which we will cover shortly. Also, there is no easyway for the network administrator to run diagnosticson the entire network. Finally, the bus network can beeasily compromised by an unauthorized network user,since all messages are sent along a common datahighway. For this reason, it is difficult to maintainnetwork security.

STAR NETWORK

In a star network, each component is connecteddirectly to the central computer or network server, asshown in figure 6-3. Only one cable is required fromthe central computer to each PC’s network interfacecard to tie that workstation to the LAN. The star isone of the earliest types of network topologies. It

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uses the same approach to sending and receivingmessages as our phone system. Just as a telephonecall from one person to another is handled by a centralswitching station, all messages must go through thecentral computer or network server that controls theflow of data. New workstations can be easily addedto the network without interrupting other nodes. Thisis one of the advantages of the star topology.

Another advantage of star topology is that thenetwork administrator can give selected nodes ahigher priority status than others. The centralcomputer looks for signals from these higher priorityworkstations before recognizing other nodes. The startopology also permits centralized diagnostics(troubleshooting) of all functions. It can do thisbecause all messages must first go through the centralcomputer. This can prove invaluable in making surethat network security has not been breached.

The main disadvantage of the star topology is itsreliance on the central computer for performingalmost all the functions of the network. When thecentral computer fails, all nodes also stop functioning,resulting in failure of the entire network.

Figure 6-3.—A star network topology.

DISTRIBUTED STAR

Figure 6-4.—A distributed star (tree) network topology.

The distributed star, or tree, topology is shown infigure 6-4. It provides many of the advantages ofboth bus and star topologies. It connects workstationsto a central point called a hub. This hub can supportseveral workstations or hubs which, in turn, cansupport other workstations. Distributed startopologies can be easily adapted to the physicalarrangement of the facility site. If the site has a highconcentration of workstations in a given area, thesystem can be configured to more closely resemble astar topology. If the workstations are widelydispersed, the system can use inexpensive hubs withlong runs of shared cable between hubs, similar to thebus topology.

RING NETWORK

In a ring network, all the components or nodes areconnected to the main cable, and the cable forms aring, as shown in figure 6-5. This topology allows anode to send a message to another node on the ring.However, the message must be transmitted througheach node until it reaches its destination. Messagesproceed from node to node in one direction only.Should anode fail on the network, data can no longerbe passed around the ring unless the failed node iseither physically or electronically bypassed. Usingbypass software, the network can withstand the failureof a workstation by bypassing it and continuing tomaintain the integrity of the network. One of the

major issues in a ring topology is the need for makingsure all workstations have equal access to thenetwork.

One of the major disadvantages of ring topologiesis the extreme difficultly of adding new workstationswhile the network is in operation. Normally, theentire network has to be brought down while a newnode is added and cabling reattached. However, thisparticular problem can be overcome by the installationof additional connectors when the network is initiallyset up. These connectors enable you to add or removenodes while the network remains intact and inoperation.

ACCESS METHODS

Another decision the designer makes is that ofwhich access method to use. Access methods are the

Figure 6-5.—A ring network topology.

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Figure 6-6.—A bus network using the CSMA/CD accessmethod.

arrangements used to make sure each workstation hasfair and equal access to the network. The accessmethod used is governed primarily by the topologyand the protocol of the network. The principal accessmethods are contention and token passing.

Contention

The contention method features Carrier SenseMultiple Access (CSMA) and Carrier Sense MultipleAccess with Collision Detection (CSMA/CD). TheCSMA/CD method is shown in figure 6-6. Access forboth is on a first-come, first-served basis. The CSMAaccess scheme is very similar to that of a citizens band(CB) radio. Stations with data to send listen to thechannel and wait until it is clear to transmit. WithCSMA/CD, when two or more workstations transmit

simultaneously, their messages will collide. As soonas a workstation detects a collision, it ceasestransmission, monitors the network until it hears noother traffic, and then retransmits. Most contentionnetworks assign a unique retry algorithm to vary thewait-and-retry period. This algorithm reduces thelikelihood that after a collision, two workstations willtransmit retries simultaneously.

Token Passing

Token passing is an orderly access method and isshown in figure 6-7. Each workstation passes on theopportunity to transmit to its closest neighbor until astation is found with a message to send. Thispermission to transmit is called a token. When aworkstation with data to send is handed a token, partof the token is changed, indicating it is carrying amessage, and then data is transmitted with the token.The token is then passed around the network, andevery station checks whether the message is intendedfor it. The receiving station copies the message fromthe token, but then passes the unchanged token alongthe network. When the transmitting station receivesthe same token, it knows the message has been passedaround the network. The transmitting station erasesthe message and puts the empty token back intocirculation on the network. The amount ofinformation that maybe transmitted during possessionof the token is limited so that all workstations canshare the cable equally.

PROTOCOLS

Network protocols are an important componentbecause they define how networks establishcommunications between elements, exchangeinformation, and terminate communications.Protocols have two major operational functions. Theyestablish the circuit for transmission (handshaking)and for the transmission itself. Transmission isconducted subject to the line discipline. The linediscipline is the sequence of operations that actuallytransmits and receives the data, handles theerror-control procedures, handles the sequencing ofmessage blocks, and provides for validation forinformation received correctly.

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Figure 6-7.—A ring network using the token passingaccess method.

Two representative protocols, which control linediscipline, are the Binary SynchronousCommunications Protocol (Bisync) and theSynchronous Data Link Control (SDLC).

Bisync

Bisync is a half-duplex protocol that transmitsstrings of characters at lower speeds over dial-upcircuits. The information movement is in onedirection at a time, with each data transfer beinganswered by an acknowledgement.

SDLC

SDLC is a control procedure that sends multipleblocks of data and returns a single acknowledgementfor many blocks, thereby increasing the amount oftime spent transmitting data. The bits that are putbefore and after the message at the transmitting endare removed at the receiving end, so only the messageis presented to the user.

The hardware chosen for the network plays a partin the choice of network protocol. Most users andmany of the vendors who build the clone type ofequipment would like to see universal interfaces,while others think that the availability of differentspecifications will lead to a proprietary set ofequipment, even though they favor the overall OSIspecifications.

LAN SYSTEMS

When you decide to install a LAN system, thetype of topology used in the initial wiring of thesystem will have a major effect on the type of systemthat can be used. There are many LAN systemsavailable, each with advantages and disadvantages. Inthe following paragraphs, we briefly examine some ofthe available LAN systems.

The Institute of Electrical and ElectronicsEngineers (IEEE) has developed a set of standards forlocal-area networks. These standards encourage theuse of common approaches for LAN protocols andinterfaces. The IEEE LAN standards were developedby a committee of engineers and classified as the 802standards. The 802 standards are broken down evenfurther to define the protocols and topology used in aLAN. Some of the standards we are concerned withare the following:

IEEE 802.3—Carrier sense multipleaccess/collision detection (CSMA/CD)

IEEE 802.4—Token Bus

IEEE 802.5—Token ring

ETHERNET

The EtherNet local-area network was developedby Xerox, the Intel Corporation, and the DigitalEquipment Corporation. It became the model for thedevelopment of the IEEE 802.3 standard. Theoriginal standard defined a maximum throughput forEtherNet of 10 Mbit/s, but it has been revised tosupport throughput of much higher rates. Whenoperating over coaxial cable, EtherNet has a 20-Mbper second throughput speed. For high-demandenvironments, such as engineering or graphics,EtherNet is often the choice. It is a bus topology anduses CSMA/CD protocol. It is available in thefollowing three versions: standard EtherNet, ThinNet,and twisted-pair EtherNet.

Standard EtherNet and ThinNet both use coaxialcable. Standard EtherNet is somewhat moreexpensive and more difficult to install than ThinNet,

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but it allows networking over greater distances withmore users. Twisted-pair EtherNet uses a distributedstar topology with wiring concentrator hubs, not thebus topology characterizing standard EtherNet andThinNet. Connecting more than 100 users on astandard EtherNet trunk or on a series of twisted-pairconcentrators is not uncommon, while ThinNet LANsusually support less than 50 users.

All versions of EtherNet create a LAN with highinterconnectivity options. A number of products areavailable for connecting EtherNet LANs tominicomputers and mainframe computers and forbridging to other LANs; examples are STARLAN,ARCnet, and IBM Token Ring Network.

STARLAN

STARLAN uses a star topology with a CSMA/CDprotocol. Its throughput speed is 1Mb per secondover twisted-pair cable. If buildings are already wiredwith twisted-pair cable meeting AT&T premisecabling specifications, STARLAN can be installedeasily. It is considered to be a low cost-per-usernetwork and its star topology makes it a flexiblenetwork.

ARCnet

ARCnet is a distributed star topology that uses atoken passing protocol and either twisted-pair orcoaxial cabling. Its throughput speed is 2.5Mb persecond. Although ARCnet does not conform to anIEEE standard, it closely resembles the 802.4 standardfor a token bus system. It can easily handle up to 75users. If user demand is low, it can handle additionalusers. It is considered an extremely reliable networkand is easy to install, expand, and modify.

IBM Token Ring Network

The IBM Token Ring Network uses a star ringtopology, and is defined by the IEEE 802.5specification. It has a throughput speed of 4 Mbitsper second and 16 Mbit per second. This allows forflexible expansion of very large networks. Because ofits speed and token passing protocol, it is a goodchoice to meet high-volume requirements. It is a

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sophisticated LAN technology developed by IBM tobe used with an IBM cabling system and is currentlythe fastest growing installed network base. The starring topology also makes use of redundant circuitsand loopbacks to handle breaks in the ring and resultsin high-fault tolerance on the network.

NETWORK OPERATING SYSTEMS

Network operating system software is necessaryto control the overall operations of the network.Careful consideration must be given to the variouspackages on the market to ensure the operatingsoftware is fully compatible with your system,topology, and needs.

NETWORK OPERATING SYSTEM BASICS

The two basic components of the networkoperating system are the network operating systemserver and the workstation. The network server isusually a dedicated computer that runs the operatingsystem software and processes all requests forservices. The workstation computer runs theapplication software needed by the workstation userand establishes communications with the networkserver.

The network server operating system consists ofthe following five subsystems: the control kernel,the network interfaces, the file systems, the systemextensions, and the system services.

Control Kernel

The control kernel is the main subsystem of thenetwork operating software. The control kernelcoordinates the various processes of the othersubsystems. Some of the functions performed by thecontrol kernel are as follows:

Optimizing access to services by users

Maintaining status information of many of theprocesses

Error reporting

Service initialization and service terminationof workstations

Network Interfaces

The network interfaces provide the low-levelsubnet protocols and basic translation for bridginghardware drivers with the network operating system.In sophisticated systems, the network interfaces canalso provide for bridging a new network into anoperating network without having to rebuild theoperating system.

File Systems

The file system controls the way the data isorganized, stored, and retrieved from the storagesystems available to the network. The files may bestored on hard drives, RAM disks, or optical storagedevices, such as CD-ROM or write once, read many(WORM) drives.

File systems are generally designed to provideuniversal applicability. This means that the filesystem can be compatible with any applicationprogram’s expectation of file input/output protocol.When adaptable interfaces are used, the file systemcan appear to emulate a number of different filesystems.

System Extensions

The system extensions define the openness of thenetwork operating system and are used by third partydevelopers to produce add-on products. Theextensions are usually high-level protocol handlersthat perform operations, such as file access protocoltranslations required by different operating systems.The extensions available also include networkmanagement, system tools, and data-base services.

System Services

Network system services contain all services thatare not easily defined by any of the other areas of thenetwork. Examples of network services are security,system reliability features, error conditions, andaccess violations.

