Digital Transmission Technology an Overview

8/16/2019 Digital Transmission Technology an Overview

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Digital Transmission System(DTS) Digital Transmission System-an overview

EETP/BSNL Silver Certification Course Page 1 of 43

For Restricted Circulation

EETP/BSNLSILVER CERTIFICATION

COURSE DIGITAL TRANSMISIION

SYSTEM

VERSION 1 JUNE 2013


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Contents

Sl. No. Name of Topic Page No.

1

INTRODUCTION

3

2

BUILDING BLOCK OF COMMUNICATION SYSTEM

6

3

EVOLUTION OF TRANSMISSION SYSTEMS

7

4

TRANSMISSION FUNDAMENTALS

10

5

TYPES OF MULTIPLEXING

16

6 PULSE CODE MODULATION 19

7

EVOLUTION OF DIGITAL MULTIPLEXING

21

8 CLASSIFICATION OF TRANSPORT NETWORK BY

GEOGRAPHY

25

9

TRANSPORT NETWORK AND THE ROAD ANALOGY

27

10

COMPONENTS OF TRANSMISSION NETWORK

27

11

DEVICES USED IN TRANSMISSION SYSTEMS

30

12

GENERAL INSTALLATION PRACTICE FOR TRANSMISSIONEQUIPMENT

39

13

ROUTINE MAINTENANCE 40

14

SUMMARY41


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1 DIGITAL TRANSMISSION SYSTEM-AN OVERVIEW

STRUCTURE

1.1

INTRODUCTION

1.2 OBJECTIVE

1.3 BUILDING BLOCK OF COMMUNICATION SYSTEM

1.4 EVOLUTION OF TRANSMISSION SYSTEMS

1.5 TRANSMISSION FUNDAMENTALS

1.6 TYPES OF MULTIPLEXING

1.7 PULSE CODE MODULATION

1.8 EVOLUTION OF DIGITAL MULTIPLEXING

1.9 CLASSIFICATION OF TRANSPORT NETWORK BY GEOGRAPHY

1.10 TRANSPORT NETWORK AND THE ROAD ANALOGY

1.11 COMPONENTS OF TRANSMISSION NETWORK

1.12 DEVICES USED IN TRANSMISSION SYSTEMS

1.13 GENERAL INSTALLATION PRACTICE FOR TRANSMISSION

EQUIPMENT

1.14 ROUTINE MAINTENANCE

1.15 SUMMARY

1.16 SELF ASSESSMENT QUESTIONS

1.17 REFERENCES

1.1 INTRODUCTION

In earlier times, communication may have involved the use of smoke signals,

drums, semaphore, flags, homing pigeons etc. In the Middle Ages, chains of beacons

were commonly used on hilltops as a means of relaying a signal. Beacon chains

suffered the drawback that they could only pass a single bit of information, so the

meaning of the message such as "the enemy has been sighted" had to be agreed upon

in advance. One notable instance of their use was during the Spanish Armada, when a

beacon chain relayed a signal from Plymouth to London. The conventional telephone

was invented independently by Alexander Bell in 1876.


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In 1792, Claude, a French engineer, built the first fixed visual telegraphy

system (or semaphore line) between Lille and Paris. However semaphore sufferedfrom the need for skilled operators and expensive towers at intervals of ten to thirty

kilometers (six to nineteen miles). As a result of competition from the electrical

telegraph, the last commercial line was abandoned in 1880. The first commercial

telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the

cities of New York and London.

Samuel Morse independently developed a version of the electrical telegraph

that he unsuccessfully demonstrated on 2 September 1837

The first transatlantic telegraph cable was successfully completed on 27 July

1866, allowing transatlantic telecommunication for the first time

Fig : 1 Advances in phone telephony

Fig : 2