NETWORK OPERATING SYSTEMSSOFTWARE

The most important job of a network operatingsystem (NOS) is to provide file service for theattached computers. This allows information retrievaland usage and the storage of data in a sharedenvironment. A NOS manages the other resourcesshared by the network and provides the followingfunctions:

Directory structure for shared hard diskstorage devices

File service for sharing and using data

Interface to the network for applicationsoftware/programs

The means by which the network managermanages the network and its users

Network security and data protection

Communications with other networks

The types of network operating systems includefull-featured, low-cost, and zero-slot operatingsystems.

Full-Featured Network Operating Systems

Most full-featured network operating systemsallow for high performance, flexibility, and excellentsecurity measures. They require a LAN administrator.They require network interface cards. Also, they canbe quite costly. Examples of a fill-featured NOS areEtherNet, Novell’s NetWare, 3Com’s 3+Share, IBM’sToken Ring Network, and Banyan’s Vines.

Low-Cost Network Operating Systems

Most low-cost network operating systems differfrom full-featured systems only in the maximumnumber of users accommodated on the network andthe number of security levels incorporated into theoperating system. In general, they are much lower incost and are easier to install and use. Examples of

6-13

low-cost systems are STARLAN, ARCnet, 10Net,and LANtastic.

Zero-Slot Network Operating Systems

Zero-slot network operating systems areappropriate only for networks with very few users andlight usage. They are an inexpensive and simplealternative to the NOSs that require expensivenetwork interface cards. Rather than requiring eachworkstation computer to have a NIC, the computer’sRS-232 serial communications port and twisted-paircables are used. Because of this, they are very slowand offer limited file transfer abilities. They may notprovide disk sharing. An example of a zero-slotsystem is LANLink.

SUMMARY—LOCAL-AREA NETWORKS

This chapter introduced you to local-areanetworks. The following information summarizes theimportant points you should have learned.

LOCAL-AREA NETWORKS— LANs are acombination of hardware and software which allowspersonal computers to share information. The totalnumber of computers and the total distance thenetwork can cover are determined by several factors,including the type of cable used and the networkoperating system software.

CABLES— Several types of cables can be used tocreate a local-area network. They are twisted-pair,shielded twisted-pair, coaxial, and fiber-optic. Thetype of cable used determines maximum data transferrates and can be a factor when the number of nodes inthe network is determined.

NETWORK INTERFACE CARD— Thenetwork interface card attaches the PC to the network.Most network interface cards have built-inmicroprocessors that control networkcommunications. This frees the PC’s main processorof time-consuming I/O operations.

NETWORK SERVERS— The modern networkserver controls all operations of the network. Theseoperations include controlling network

communications, storing and retrieving files fromshared memory resources, and controlling commonprinters. In older systems, each of these functionsrequired a separate server.

WORKSTATIONS— Workstations are thepersonal computers connected to the network. Evenif a PC is part of a network, it can still be used in astand-alone configuration.

OPEN SYSTEM INTERCONNECTIONOSI REFERENCE MODEL— The open systemsinterconnection reference model defines the protocolsnetwork hardware and software manufacturers use tocreate a network operating system. There are sevenlayers in the OSI model. These layers are containedin the five levels.

HARDWARE LEVEL— The hardware levelcontains the first two layers of the OSI referencemodel. These are the physical layer and the data linklayer. The physical layer defines the electrical,mechanical, and functional interfaces for thetransmission of data through the cable. The data linklayer is responsible for error detection and correctionof the transmitted data.

TRANSPORT LEVEL— The next three layers ofthe OSI reference model are contained in the transportlevel, also referred to as the subnet. The three layersof the transport level are the network layer, thetransport layer, and the session layer.

The network layer monitors network activity andcontrols which path the data is to be transmitted over.The software, controlling the network interface card,stores the data to be transmitted, builds the datapackets, and routes the data to the correct destination.

The transport layer ensures the integrity of thedata packets. The session layer provides for theinterface between two or more applications tocommunicate across the network.

PRESENTATION LEVEL/LAYER— Thepresentation level contains the presentation layer. Thepresentation layer formats the data presented to theapplication level. The presentation layer provides

6-14

standardized data encoding, data compression, anddata encryption and decryption as required.

APPLICATION LEVEL/LAYER— The finallevel of the OSI reference model is the applicationlevel and it consists of the application layer. Thislayer directly supports the users and application tasks.

USING THE OSI MODEL— When you use alayered architecture (such as the OSI reference model)to design a communications network, it is possible toupdate specific items in the network without having toreplace the entire system.

LAN TOPOLOGIES— The physicalarrangement of the components of a LAN is called itstopology. The three basic topologies used in buildinga LAN are the linear bus, the star, and the ring.Hybrid topologies can be created by combiningdifferent features of each.

LINEAR BUS— The linear bus topologyconnects all the nodes to a common straight cable.All the nodes on the network share the common bus.This topology is very reliable since a failure of one ormore nodes does not affect the bus. Thedisadvantages of the linear bus are the need forminimum distances between nodes to avoid signalinterference, and the loss of data caused bythe simultaneous transmission by two nodes.

STAR NETWORK— In a star network, eachnode is connected directly to the central computer.All communications between the nodes have to passthrough the central computer. Star networks allow thenetwork administrator to give selected nodes higherpriority and also allow centralized running ofdiagnostic programs.

RING NETWORK— In a ring network, all nodesare connected to a common cable, and the cable startsand ends at the network server. In this type ofnetwork, communications are always in one direction,and the data being transmitted is passed through eachnode in the ring. A major disadvantage of thisnetwork is that when a node fails, it can completelyhalt all communications on the network.

6-15

ACCESS METHODS— Once the topology of anetwork is determined, the method by which thenodes access the network must be determined. Insome cases, the access method is determined by thetopology of the network. Some of the access methodsused in networks are as follows: carrier sensemultiple access (CSMA), carrier sense multiple accesswith collision detection (CSMA/CD), and tokenpassing.

In the CSMA method, each node monitors thenetwork line for activity. When the node detects thatthere is no activity on the network, it will send itsdata. A problem occurs when two or more nodesattempt to use the network at the same time. Thissituation causes a collision of the data packets and apossible loss of data. In the CSMA/CD method, whena collision is detected, each node ceases transmissionand retransmits when it senses that activity on thenetwork is completed.

In token passing, a software token is passed toeach node in an orderly manner. The method issimilar to the Roll Call mode of operation of the Link-11 system described in chapter 2. When the nodewith the token has no data to transmit, it passes thetoken to the next unit. When the node has data, ittransmits it when it receives the token, and when itcompletes its transmission, passes the token to thenext node.

LAN SYSTEMS— There are several types ofsystems that can be installed in a LAN. The topologyused has a major effect on the system the LAN will becapable of using. A few of the LAN systemsavailable are as follows: EtherNet, STARLAN,ARCnet, and the IBM Token Ring. EtherNet isgenerally a linear bus network using the CSMA/CDprotocol for network access. STARLAN is a startopology that also uses the CSMA/CD accessprotocol. ARCnet is a distributed star network thatuses a token passing access protocol. The IBM TokenRing network is a star ring network that uses the tokenpassing access protocol.

NETWORK OPERATING SYSTEMBASICS— The network operating system has fivebasic subsystems to control the operation of the

network. These are the control kernel, the networkinterfaces, the file systems, the system extension, andthe system services.

The control kernel coordinates the variousfunctions and processes of the network. The networkinterfaces provide the low-level subnet protocols forbridging hardware devices with the network operatingsystem. The file systems module controls themethods of organizing, storing, and retrieving data

from the various types of storage systems used by thenetwork. System extensions are defined by thenetwork operating system manufacturers to allowthird party modifications to the operating systems.This allows the network user to customize thenetwork. The network services module contains allthe functions that do not fit in any of the othersubsystems. These include, but are not limited to,system security, system reliability, error conditions,and access violations.

6-16

APPENDIX I

GLOSSARY

This glossary defines abbreviations and acronyms as they are used in this training manual.

ACDS— Advance combat direction system

AFTS— Audio-frequency tone shift

ASCII— American Standard Code for information

interchange

BER— Bit error rate

BFSK— Binary frequency-shift keying

CAINS— Carrier Aircraft Inertial Navigation System

CCA— Circuit card assembly

C2— Command and Control

C2P— Command and Control Processor

CDG— Control/display group

CDS— Combat Direction System

CRT— Cathode-ray tube

CPS— Continuous phase shift

CS— Carrier suppression

CSMA— Carrier sense multiple access

CU— Control unit

dB— Decibel

dBm— Milliwatt reference (600-ohm load)

DDC— Digits!-to-digital converter

DDPG— Digital data processor group

DIV— Diversity

DLRP— Data link reference unit

DPG— Data processing group

DTS— Data Terminal Set

EDAC— Error detection and correction

elf— Extremely low frequency

ehf— Extremely high frequency

EMMSD— Embedded mass memory storage device

EMS— Embedded memory subsystem

ETA— Estimated time of arrival

ETD— Estimated time of departure

FDM— Frequency-division multiplexing

FFT— Fast Fourier transform

FSK— Frequency-shift keying

Ghz— Gigahertz

GRU— Gridlock reference unit

hf— High frequency

HPAG— High power amplifier group

HPIB— Hewlett Packard Interface Bus

Hz— Hertz

ID— Track identification

AI-1

IDA— Input data acknowledge

IDR— Input data request

IEEE— Institute of Electrical and Electronics Engineer

I/O— input/output

ISO— International Standards Organization

JTIDS— Joint Tactical Information Distribution System

JU— JTIDS unit

kHz— Kilohertz

lf— Low frequency

LAN— Local area network

log— Logarithm

LMS— Link monitor system

LSB— Lower sideband

mf— Medium frequency

MHZ— Megahertz

MODEM— MOdulator Demodulator

MCM— Monitor control message

MRM— Monitor reply message

MUF— Maximum useable frequency

NATO— National Alliance Treaty Organization

NIC— Network interface card

NICP— Network interface computer program

NCS— Net control station

NTR— Network time reference

ODR— Output data request

OSI— Open system interconnection

AI-2

POFA— Programmed operational and fictionalappraisal

PIU— Power interface unit

PU— Participating unit

R/T— Receiver/transmitter group

RGB— Red, green, and blue

RCV— Receive

SDLC— Synchronous data link control

SDU— Secure data unit

SNR— Signal-to-noise ratio

SGS— Shipboard Gridlock System

shf— Superhigh frequency

SINS— Ship’s Inertial Navigation System

TADIL— Tactical Data Information Link

TDM— Time-division multiplexing

TDMA— Time-division multiple access

TQ— Track quality

uhf— Ultra-high frequency

USB— Upper sideband

µsec— Microsecond

UTM— Universal test message

vhf— Very high frequency

vlf— Very low frequency

XMT— Transmit

APPENDIX II

REFERENCES USED TO DEVELOPTHE TRAMAN

NOTE: Although the following references were current when thisTRAMAN was published, their continued currency cannot be assured.Therefore, you need to be sure that you are studying the latest revision.

Chapter 1

Black, Uyless D., Data Networks, Concepts, Theory, and Practice,Prentice-Hall, Inc., Englewood Cliffs, NJ, 1989.

Navy Electricity and Electronics Training Series, Module 11, MicrowavePrincipals, NAVEDTRA 172-16-00-84, Naval Education and TrainingProgram Management Support Activity, Pensacola, FL, 1984.

Navy Electricity and Electronics Training Series, Module 17, Radio-FrequencyCommunications Principles, NAVEDTRA 172-17-00-84, Naval Educationand Training Program Management Support Activity, Pensacola, FL, 1984.

Understanding Link-11, A Guidebook for Operators, Technicians, and NetManagers, Navy Center for Tactical Systems Interoperability, San Diego,CA, 1991.

Chapter 2

Data Communication System AN/USC-30, NAVSEA 0967-563-9010,Government Telecommunications Division, Collins Radio Group, RockwellInternational, Dallas, TX, 1975.

Instruction Manual, Data Terminal Set, AN/USQ-59(V)2, SPAWAR 0967-LP-563-9020, Space and Naval Warfare Systems Command, Washington, D. C.,1973.

Link-11 Seminar for Operators and Technicians, Instructor Notes, Link-11Waterfront Seminar, Logicon, Inc., San Diego, CA, 1990.

Operation and Maintenance Instructions, Organizational Level, Link 11 DataTerminal Set AN/USQ-76(V)3, SPAWAR EE640-DW-OMI-01B/E110-USQ76V3, Space and Naval Warfare Systems Command, Washington,D.C., 1990.

System Operation and Maintenance Manual, AN/USQ-74, 74A, Data TerminalSet, SPAWAR EE600-AA-OMI-010, Space and Naval Warfare SystemsCommand, Washington, D. C., 1990.

AII-1

Technical Manual, Operation, and Maintenance with Illustrated PartsBreakdown, Data-Terminal Set Control, C-9063/USQ-59, NAVSEA 0967-LP-563-9050, Naval Sea Systems Command, Washington, D. C., 1977.

Technical Manual, Operation, Maintenance Manual with Illustrated PartsBreakdown, Digital To Analog Converter, CV-2969A(P)/U, NAVSEA 0967-LP-563-9070, Naval Sea Systems Command, Washington, D. C., 1977.

Technical Manual, Installation, Operation, and Maintenance with IllustratedParts Breakdown, Computer Adapter MX-9222/U, NAVSEA 0967-LP-563-9060, Naval Sea Systems Command, Washington, D. C., 1977.

Technical Manual, Operation, Maintenance with Illustrated PartsBreakdown, Address Control - Indicator, C-9062/U, NAVSEA 0967-LP-563-9040, Naval Sea Systems Command, Washington, D. C., 1977.

Understanding Link-11, A Guidebook for Operators, Technicians, and NetManagers, Navy Center for Tactical Systems Interoperability, San Diego,CA, 1991.

Chapter 3

LMS-11 Troubleshooter’s Guide for Link-11 Operations, Logicon, Inc., SanDiego, CA, 1990.

Operator/O-Level Maintenance Training Course, Trainee Guide for the LinkMonitor System, AN/TSQ-162(V)1, Logicon, Inc., San Diego, CA, 1990.

Technical Manual, System Operation and Maintenance Instructions,Organization Level, Link Monitor System, AN/TSQ-162(V)1, SPAWAR EE-190-AB-OMI-010/TSQ-162(V)1, Space and Naval Warfare SystemsCommand, Washington, D. C., 1989.

User’s Manual, Link-11 Monitor System, Rack-mountable (LMS-11R), Logicon,Inc., San Diego, CA, 1990.

Chapter 4

Operation and Maintenance Manual for the Link Monitor System (LMS-4) forLink-4A, Logicon, Inc., San Diego, CA, 1990.

Technical Manual, Volume 1, Digital Data Communications Control Set,AN/SSW-1D(U), NAVSEA 0967-LP-555-4010, Naval Sea SystemsCommand, Washington, D. C., 1973.

Chapter 5

Operating and Service Manual, C-12428/USQ-125 Control Unit, CedarTechnology Inc., Longmont, CO, 1995.

AII-2

Operation and Maintenance Instructions, MK512PV, Link-11 Data Terminal,General Atronics Corp., Philadelphia, PA, 1992.

Preliminary Technical Manual, System Maintenance, Organization Level,AN/UYQ-62(V)1, 2, Command and Control Processor (C2P) Subsystem,SPAWAR EE600-AB-SLM-010, Space and Naval Warfare SystemsCommand, Washington, D. C., 1992.

Understanding Link-16, A Guidebook for New Users, Logicon, Inc., San Diego,CA, 1994.

Chapter 6

Black, Uyless D., Data Networks, Concepts, Theory, and Practice,Prentice-Hall, Inc., Englewood Cliffs, NJ, 1989.

Hancock, Bill, Designing and Implementing Ethernet Networks, QEDInformation Sciences, Inc., Wellesley, MA, 1988.

Heath, Steve, Effective PC Networking, Butterworth-Heinemann Ltd., Oxford,England, 1993.

Woodward, Jeff, The ABC’s of Novell NetWare, Sybex Inc., Alameda, CA,1989.

Durr, Michael, Networking Personal Computers, 3d ed, Que Corp., Carmel, IN,1989.

AII-3

INDEX

A

Access methods, 6-9Arcnet, 6-12Air traffic control, 4-2Amplitude modulation, 1-6Antennas, 2-4Antenna couplers, 2-4AN/USQ-125 Data Terminal Set, 5-1Asynchronous transmission, 1-5Audio tone generation, 2-11Automatic Carrier Landing System, 4-2

B

Building a link message, 2-7Broadcast, 2-7

C

Carrier Aircraft Inertial Navigation System(CAINS), 4-6CDS computer, 2-3CP-2205 (P)(V) USQ-125 Data Terminal, 5-1Command and Control Processor, 5-11

purpose, 5-11system configuration, 5-12

Communications systems, 1-1

D

Data Terminal Set (DTS), 2-3Decibel measurement system, 1-3Digital data communications techniques,

1-5Distributed star network, 6-9

E

Error detection and correction (EDAC),2-10

Ethernet, 6-11Establishing a LINK-11 net, 2-5

FFrequency modulation, 1-6

H

Hamming bits, 2-11

I

IBM Token Ring Network, 6-12Information segment, 2-8Intercept vectoring, 4-2

J

Joint Tactical Information Distribution System(LINK-16), 5-4

architecture, 5-6capabilities, 5-6data exchange, 5-6equipment configuration, 5-8features, 5-4model 5, 5-8nets, 5-5terminal, 5-9transmission protocols, 5-5

L

Landlines, 1-2LANS, 6-1

hardware, 6-2network interface card, 6-2network servers, 6-2systems, 6-11topologies, 6-7workstations, 6-4

Linear bus network, 6-7Link protocol & interface control, 2-12LINK-4A

CDS system, 4-1components, 4-4control messages, 4-3message formats, 4-3overview, 4-1reply message, 4-3

LINK-11, 2-1

INDEX -1

communication switchboard, 2-4controls & indicators, 2-13DTS, 2-9functions, 2-10message formats, 2-8myths & facts, 3-1net operational modes, 2-5overview, 2-2POFAs, 3-3security device, 2-3

LMS-4, 4-6LMS-11, 3-6

accessory group, 3-9carrier suppression, 3-18control/display group, 3-8data processing group, 3-7link monitor mode, 3-9net display, 3-12operation & displays, 3-9

L

LMS-11 (cont)pu display, 3-15spectrum display, 3-17status display, 3-11system configuration, 3-7system initialization, 3-9

M

Message data frames, 2-8Modems, 1-8Modulation/demodulation, 1-6Multibit modulation,Multiplexing, 1-9

N

1-7

Net Control Station (NCS)input/output operations, 2-18modulator/demodulator, 2-18radio set interface, 2-19

Net synchronization, 2-6Net test, 2-6Network Operating Systems

control kernel, 6-12file systems, 6-13network interfaces, 6-13

system extensions, 6-13system services, 6-13

Network operating system

O

software, 6-13

Online & offline system test options, 5-3Open system interconnection reference model, 6-4

application, 6-7hardware level, 6-5presentation level, 6-5transport level, 6-5

P

Phase reference frame, 2-7Preamble, 2-7Programmed Functional and Operational Analysis(POFAs), 3-3

analyzing multistation, 3-5analyzing single station, 3-3multistation, 3-4multistation procedures, 3-5setup, 3-3

Precision course direction, 4-2Protocols, 6-10

R

Radio silence, 2-7Recognizing LINK-11 net problems, 3-18Receive cycle, 2-5Ring network, 6-9Roll call, 2-6

S

Shipboard Gridlock System, 2-3Short broadcast, 2-7Starlan, 6-12Star network, 6-8Start code, 2-8Stop code, 2-8Synchronous transmission, 1-5

T

Transmission cycle, 2-4Types of communication channels, 1-2

INDEX-2

Assignment Questions

Information: The text pages that you are to study areprovided at the beginning of the assignment questions.

ASSIGNMENT 1

Textbook Assignment: “Fundamentals of Data Communications,” chapter 1, pages 1-1 through 1-13;“The Link-11 System,” chapter 2, pages 2-1 through 2-8.

1-1 .

1-2.

1-3.

1-4.

Which of the following are componentsof a communications system?

1. Receivers2. Transmitters3. Communications channels4. All of the above

The conversion of da. to a form that canbe sent over a communications channel isa fiction of which of the followingcommunication system components?

1. The transmitting equipment2. The receiving equipment3. The communications channel4. The computer

Data sent over a communications channelmay be in which of the following forms?

1. Analog only2. Digital only3. Either analog or digital, depending

on the type of system4. Alphanumeric characters

A communications channel signal thatvaries continuously between a minimumand a maximum value is what type ofsignal?

1. Analog2. Digital3. Numeric quantities4. Alpha characters

1-5 .

1-6.

1-7.

1-8.

To convey data, analog signals can bevaried in which of the following ways?

1. Phase only2. Amplitude only3. Frequency only4. Phase, amplitude, or frequency

Which of the following communicationssignal types have a limited set of valuesand

1 .2 .3 .4 .

are transmitted as discrete pulses?

AnalogDigitalBoth 1 and 2 aboveAlphanumeric characters

In which of the following types ofcommunications channels is data in asingle direction ONLY?

1. Simplex2. Duplex3. Half duplex4. Full duplex

Which of the following communicationschannels can transmit and receive datasimultaneously?

1. Simplex2. Duplex3. Half duplex4. Full duplex

1

1-9 . Which of the following types ofcommunications channels transmits datain one direction, pauses, and then receivesdata coming in the opposite direction?

1. Simplex2. Duplex3. Half duplex4. Full duplex

1-10. What device converts digital data signalsto tones and converts tones back to digitaldata signals?

1-11.

1-12.

1. Modem2. Modulator3. Demodulator4. Data converter

The modulator of a modem performswhich of the following functions?

1. It converts data to be transmitted intodiscrete modifications of the tone orcarrier signal

2. It converts data-carrying tones intodigital data

3. It receives digital data from thedemodulator

4. It receives analog data from thecomputer

The operational characteristics of a radiocommunications system are determined bywhat means?

1. Low frequency2. High frequency3. Carrier frequency4. Radio frequency band

1-13.

1-14.

1-15.

1-16.

1-17.

The tactical digital information links thatthe Navy uses generally require which ofthe following radio frequency bands?

1 . L F2 . H F3. UHF4. Both 2 and 3 above

The data signals must be (a)to the carrier signal at the transmitter and

(b) from the carrier signalin the receiver.

1. (a) Modulated (b) modulated2. (a) Modulated (b) demodulated3. (a) Demodulated (b) modulated4. (a) Demodulated (b) demodulated

A radio frequency of 8,090 KHz is inwhich of the following frequency bands?

1 . L F2 . M F3 . H F4 . U H F

Which of the following radio frequencybands is limited to line-of-sightcommunications?

1 . L F2 . M F3 . H F4 . U H F

Decibels are the unit of measure of whatfunction of an amplifier, communicationsequipment, or a system?

1 . G a i n2. Power3. Output4 . I n p u t

2

1-18. The bel most often expresses what valueof any component, circuit, or system?

1. Power2. Input3. Output4. Ratio of input to output power

1-19. In the formula for bel, which of thefollowing symbols would represent theoutput of an amplifier?

1. N2. PI3. P24. Log10

1-20. In the formula for bel, which of thefollowing symbols would represent thegain of an amplifier in bels?

1. N2. P13. P24. Log10

1-21. The decibel is equal to what total numberof bels?

1 . 1/10 bel2 . 1/100 bel3 . 1 bel4 . 10 bels

1-22. A value of 1.5 bels is equal to what totalnumber of decibels?

1 . 15.2 . 153 . 1504 . 1,500

1-23. What is the power ratio of an increase ofa reference signal of 30 dBs?

1. 102. 1003. 1,0004. 10,000

1-24. A power gain of -6 dB from a referencesignal of 1,000 watts results in whatmaximum value of output signal?

1. 250 watts2. 500 watts3. 2,500 watts4. 5,000 watts

1-25. When dBm is measured, which of thefollowing signals is normally thereference signal?

1. Input2. Output3. 1 milliwatt4. 1 millivolt

1-26. Which of the following signal levelsindicates that the output signal is greaterthan the input signal?

1 . 5 dB2 . +3 dB3 . Either 1 or 2 above4 . -2 dB

1-27. A reading of 0 dB indicates that theoutput signal has what relationship to thereference signal?

1. Equal to2. Less than3. Greater than4. Not equal to

3

1-28. A reading of 0 dBm indicates an outputvalue equal to which of the followingsignal values?

1 . 0 dB2. 1 milliwatt3 . 500 watts4. 1,000 watts

1-29. What framed transmission method is usedto transmit ASCII characters?

1. Asynchronous2. Synchronous3. Message framed4. Alphanumeric framed

1-30. What framed transmission method is usedto transmit long streams of uninterrupteddata bits?

1. Asynchronous2. Synchronous3. Character framed4. Alphanumeric framed

1-31. When the asynchronous transmissionmethod is used, each character sent haswhich of the following control bits?

1. Start bit(s) only2. Stop bit(s) only3. Both start and stop bits4. Parity bits

1-32. When the asynchronous transmissionmethod is used, synchronization betweenthe transmitting and the receiving devicesis achieved by which of the followingmethods?

1. Synchronization reference signal inthe preamble

2. Synchronization reference in the stopcodes

3. Character-by-charactersynchronization

4. External timing signals sentconcurrently on a separate line

1-33. Which of the following codes indicatesthe beginning sequence of a synchronousmessage?

1. Sync bits2. Stop code3. Parity code4. Preamble

IN ANSWERING QUESTIONS 1-34 THROUGH1-38. MATCH THE TERM OR STATEMENT INCOLUMN A WITH THE BASIC MODE OFMODULATION LISTED IN COLUMN B.RESPONSES IN COLUMN B MAY BE USEDMORE THAN ONCE.

A. TERMS/STATEMENT B. MODES

1-34. The change in 1. Phasesignal amplitude modulationindicates achange in the 2. Amplitude1 or 0 bits modulationbeing transmitted

3. Frequencymodulation

1-35. Differentialquadraturephase-shiftkeying

4

1-36.

1-37.

1-38.

1-39.

1-40.

1-41.

1-42.

BFSK

AFTS

Interrupting the cycle at adegree point and changing thedirection or amplitude ofthe sine wave

Which of the following modes ofmodulation can be used to modify carriersignals to convey data?

1. Phase modulation2. Frequency modulation3. Amplitude modulation4. All of the above

In which of the following modulationmethods are frequencies above and belowa center frequency used to indicate a logic1 or 0 bit?

1. Amplitude modulation2. Frequency-shift keying3. Audio frequency tone shift4. Differential quadratic phase-shift

keying

Which of the following modulationmethods uses two discrete audio tonesthat are modulated to a constant frequencycarrier signal?

1 . F S K2. BFSK3. AFTS4. Phase-shift keying

When quadrature phase-shift modulationis used, a single tone transmits what totalnumber of binary bits of data for eachphase Shift?

1 . O n e2. Two3. Three4 . F o u r

1-43. When quadrature phase-shift modulationis used, which of the following phaseshifts indicates a binary 00?

1 . +135 degrees2 . –135 degrees3 . +225 degrees4 . –225 degrees

1-44. Which of the following components of amodem converts the data bits into acarrier signal?

1. The transmitter2. The receiver3. Both 1 and 2 above

1-45. Each modem transmitter circuit outputsseveral carrier signals.

1 . T rue2. False

1-46. Which of the following circuits allowsONLY the desired carrier signal to bereceived from the communicationschannel?

1. The demodulator2. The data decoder3. The band pass falter4. The receiver control circuit

1-47. The term baud describes the number ofcharacters per second transmitted over acommunications channel.

1 . T rue2. False

5

1-48.

1-49.

1-50.

1-51.

1-52.

Multiplexing data being transmitted over acommunications channel performs whichof the following functions?

1. It increases the baud rate2. It allows multiple users of the same

channel3. Either 1 and 2 above4. It changes the carrier frequency

Which of the following multiplexingmethods divide(s) the asynchronousmessage into a fixed number of timeslots?

1. Time division2. Frequency division3. Both 1 and 2 above4. Quadrature phase-shift

Which of the following multiplexingmethods transmits several tones over asingle communications channel?

1. Time division2. Frequency division3. Both 1 and 2 above4. Quadrature phase-shift

Link-11 is designated as which of thefollowing types of tactical datainformation link?

1. TADIL A2. TADIL C3. TADIL J4. Teletype

Link-11 communications can operate withwhich of the following radios?

1. HF only2. UHF only3. Either HF or UHF4. VHF only

1-53. When Link-11 is operated with UHFradio, it is capable of over-the-horizoncommunications.

1 . T rue2. False

IN ANSWERING QUESTIONS 1-54 THROUGH1-61, SELECT FROM THE FOLLOWING LISTTHE EQUIPMENT THAT PERFORMS THEFUNCTION DESCRIBED IN THE QUESTION.ITEMS IN THE LIST MAY BE USED MORETHAN ONCE.

A.B.C.D.E .F .

CDS Digital ComputerSGS ComputerCryptographic DeviceData Terminal SetCommunications SwitchboardRadio Set

1-54. Selects the HF or the UHF transceiver.

1 . B2 . C3 . D4 . E

1-55. Encrypts parallel data from the CDScomputer and passes the encrypted data tothe data terminal set.

1 . A2 . B3 . C4 . D

1-56. Correlates reported positions of local andremote tracks.

1 . A2 . B3 . C4 . D

6

1-57. Outputs 24-bit data words to the securityequipment via the SGS computer.

1 . A2 . B3 . C4 . D

1-58. Multiplexes and modulates parallel datainto audio tones.

1 . A2 . B3 . C4 . D

1-59. Receives the audio tone package from thedata terminal set and transmits the tones.

1 . C2 . D3 . E4 . F

1-60. Demodulates the audio tones and checksthe six hamming bits for transmissionerrors.

1 . A2 . B3 . C4 . D

1-61. Decrypts the 24-bit data word and sends itto the CDS computer.

1 . A2 . B3 . C4 . D

1-62. Which of the following functions isperformed by an antenna coupler?

1. Amplification of the HF radio signal2. Impedance matching of the antenna

and the radio set3. Conversion of atmospheric

electromagnetic energy to RF current4. Coupling of the data terminal set to

the radio

1-63. The size of an antenna is determined bywhich of the following factors?

1. The operating power2. The operating frequency3. The range of the receiver4. The type of data being transmitted

1-64. The frequency range of an antenna can beextended by adding which of thefollowing factors?

1. A resistive load only2. A capacitive load only3. An inductive load only4. Either a capacitive or an inductive

load

1-65. Which of the following functions is NOTperformed by the data terminal set?

1. Generating the radio key-line signal2. Converting digital data to audio tones3. Encrypting CDS computer data4. Converting audio tones to digital data

1-66. The data terminal set communicates withthe radio set via which of the followingdevices?

1. The communications switchboard2. An antenna coupler3. The cryptographic device4. The SGS computer

7

1-67. Which of the following radio frequencymodulation methods is used to minimizepropagation-caused signal loss during HFLink-11 operations?

1. Quadrature phase-shift modulation2. Frequency modulation3. Phase modulation4. Amplitude modulation independent

sideband

1-68. Which of the following individuals isresponsible for assigning primary andsecondary Link-11 frequencies before thedeployment of a task force?

1. The aircraft carrier commandingoff icer

2. The net control station tracksupervisor

3. The task force commander4. The fleet CinC

1-69. When a Link-11 net is established, whichof the following sequences of operationsshould be followed to determine readinessof all units to enter the net?

1. Net Test, Net Sync, roll call2. Net Sync, Net Test, roll call3. Net Sync, Net Test, Broadcast4. Roll call, Net Test, Net Sync

IN ANSWERING QUESTIONS 1-70 THROUGH1-75, SELECT FROM THE FOLLOWING LISTTHE OPERATING MODE DESCRIBED IN THEQUESTION.

A. Net SynchronizationB. Net TestC. Roll CallD. BroadcastE. Short BroadcastF. Radio Silence

1-70.

1-71.

1-72.

1-73.

1-74.

What Link-11 operating mode establishesa uniform time base from which all netcommunications are normally initiated?

1 . A2 . B3 . C4 . D

In what Link-11 operating mode is eachpicket unit interrogated, in turn, by NCS?

1 . A2 . B3 . C4 . D

What Link-11 operating mode provides anoverall evaluation of net and equipmentperformance?

1 . B2 . C3 . D4 . E

In what Link-11 operating mode will oneparticipating unit transmit datacontinuously to all other net members?

1 . C2 . D3 . E4 . F

In what Link-11 operating mode are theradio set key line and data terminal audiooutput inhibited?

1 . C2 . D3 . E4 . F

8

1-75. In what Link-11 operating mode is asingle data transmission sent only whenthe operator depresses the TRANSMITSTART button?

1 . B2 . C3 . D4 . E

9

ASSIGNMENT 2

Textbook Assignment: “The Link-11 System,” chapter 2, pages 2-7 through 2-23; “Link-11 FaultIsolation,” chapter 3, pages 3-1 through 3-7.

2-1 .

2-2 .

2-3 .

2-4 .

Net Sync continuously broadcasts whichof the following signals?

1. Phase reference frames2. Start codes3. Stop code4. Preamble frames

When the stored sync mode is operating,the picket station uses which of thefollowing signals to establish a time base?

1 .2 .

3 .4 .

An external frequency standardThe internal frequency standard in theDTSThe sync signal received from NCSThe sync signal received form anotherpicket

Which of the following functions is NOTtested when Net Test is running?

1. DTS to radio interface2. CDS computer to DTS interface3. Radio to antenna interface4. Radio receiver function

The preamble of a Link-11 messageconsists of a total of how many frames?

1. Five2. Six3. Seven4. Eight

2-5 .

2-6 .

2-7 .

During transmission of the preamble, the605 Hz tone is transmitted at which of thefollowing power levels?

1. –6 dB2. +6 dB3. –12 dB4. +12 dB

To enable the DTS to detect frametransitions during the preamble, the 2,915HZ sync tone is phase shifted how manydegrees at each frame?

1. 902 . 1 8 03 . 2 7 04 . 3 6 0

The phase reference frame of a Link-11header performs which of the followingfunctions?

1. Provides synchronization betweenthe DTS and the CDS computer

2. Provides a time reference for theDTS

3. Provides a time reference for theradio

4. Provides the reference to extractthe data in the next frame

10

2-8 . During a receive cycle, the start codecauses which of the following actions?

1 . The CDS computer to send theprepare to receive data interrupt

2 . The DTS to send the prepare toreceive data interrupt

3 . The CDS computer to send theprepare to transmit data externalfunction

4 . The DTS to send the prepare totransmit data interrupt

2-9 . Exactly how many data bits arecontained in each Link-11 informationframe?

1 . 182 . 243 . 304 . 32

2-10. The control stop code is generated bywhich of the following units?

1. NCS only2. Picket station only3. Either NCS or a picket station,

indicating the end of a controlm e s s a g e

2-11. During a receive cycle, the stop code(control or picket) causes which of thefollowing actions?

1 . The CDS computer to send the endof data interrupt

2 . The DTS to send the end of dataexternal function

3 . The CDS computer to send the endof receive interrupt

4 . The DTS to send the end ofreceive interrupt

2-12. A call-up, or interrogation messageconsists of a total of how many frames?

1 . Five2 . Six3 . Seven4 . Eight

2-13. The two frames following a controlstop code indicate which of thefollowing?

1 . The address of the NCS2 . The next picket address in the roll

call3 . The end of the NCS message4 . The last address interrogated

2-14. A picket reply message is sent in whichof the following sequences?

1 . Preamble, phase reference, data,stop code

2 . Preamble, phase reference, startcode, data, control stop code

3 . Preamble, phase reference, startcode, data, picket stop code

4 . Phase reference, preamble, startcode, data, stop code

2-15. The DTS operates in full duplex whenit performs which of the followingoperations?

1. System testing2. Net Test3. Normal operations4. Short Broadcast

2-16. The DTS performs which of thefollowing functions?

1. Data encryption2. Error detection and correction3. Track gridlock4. Transmitting data tones on a

carrier frequency

11

2-17. The six hamming bits added to the dataword enables the DTS to correct whatmaximum number of data bits?

1 . O n e2 . T w o3. Three4 . F o u r

2-18. The DTS is operating in the detect andcorrect mode. A data word is receivedby the CDS computer with bit 24=0,and bit 25=1. Which of the followingconditions is indicated by this bitcombination?

1. No errors detected2. Parity error(s) detected3. Odd bit error(s) detected,

correction attempted4. Even errors detected, no correction

attempted

2-19. The DTS develops a composite signalconsisting of what total number offrequency division multiplexedaudio-frequency tones?

1 . 22 . 113 . 164. 30

2-20. Bits 4 and 5 of a 30-bit data word arecarried by which of the following audiotone frequencies?

1 . 935 Hz2 . 1,155 Hz3 . 1,265 Hz4. 1705 Hz

2-21. What is the basic unit of the Link-11transmission?

1 . B i t2. Tone3. Frame4. Doppler

2-22. In a single frame, the DTS can toleratewhat maximum phase shift errorwithout generating an error code?

1 . + 44 degrees only2 . – 44 degrees only3 . ± 44 degrees4 . ±135 degrees

2-23. A phase-shift error of +105 degrees inany one of the data tones will cause asingle bit to be erroneous.

1 . T rue2. False

2-24. Which of the following Link-11 signalsallows the receiving unit to correcterrors caused by the relative motionbetween the sending and receivingunits?

1. Sync tone2. Doppler tone3. Data carrying tones4. Motion correct tone

2-25. During the preamble, the 2,915 Hz tonesets which of the following references?

1 . Frame timing when the DTS is incorrected timing

2 . Signal power levels when the DTSis in corrected timing

3 . Frame timing when the DTS is instored timing

4 . Signal power levels when the DTSis in stored timing

12

2-26. During the reception of the datasegment of a Link-11 message, the 605Hz Doppler tone should be at which ofthe following power levels?

1 . +12 dB2 . + 6 dB3 . – 6 dB4 . 0 dB

2-27. When the DTS is in the OPERATEmode, exactly how many fault-sensingsensors can cause the SUMMARYFAULT lamp to light?

1 . 1 12 . 1 43 . 2 34 . 2 7

2-28. The LAMP TEST switch on the ModeControl Panel will cause all of which ofthe following lamps to light?

1. Those on the mode control panelonly

2. Those on the TADIL A controlpanel only

3. Those on the address control unitonly

4. Those on the mode control panel,the TADIL A control panel, andthe address control unit

2-29. When theFULL-DUPLEX/HALF-DUPLEXswitch is in the FULL-DUPLEXPOSITION, the transmit sidetone isprocessed for input to the computer.

1 . True2 . False

2-30. When the SIDEBAND SELECT switchis in the DIV position, which sidebandsignal is processed for input to thecomputer?

1. USB only2. LSB only3. The combination of the USB and

LSB signals4. Either the USB or the LSB,

depending on the signal quality ofeach sideband

2-31. During normal Link-11 operations, theDATA RATE switch on the modecontrol panel should be in which of thefollowing positions?

1 . 1,200 bps2 . 2,400 bps3 . DUAL 1,200 bps4 . TADIL A

2-32. The SYNC MODE switch on the modecontrol panel is used in conjunctionwith which of the following switches onthe TADIL A control panel?

1. OPERATE/RADIO SILENCEswitch

2. NET CONTROL/PICKET switch3. TIMING STORED/CORRECTED

switch4. ERROR CORRECT/LABEL

switch

13

2-33. When the SYNC MODE switch isplaced in the FAST position,synchronization is obtained by which ofthe

1 .

2 .

3 .

4 .

following methods?

Use of the frame timing referencestored during Net SyncUse of the frame timing referenceobtained from the preamble of thecurrent message onlyUse of the frame timing referenceobtained at each data frame of thecurrent message onlyUse of both the frame timing

2-34.

2-35.

reference obtained during thepreamble and the frame timing ofeach frame of the current message

The NET BUSY indicator of theTADIL A control panel is activated bywhich of the following signals?

1. Signal presence2. Receive mode3. Transmit mode4. Start code detected

When the SYNC COMPT indicator islighted after the DTS has achievedwhich of the following conditions?

1 . It is in sync with the radio2 . It is using stored synchronization

signals3 . It is in sync with NCS4 . It is testing the internal sync

circuits

2-36. The TIMING/STORED/CORRECTEDswitch is set to the STORED position.Which of the following signals will theDTS use for frame timingsynchronization?

1 .

2 .

3 .

4 .

The frame timing reference storedduring Net SyncThe frame timing referenceobtained from the preamble of thecurrent message onlyThe frame timing referenceobtained at each data frame of thecurrent message onlyBoth the frame timing referenceobtained during the preamble andthe frame timing of each frame ofthe current message

2-37. When the ERROR CORRECT/LABELswitch is in the CORRECT position, theDTS is capable of performing which ofthe following operations?

1. Detecting and correcting an evennumber of bit errors in thereceived data word

2. Detecting and correcting an oddnumber of multiple bit errors inthe received data word

3. Detecting and correcting a singlebit error in each received dataw o r d

4. Detecting and correcting a singlebit error in the received datamessage

14

2-38. When you depress the TRANSMITRESET switch on the TADIL A controlpanel, it causes the DTS to performwhich of the following operations?

1 .

2 .

3 .4 .

To immediately stop alltransmissionsTo inhibit the generation of outputdata requests, generating a stopcode and ending the currenttransmissionTo place the radio in radio silenceTo inhibit the generation of inputdata requests, generating a stopcode and ending the currentreception

2-39. The DTS is configured as a picketstation in roll call mode. When youdepress the TRANSMIT INITIATEswitch on the TADIL A control panel,it will cause the DTS to perform whichof the following operations, if any?

1. To immediately transmit the data2. To allow the unit to enter the net3. To assume control of the net as

NCS4. None

2-40. On the NCS platform, the MISS CALLindicator on the TADIL A panel willlight when a picket fails to respond totwo successive interrogations.

1 . True2 . False

2-41. The address entered into the OWNSTATION ADDRESS switches performwhich of the following DTS functions?

1. Transmits the entered address to allother members of the net

2. Transmits the entered address toNCS only

3. Receives messages that match theentered address

4. Transmits tactical data when theinterrogation message addressmatches the entered address

2-42. On the NCS platform operating in aLink-11 net where the units areapproximately 100 miles apart, whichof the following values should beentered into the RANGE IN MILESswitches?

1 . 0 miles2 . 25 miles3 . 50 miles4 . 100 miles

2-43. With a single address control indicator,an NCS platform can control whatmaximum number of participatingunits?

1. 52 . 1 03 . 1 54 . 2 0

2-44. Data exchange between the Link-11DTS and the CDS computer iscontrolled by the DTS using which ofthe

1 .2 .3 .4 .

following control signal protocols?

External interruptsExternal functionsInput data requestsOutput data requests

15

2-45. During a receive data cycle, the DTSperforms which of the following actionswhen frame two of the stop code isdetected?

1 .

2 .

3 .

4 .

The frame is processed as a dataframe and sent to the computerThe DTS processes the stop codeand resets itself onlyThe DTS sends the end of receiveexternal interrupt to the computerThe computer processes the stopcode and closes the input databuffer

2-46. Which of the following actions isperformed by the DTS when a controlstation stop code is received?

1 .

2 .

3 .

4 .

The DTS compares the next twoframes received with the ownstation addressThe DTS resets all I/O timingcircuitsThe DTS sends the next twoframes received to the CDScomputerThe DTS sends the prepare totransmit data interrupt to thecomputer

2-47. When the DTS recognizes own stationaddress, it transmits which of thefollowing signals first?

1 .2 .3 .4 .

Prepare to transmit interruptThe first frame of the preambleThe phase reference frameInput data request

2-48. At the start of a transmit cycle, theoutput data request is first set activeduring which of the following frames?

1. The first frame of the preamble2. The first frame of the start code3. The phase-reference frame4. The second frame of the start code

2-49. Which of the following eventsplace when the CDS computeranswer an ODR from the DTSthe specified time limit?

takesdoes notwithin

1 . The DTS generates the stop code2 . The DTS hangs-up3 . The computer generates an

external function to clear the DTS4 . The computer sends a stop code to

the DTS

2-50. Which of the following events occur ifan interrogated picket station does notanswer an initial interrogations from theNCS within 15 frame intervals?

1. NCS interrogates the next station2. NCS waits another 15 frame

intervals3. Link-11 network hangs up4. NCS retransmits the interrogation

to the unit that did not reply

2-51. A total of how many data tones are inthe composite tone package developedby the DTS?

1 . 132 . 143 . 154 . 16

16

2-52.

2-53.

2-54.

2-55.

The intelligence (data bits) in a datatone is stored by which of the followingmethods?

1 . Phase shifting the tone by apredetermined amount with respectto the following frame

2 . Phase shifting the tone by apredetermined amount with respectto zero degrees

3 . Phase shifting the tone by apredetermined amount with respectto the preceding frame

4 . Increasing or decreasing theamplitude of the data tone withrespect to the preceding frame

During receive operations, exactly howmany EDAC bits are extracted from thereceived data tones?

1 . O n e2 . T w o3. Five4 . S i x

The EDAC bits enable the DTS tocorrect a total of how many received biterrors?

1 . O n e2 . T w o3. Three4 . F o u r

When operating the Link-11 with aUHF radio set, you should place theSIDEBAND SELECT switch in whatposition only?

1 . LSB2 . USB3 . DIV4 . AUTO

2-56. With Link-11 transmitting on the HFrange and the sideband select switch setto the AUTO position, which of thefollowing priorities are used by theDTS to find the data word with noerrors to send to the computer?

1 . LSB, USB, DIV2 . LSB, DIV, USB3 . DIV, LSB, USB4 . DIV, USB, LSB

2-57. The Link-11 radio set is in the transmitmode when the key line is clear.

1 . T rue2. False

2-58. Which of the following conditionswould NOT be a valid reason forchanging the unit functioning as NCS ina Link-11 net to improve netcommunications?

1 . The current NCS has one PUaddress entered wrong

2 . The current NCS has poor receiversensitivity and is polling on top ofPU responses

3 . Several PUS are in a propagationshadow

4 . Several PUs are out of range ofthe current NCS unit

2-59. Changing frequencies will always solveLink-11 problems.

1 . True2 . False

17

2-60. When you keeping the radio set tunedto output maximum power, it causeswhich of the following problems?

1 .

2 .

3 .

4 .

Increases RFI/EMI on thetransmitting unit onlyIncreases receive data errors onreceiving units by saturating thedata terminal sets onlyIncreases RFI/EMI on thetransmitting unit and increasesreceive data errors by DTSsaturationDecreases RFI/EMI on thereceiving units

2-61. When the NCS enters dummy PUs,which of the following net conditionswill exist, if any?

1. Net efficiency increases2. Netcycle time decreases3. Netcycle time increases4. None, dummy PUs have no effect

on the net

2-62. What following NCS action is the mosteffective net management techniquewhen a PU is having troublemaintaining Link-11 communications?

1 .

2 .

3 .

4 .

Continuing normal Link-11operations while the trouble PUDirecting the PU to go to radiosilence so that the PU does notrespond to interrogationsDirecting all units to change froma HF frequency to an UHFfrequencyRemoving the troubled PU fromthe polling sequence until theproblem is corrected and thetroubled PU is ready to reenter thenet

2-63. When you set up the DTS to run asingle-station POFA, the DTS must beconfigured to operate in which of thefollowing modes?

1. Simplex2. Half duplex3. Full duplex4. POFA TEST mode

2-64. When you run a single-station POFAwith the radio set which of thefollowing equipments is/are NOTchecked?

1. Security device I/O path2. Antenna coupler3. DTS-to-radio audio lines4. Radio-to-DTS audio lines

2-65. Running a single-station POFA canassist the technician in isolating aproblem in which of the followingsections of the DTS?

1. Receive timing2. Doppler correction3. DTS to antenna interface4. Transmit timing

2-66. A single-station POFA should print theinterrupts in what sequence?

1. End of receive, prepare to transmit,prepare to receive

2. Prepare to receive, prepare totransmit, end of receive

3. Prepare to transmit, end of receive,prepare to receive

4. Prepare to transmit, prepare toreceive, end of receive

18

2-67. A single-station POFA error printoutthat lists bit errors that are less than 10percent of the total words transmitted isconsidered a successful POFA.

1 . True2 . False

2-68. A single broken line in the switchboardbetween the DTS and the crypto devicecould cause which of the followingproblems?

1. One bit always set to a logic “1”2. One bit always set to a logic “0”3. All bits randomly set to a logic “1”4. All bits randomly set to a logic “1”

or a logic “0”

2-69. The multi-station POFA is run in whichof the following modes?

1 . Net test2 . Roll call3 . Broadcast4 . Short broadcast

2-70. When a multi-station POFA is run,what total number of data words are ineach block of data transmitted?

1 . 1152 . 2303 . 3454 . 460

2-71. When a multi-station POFA is run, allunits participating in the test should bepositioned within how many miles ofeach other?

2-72. A multi-station POFA should be runusing which of the followingfrequencies?

1. Any HF frequency2. Any UHF frequency3. The current operational frequency

only4. The current secondary frequency

only

2-73. The multi-station POFA should be runfor what minimum amount of time?

1 . 5 minutes2 . 7 minutes3 . 10 minutes4 . 15 minutes

2-74. At the completion of the multi-stationPOFA, the technician should recordwhich of the following information onthe error printout?

1. Distance and bearing of all PUS2. Frequency used3. Start and stop time of the POFA4. All of the above

2-75. A multi-station POFA should beconsidered successful when the linkquality factor is which of the followingvalues?

1. Greater than 90 percent2. Greater than 100 percent3. Greater than 95 percent but less

than 100 percent4. It must equal 100 percent

1 . 252 . 503 . 754 . 100

19

ASSIGNMENT 3

Textbook Assignment: “Link-11 Fault Isolation,” chapter 3, page 3-7 through 3-22, “Link-4A,” chapter 4,page 4-1 through page 4-10.

3-1 .

3-2 .

3-3 .

A multi-station POFA is consideredsuccessful when the computed receiveerror factor is less than what percentage?

1. 12. 53. 104. 15

Which of the following buffers isacceptable to be received from anunrecognized station (UNREC STA)when a multi-station POFA is running?

1. One buffer of 200 words2. One buffer of 230 words3. Two buffers of 200 words4. Two buffers of 230 words

The heading PARITY STATUS OFCORRECT WORDS lists which of thefollowing error conditions?

1 .

2 .

3 .

4 .

Words determined to be correct by theDTS parity check and found in errorby the computer parity checkWords determined to have an errorand corrected by the DTSWords determined to be correct byboth the DTS and the computer paritychecksWords determined to be in error bythe DTS parity check and foundcorrect by the computer parity check

20

3-4 .

3-5 .

3-6 .

3-7 .

The LMS-11 is designed to performwhich of the following functions?

1 .

2 .

3 .

4 .

Isolate problems to the componentlevel in the Link-11 systemProvide a history of Link-11 netperformanceProvide a real-time visual display ofthe Link-11 net operationsProvide the technician with a visualdisplay of the Link-11 operation ofone unit

The LMS-11 is a complex systemdesigned to be permanently installed.

1. True2. False

The DPG and CDG equipment cases ofthe LMS-11 provide isolation from whichof the following environmental forces?

1 .2 .3 .4 .

Shock onlyVibration onlyShock and vibrationPower surges

The LMS-11 system printer is part ofwhich of the following equipmentgroups?

1. Accessory2. Control/display3. Data processing4. Support

3-8 .

3-9 .

3-10.

3-11.

Which of the following components isNOT part of the LMS-11 data processinggroup?

1. Control processing unit2. Dual 5.25-inch disk drive unit3. Audio interface unit4. Power control unit

The HP9920U computer of the LMS-11contains what additional amount ofrandom access memory (RAM)?

1. 2 MB2. 4 MB3. 8 MB4. 12 MB

The data communications interface of theLMS-11 provides which of the followingfunctions?

1. Parallel synchronous interface withthe Link-11 data terminal

2. Parallel asynchronous interface withthe printer

3. Serial synchronous interface with theLink-11 data terminal

4. Serial asynchronous interface with theprinter

The audio interface unit of the LMS-11connects which of the following signals?

1. USB from HF radios only2. LSB from HF radios only3. USB from UHF radios only4. USB and LSB from HF radios and

USB from UHF radios

3-12. To determine the phase shift of theLink-11 audio tones the LMS-11 useswhich of the following methods?

1. Fast Fourier Transform formula2. Comparison of the raw analog signal

with the preceding frame3. Phase shift signal from the DTS4. Digital data from the crypto device

3-13.

3-14.

3-15.

3-16.

The control/display group consists ofwhich of the following units?

1. Composite video and color outputcircuit card assemblies

2. Data communications interface andHP interface bus

3. Color monitor and keyboard4. Color printer and interconnecting

cables

The color display monitor of the LMS-11uses composite video when it performswhich of the following operations?

1. Normal LMS-11 operations2. Start-up operations only3. Testing operations only4. Both start-up and testing operations

The functional keys on the LMS-11keyboard are color-coded to facilitateoperator selections and entries.

1. True2. False

During initialization of the LMS-11,which of the following operator entries isNOT a required entry?

1. Date and time2. Own-ship PU number3. Net mode4. Data rate

21

3-17.

3-18.

3-19.

3-20.

When initializing the LMS-11, theoperator enters 127 frames as theCALL-TIMEOUT value. What type oflink-11 operation is indicated by thisentry?

1. NET TEST2. Normal roll call mode3. Satellite link operation4. NET SYNC

The link monitor mode of the LMS-11displays what information?

1. Quantitative information concerningthe operation of a maximum of 21PUs

2. Detailed characteristics of the receivedsignal from a specified PU

3. Real-time link activity4. A graphic representation of the power

levels of the received Link-11 tonesfrom a single PU

Which of the following function keys onthe LMS-11 keyboard would the operatordepress to select the link monitor mode?

1. LM2. NET3. SPECT4. PU

A PU address of 77 is used by theLMS-11 to indicate which of thefollowing units, if any?

1. PU 772. NCS3. Task force flagship4. None, 77 is an illegal address

IN ANSWERING QUESTIONS 3-21 THROUGH3-25, REFER TO FIGURE 3-7 IN THE TEXT.

3-21. Which of the following colors indicatesthe preamble on the link monitor screenof the LMS-11?

1. Yellow2. Red3. Cyan4. Green

3-22. A small cyan line in the middle of thedata field indicates the LMS-11 hasdetected which of the following signals?

1. Start code2. Stop code3. Phase-reference frame4. Noise

3-23. Which of the following displays is usedto indicate the LMS-11 is listening?

1. A thick blue line2. A thin blue line3. A thick red line4. A thin red line

3-24. The phase-reference frame is displayed onthe link monitor display in what manner?

1 .

2 .

3 .

4 .

As a small green line between thestart code and the dataAs a small red line between the startcode and the dataAs a small green line between thepreamble and the start codeAs a small red line between thepreamble and the start code

3-25. Data frames are represented on the linkmonitor display by which of thefollowing colors?

1. Yellow2. Red3. Cyan4. Green

22

3-26. An NCS report can be easily identified onthe link monitor display by which of thefollowing features?

1. A two digit address at the end of areport only

2. No call-up message between the endof the preceding report and the NCSreport only

3. Both a two digit address at the end ofthe report and no call up messagebetween the preceding report and theNCS report

4. A call message to the NCS address

3-27. An address shown in red on theXMT-ADDRS line of the status displayindicates which of the followingconditions?

1. The addressed unit is beinginterrogated

2. The addressed unit failed to answertwo interrogations

3. The addressed unit has replied to aninterrogation

4. The unit indicated is the next addressin the polling sequence

3-28. When the LMS-11 is operating in the linkmonitor mode and a phase referenceframe is detected which, of the followingstatus indicators is active?

1. CC12. PHA3. PRE4. EOT

3-29. When the LMS-11 is operating in the linkmonitor mode and the first frame of apicket stop code is detected, which of thefollowing status indicators is active?

1. CC12. PHA3. PRE4. EOT

3-30.

3-31.

3-32.

3-33.

In the status box, the number in the%DATA indicates which of the followingquantities?

1 .

2 .

3 .

4 .

The total percentage of data in the lastmessage receivedThe percentage of data that is errorfree in the last message receivedThe total percentage of data receivedduring the most recent net cycleThe percentage of data that is errorfree received during the most recentnet cycle

The net cycle time that is displayed in theLMS-11 status box indicates which of thefollowing time cycles?

1. Start code to start code of NCS only2. Control stop to control stop of NCS

only3. Control stop to control stop of an

operator selected PU only4. Control stop to control stop from

either NCS (default) or an operatorselected PU

The Net Display mode of the LMS-11 iscapable of displaying two separate typesof information.

1. True2. False

In the summarize mode of the netdisplay, a summary of quantitativeinformation is displayed for a maximumof how many PUs?

1. 102. 113. 204. 21

23

3-34.

3-35.

3-36.

3-37.

The net display mode can only bedisplayed when Link-11 is operating inwhich of the following modes?

1. Net test2. Net sync3. Roll call4. Short broadcast

The PU field in the header of the NetDisplay screen of the LMS-11 is used todefine which of the following units, ifany, while in the summarize mode?

1. NCS2. Own station PU number3. The PU whose recurring transmission

is used to define a cycle4. None, the PU field is only used

during the History mode

To change the Net Display screen fromthe summarize mode to the PU historymode, the operator would take which ofthe following actions?

1. Depress the history mode key on thekeyboard

2. Enter a zero into the SUMMARIZEfield of the Net Display header

3. Enter the PU number of the unit to bemonitored in the PU field

4. Both 2 and 3 above

On the Net Display screen, which of thefollowing values indicates anunacceptable SNR?

1. 5 dB2. 15 dB3. 25 dB

3-38. When the LMS-11 is in the Net Display(summarize) mode, which of thefollowing conditions will cause theFRAME CNT value of a picket to bedisplayed in yellow and followed by a“?”?

1. The average number of frames pertransmission exceeds seven

2. The average number of frames pertransmission is six or less

3. The average signal to noise level pertransmission exceeds 20 dB

4. The average signal to noise level pertransmission is less than 20 dB

3-39. The %THRU column of the Net Displayscreen displays which of the followingvalues?

1. The percentage of data received bythe listed PU that is error-free

2. The percentage of message dataframes received by the LMS-11 thatcontain errors

3. The percentage of message dataframes received by the LMS-11 thatare error-free

4. The percentage of control code framesreceived

3-40. Which of the following power levelslisted in the REL 605 column of the NetDisplay screen indicates normal operationof the link?

1. -3 dB2. –6 dB3. +3 dB4. +6 dB

4. 34 dB

24

3-41.

3-42.

Using TADIL A specifications, what isthe maximum allowable variation ofpower in the Link-11 data tones?

1. 1.0 dB2. 1.5 dB3. 3.0 dB4. 4.0 dB

The PU display of the LMS-11 presents agraphic representation of the relativepower and phase error of the Link-11signal received from a specified unit.

1. True2. False

3-43.

3-44.

What information is displayed on therelative power bar graph of the PUdisplay?

1.2.

3.

4.

The power of the 605 Hz tone onlyThe relative power of the data toneswith respect to the 605 Hz toneThe relative power of each data tonewith respect to the average power ofall the data tonesThe relative power of each data tonewith respect to an internal standard

When the relative power bar graph isread, a data tone that is +2 dB greaterthan the average will be displayed inwhich of the following colors?

1. Cyan2. Green3. Yellow

3-45. The phase error bar graph of the LMS-11display what information about theLink-11 signal?

1. Relative power2. Mean deviation of the phase error

only3. Standard deviation of the phase error

only4. Both the mean and standard

deviations of the phase error

3-46. A standard phase deviation of 15 degreesfor a data tone will be represented on thebar graph in which of the followingcolors?

1. Cyan2. Green3. Yellow4. Red

3-47. The LMS-11 will indicate that the datareceived is bad if the standard deviationfalls in which of the following ranges?

1. A positive value less than 45 degrees2. A positive value greater than 45

degrees3. A negative value less than 45 degrees4. Both 2 and 3 above

3-48. The incidence of bit errors will increaseas the signal-to-noise ratio increases.

1. True2. False

4. Red

25

3-49. Which of the following tones aregraphically displayed by the LMS-11Spectrum Display?

1. 30 tones that are the odd harmonics of55 Hz

2. 30 tones that are the even harmonicsof 55 Hz

3. The 605 Hz tone and the noise toneonly

4. The 15 data tones only

3-50. Under ideal conditions, at what levelshould the data tones be displayed on thebar graph of the spectrum display?

1 . 0 dB2 . +6 dB3 . -6 dB4 . +4 dB

3-51.

3-52.

When the operator enters PU 00 into thePU address field of the spectrum displaywhat effect, if any, will it have on theoperation of the LMS-11?

1. 00 is an illegal address; therefore, nodata will be displayed

2. The LMS-11 will continuouslyupdate the display for each unit inthe net

3. The LMS-11 will update the displayfor NCS only

4. No effect, the LMS-11 will continueto update the last legal addresse n t e r e d

Carrier suppression can only be testedwhen the Link-11 system is operating inwhich of the following modes?

1. Net Test2. Net Sync3. Roll call4. Broadcast

26

3-53. When reading the LMS-11 spectrum

3-54.

3-55.

3-56.

display, the technician notices that onlythe 605 Hz tone and the 2195 Hz tonesare displayed. Which of the followingsetup entries would cause this display?

1. The RESTRICT field set to preambleo n l y

2. The RESTRICT field set to data only3. The PU field is set to a unit not in

the ne t4. The SIDEBAND SELECT is set to

USB only

Link-4A is what type of tactical digitalinformation link?

1. Ship-to-aircraft2. Ship-to-submarine3. Ship-to-shore4. Ship-to-ship

Link-4A is used to transmit which of thefollowing types of information?

1. High-speed computer-to-computertactical information

2. Tactical information from a CDSship to a non-CDS ship

3. Aircraft control and targetinformation

4. All of the above

Link-4A data is transmitted by usingwhich of the following methods?

1. Frequency-shift keying2. Phase-shift keying3. Audio frequency tone shift4. Quadrature differential phase-shift

keying

3-57. What is the maximum number of aircraftthat can be controlled by s single Link-4Acontrolling station?

1 . 252 . 503 . 754 . 100

3-58. Aircraft control messages from theLink-4A controlling station are developedby the CDS computer using which of thefollowing types of information?

1. Radar-derived target data2. Reply data from aircraft3. Other sources of tactical information4. All of the above

3-59. Link-4A uses which of the followingfrequency bands for data exchange?

1. HF only2. UHF only3. VHF only4. Both UHF and VHF

3-60. In which of the following Link-4A modesof operation is an aircraft directed to aspecific location to be at an optimumposition for an attack?

1. Precision course direction2. Automatic carrier landing3. Air traffic control4. Intercept vectoring

system

3-61. In which of the following Link-4A modesof operation is used to maintain safeflight patterns and assigns priority forlanding approach?

1. Precision course direction2. Automatic carrier landing system3. Air traffic control4. Intercept vectoring

3-62. Which of the following Link-4A modesof operation is used for the remoteguidance of bombers, reconnaissanceaircraft, and drones?

1. Precision course direction2. Automatic carrier landing system3. Air traffic control4. Intercept vectoring

3-63. In which of the following Link-4A modesof operation is used to land an aircraft onthe flight deck of a carrier?

1. Precision course direction2. Automatic carrier landing system3. Air traffic control4. Intercept vectoring

3-64. The CAINS aircraft alignment data loadedinto the navigation computer of theaircraft consists of which of the followingtypes of data?

1. The latitude and longitude of theship only

2. The ship’s velocity only3. The latitude, longitude, and ship’s

velocity4. The waypoint data

3-65. The CAINS alignment and waypoint datais initially loaded into the aircraft usingwhich of the following methods?

1. Hard-wired deck edge outlet boxesonly

2. UHF RF transmission only3. Either hard-wired deck edge outlet

boxes or UHF RF transmission4. HF transmission only

27

3-66. The standard CDS control message is (a) milliseconds in duration, whilethe reply message is (b)milliseconds in duration.

1. (a) 2 (b) 142. (a) 2 (b) 183. (a) 14 (b) 144. (a) 14 (b) 18

3-67. The CAINS receive cycle duration isequal to what total number ofmilliseconds?

1 . 22 . 43 . 144 . 18

3-68. The transmit frame is divided into whattotal number of 200 µsec time slots?

1 . 1 32 . 5 63 . 7 04 . 2 0 0

3-69. What total number of time slots make upthe sync preamble of each transmit frame?

1 . 1 32 . 4 23 . 5 64 . 7 0

3-70. What total number of transmit frame timeslots contain message data bits?

1 . 82 . 133 564. 70

28

3-71.

3-72.

3-73.

3-74.

3-75.

Which of the following transit framesignals causes the transmitter to turn offand starts the receive cycle?

1. Stop pulse2. Sync burst3. Guard interval4. Transmitter un-key

The reply message contains what totalnumber of data time slots?

1 . 1 32 . 4 23 . 5 64 . 7 0

Which of the following Link-4A testmessages is used to provide aircraft withthe means to verify proper operation ofLink-4A?

1. The monitor reply message2. The monitor control message3. The universal test message4. All of the above

Which of the following Link-4A testmessages causes internal testing of thedata terminal set?

1. The monitor reply message2. The monitor control message3. The universal message4. All of the above

Which of the following AN/SSW-1()subassemblies provides system timing forLink-4A operations?

1. Digital-to-digital converter2. Monitor test panel3. Coordinate data transfer control4. Pulse amplifiers

ASSIGNMENT 4

Textbook Assignment: “New Technology in Data Communications,” chapter 5, pages 5-1 through 5-17;“Local-Area Networks,” chapter 6, pages 6-1 through 6-18.

4-1 .

4-2 .

4-3 .

4-4 .

Which of the following pieces of 4-5 .equipment is replacing older Link-11 dataterminal sets?

1. AN/USQ-36 Data Terminal Set2. C2P3. AN/USQ-125 Data Terminal Set4. Link-16

The CP-2205(P)(V)/USQ-125 processor 4-6 .board performs which of the followingfunctions?

1. Modulation/demodulation2. Error detection and correction3. Radio set interface4. All of the above

Which of the followingCP-2205(P)(V)/USQ-125 components 4-7 .provides for communications with theCDS computer?

1. Processor board2. Interface board3. Power supply4. Modulator

The CP-2205(P)(V)/USQ-125 is capableof data encryption. 4-8 .

1. True2. False

The single-tone waveform link capabilityof the CP-2205(P)(V)/USQ-125 provideswhich of the following functions?

1. Interface with a satellite modem2. Increases UHF transmission range3. Increases HF transmission range4. Reduces HF propagation anomalies

Which of the following options of theCP-2205(P)(V)/USQ-125 incorporates aroutine to calculate the optimumfrequency?

1. Enhanced link quality analysis2. Maximum useable frequency3. Single-tone waveform link4. Multi-frequency link

Which of the following options of theCP-2205(P)(V)/USQ-125 transmitsLink-11 data through a standard wire-linemodem?

1. Enhanced link quality analysis2. Maximum useable frequency3. Single-tone waveform link4. Multi-frequency link

Which of the following options of theCP-2205(P)(V)/USQ-125 incorporatesmost of the functions of the LMS-11?

1 .2 .3 .4 .

Enhanced link quality analysisMaximum useable frequencySingle-tone waveform linkMulti-frequency link

29

4-9 . Which of the following options of theCP-2205(P)(V)/USQ-125 improvesLink-11 operations by using fourfrequencies simultaneously?

1. Enhanced link quality analysis2. Maximum useable frequency3. Single-tone waveform link4. Multi-frequency link

4-10. The normal configuration of themulti-frequency link options uses (a)HF frequencies and (b) UHFfrequencies.

1. (a) 2 (b) 12. (a) 3 (b) 13. (a) 2 (b) 24. (a) 3 (b) 2

IN ANSWERING QUESTIONS 4-11 THROUGH4-15, SELECT FROM THE FOLLOWING LISTTHE SYSTEM TEST OPTIONS OF THECP-2205(P)(V)/USQ-125 DATA TERMINALSET DESCRIBED IN THE QUESTION. NOTALL ITEMS IN THE LIST ARE USED.

A. Radio echo testB. Loopback test 1C. Loopback test 2D. Loopback test 3E. Loopback test 4F. DTS fault isolation test

4-11. This option is selected when a singlestation POFA is running without theradio.

1. A2. B3. C4. D

4-12.

4-13.

4-14.

4-15.

4-16.

30

When this test is run, the computerinterface is disabled and a test message isrepeatedly sent to the radio set.

1. A2. B3. C4. D

This test is used to verify the operation ofthe computer interface, crypto device andthe data terminal interface circuits.

1. C2. D3. E4. F

This test places the DTS in full-duplexmode to run a single station POFA withthe radio.

1. A2. B3. C4. D

This option performs an internal self-testof the DTS audio circuits.

1. A2. B3. C4. D

The data terminal can be controlled froma remote location by use of which of thefollowing pieces of equipment?

1 .2 .

3 .

4 .

A 286 personal computer onlyA 386 or better personal computeronlyThe C-12428/USQ-125 Control UnitonlyEither a 386 or better personalcomputer, or the C-12428/USQ-125Control Unit

4-17.

4-18.

4-19.

4-20.

4-21.

Link-16 uses which of the followingcommunications protocols?

1. Netted or roll call2. Time division, command and response3. Time division multiple access4. Frequency-division multiplexing

Each unit participating in a Link-16 net isidentified by assigning it what type ofdesignator?

1. A PU number2. A JU number3. A link identifier4. A hull number

What is the durationa Link-16 message?

1. 7.8125 msec2. 7.8125 µsec3. 7.8125 seconds

of each time slot in

4. It varies, accordingdata transmitted

to the amount of

During the transmission of data, exactlyhow often does Link-16 changefrequency?

1. Every 13 µsec2. Every 13 msec3. Daily4. When the frequency is excessively

noisy

Link-16 is configured for a stacked net.At any one time, what number of netscan a single terminal transmit and receivedata?

1. One2. Two3. Three4. Four

4-22,

4-23.

4-24.

4-25.

4-26.

A Link-16 data word is comprised ofwhat number of data bits?

1. 502. 603. 704. 80

A Link-16 fixed format message is whichof the following message types?

1. V-series2. R-series3. M-series4. J-series

Which of the following message types areused for Link-16 voice communications?

1. Fixed format2. Free text3. Variable format4. Unformatted

Which of the following message types areused to exchange tactical data?

1. Fixed format2. Free text3. Variable format4. Unformatted

Compared to Link-11, Link-16 isnodeless for which of the followingreasons?

1 .

2 .3 .

4 .

Once the net is established, all unitsmust participateOnly one unit controls the netOnce the net is established, operationscan continue regardless of theparticipation of any particular unitA computer is not required toparticipate in the net

31

4-27.

4-28.

4-29.

4-30.

4-31 .

Which of the following JTIDS securityfeatures is designed to prevent jamming?

1. Data encryption2. Waveform encryption3. Introduction of jitter and noise4. Frequency hopping

Which of the following networkparticipation groups is normally excludedfrom Navy command and control units?

1. Weapons coordination2. Air control3. Fight-to-fighter4. Secure voice

JU numbers 00001 through 00177 arenormally assigned to which of thefollowing units?

1. Link-4A and Link-16 capable units2. Command and Control units3. Link-16 capable aircraft4. Link-11 and Link-16 capable units

Which of the following Link-16 tracknumbers would designate the sameLink- 11 track?

1. 00000 through 077772. 01000 through 777773. 00200 through 077774. 00500 through 77777

A Link-16 track that has a reported trackquality of 15 indicates the track is withinexactly how many feet of the reportedposition?

1. 102. 253. 504. 75

4-32.

4-33.

4-34.

4-35.

4-36.

32

Which of the following trackidentifications has been added for usewith the Link-16 system?

1. Neutral2. Hostile3. Assumed hostile4. Unknown

The identifier “Unknown assumed enemy”has been changed in the Link-16 systemto what identifier?

1. Neutral2. Suspect3. Unknown assumed hostile4. Hostile

Link-16 has added which of the followingdata fields to friendly aircraft statusreports?

1. Ordnance inventory2. Equipment Status3. Fuel available for transfer4. All of the above

The Relative Navigation function of theLink-16 system is required for which ofthe following functions?

1 .2 .3 .4 .

Maintain synchronizationMaintain positionDetect course errorsCorrect the navigation plot

Which of the following equipmentconfigurations fully implements thecapabilities of the Link-16 system?

1. Model-32. Model-43. Model-54. Model-6

4-37.

4-38.

Using the Link-16 Model-5 system, linkdata generated by the ACDS computer isnormalized to be independent of anyparticular system.

1. True2. False

Using the Link-16 Model-5 system, whatcomponent formats link data fortransmission over any one of the threedata links?

1 .2 .3 .4 .

ACDSC2PLink-16 data terminalTDS computer

IN ANSWERING QUESTIONS 4-39 THROUGH4-42, SELECT FROM THE FOLLOWING LISTTHE EQUIPMENT COMPONENT GROUPS OFTHE JTIDS TERMINAL THAT PERFORMSTHE FUNCTION OR OPERATION DESCRIBEDIN THE QUESTION. NOT ALL ITEMS INTHE LIST

A.B.C.D.E.

ARE USED.

Digital data processor groupReceiver/transmitter groupHigh power amplifier groupPower interface unitSecure data unit

4-39,

4-40.

A removable assembly that storescryptovariables.

1. B2. C3. D4. E

Generates a 75 MHz intermediatefrequency for internal communications.

1. A2. B3. C4. D

4-41.

4-42.

4-43.

4-44.

4-45.

Provides the interface with the antenna.

1. A2. B3 . C4. D

Performs digital-to-analog andanalog-to-digital conversion of voicesignals.

1. A2. B3 . C4. D

The Command and Control (C2P) systemcontrols and manages the interface ofwhich of the following data links?

1. Link-4A only2. Link-11 only3. Link-16 only4. All tactical data links

Messages received by the C2P over theLink-11 net cannot be retransmitted overthe Link-16 net.

1. True2. False

A local area network (LAN) performswhich of the following functions?

1. Enables users to share data2. Enables users to share peripheral

devices3. Allows users to send and receive

messages via computer4. All of the above

33

4-46. Which of the following items is a node ina LAN system?

1. Twisted pair cable2. Communications media3. Fiber-optic cable4. Large capacity hard drive

4-47. Twisted pair cable used in a LAN haswhich of the following advantages?

1. High data transmission speeds2. Low costs and easy to install3. Shielded from electrical interference4. Secure data transmission

4-48. In designing a LAN system that requiresthe transmission of digital data, audio,and video simultaneously, which of thefollowing cables would be best suited tothe system?

1. Twisted pair2. Shielded twisted pair3. Broadband coaxial4. Baseband coaxial

4-49. Which of the following types of cable isimmune to interference from electricaland electronic devices?

1. Fiber optic2. Baseband coaxial3. Shielded twisted pair4. Telephone cable

4-50. Which of the following devices providesthe interface between the LAN and apersonal computer?

1. Network server2. NIC3. Disk server4. Cables

4-51. Which of the following devices is used to

4-52.

4-53.

4-54.

4-55.

manage the shared resources of the LAN?

1. Network server2. NIC3. Disk server4. Network monitor

Early disk servers suffered from which ofthe following problems?

1. Lack of security2. No data organization3. No disk management4. All of the above

Ensuring data integrity by preventingmultiple users access to the same recordat the same time is known as whatprocess?

1. File locking2. Field locking3. Record locking4. Disk lockout

Which of the following servers wasdeveloped to provide reliable diskmanagement in a LAN?

1. Disk server2. Print server3. File server4. Network server

Which of the following functions is NOTperformed by the file server?

1. Routing files to a central printer forprinting

2. Processing of the network controlsoftware

3. Converting high-level disks calls froma workstation to low-level diskcommands

4. Maintaining the list of user privilegesand authorizations

34

4-56.

4-57.

4-58.

4-59.

4-60.

When designing and building a LANsystem, all workstations must be from thesame manufacturer.

1. True2. False

The OSI reference model is used todefine which of the followingcommunications standards?

1. Interconnection of communicationsfacilities

2. Software3. Hardware4. Protocol

What layer of the OSI reference modeldescribes the electrical, the mechanical,and the functional interface of thecommunications channel?

1. Physical layer2. Data link layer3. Network layer4. Transport layer

What layer of the OSI reference modelestablishes and deletes host-to-hostconnections across the network?

1. Data link layer2. Network layer3. Transport layer4. Session layer

As a translator for the network, whatlayer of the OSI reference model providesa common representation for data whichcan be used between the applicationprocesses?

1 .2 .3 .4 .

Network layerTransport layerSession layerPresentation level/layer

4-61. Which of the following layers of the OSIreference model provides error-freetransmission of information over thephysical medium?

1. Physical2. Data link3. Network4. Transport

4-62. Communications between users on twodifferent machines are established bywhat layer of the OSI reference model?

1. Data link2. Network3. Transport4. Session

4-63. Based on network conditions and priorityof service, what layer of the OSIreference model decides which physicalpathway the data should take?

1. Physical2. Data link3. Network4. Transport

4-64. The application level/layer of the OSIreference model provides the protocolsfor which of the following user items?

1. Media interface2. Electronic mail3. Routing of messages between

networks4. Data compression

4-65. Which of the following is NOT a LANtopology?

1. Linear2. Ring3. Star4. EtherNet

35

4-66.

4-67.

4-68.

4-69.

Which of the following features of alinear bus topology should be consideredan advantage?

1 .

2 .

3 .4 .

Signal interference when nodes aretoo close to each otherSystem remains operable even whenone or more nodes failVery secure systemVery easy to run system diagnosticsfrom the LAN administrator

In which of the following networktopologies, if any, is each nodeindividually connected to the networkserver?

1. Linear bus2. Star3. Ring4. None of the above

Which of the following network accessmethods requires each node to wait forpermission to transmit data?

1. CSMA2. CSMA/CD3. Token passing4. Contention

On a network that uses the CSMA/CDaccess method, which of the followingactions, if any, will be taken when a datacollision is detected?

1 .

2 .

3 .

4 .

The workstation will ceasetransmission and wait until the line isclearThe workstation will continue totransmit dataThe network server will assign thenext open time to the workstation thatsuffered the collisionNo action is taken; the data is lost

4-70. In a LAN using a token ring topology,the interface and protocols are defined bywhich of the following IEEE standards?

1. IEEE 8022. IEEE 802.33. IEEE 802.44. IEEE 802.5

4-71. Which of the following LAN systemsuses a token ring topology and has a datathroughput of 4 Mbits and 16 Mbits persecond?

1. EtherNet2. STARLAN3. ARCnet4. IBM Token Ring

IN ANSWERING QUESTIONS 4-72 THROUGH4-74, SELECT FROM THE FOLLOWING LISTTHE NETWORK OPERATING SYSTEMCOMPONENT THAT PERFORMS THEFUNCTION DESCRIBED IN THE QUESTION.NOT ALL ITEMS IN THE LIST ARE USED.

A. Control kernelB. Network interfacesC. File systemsD. File extensions

4-72. Provides low-level subnet protocols andtranslation for bridging hardware driverswith the network operating system.

1. A.2. B.3. C.4. D.

4-73. Controls data organization, storage, andretrieval on the various storage systemsavailable to the network.

1. A.2. B.3. C.4. D.

36

4-74. The main subsystem of the networkoperating software.

1. A.2. B.3. C.4. D.

4-75. Which of the following network operatingsystems, if any, is designed for very fewusers and light usage?

1. Full featured2. Low cost3. Zero slot4. None of the above

37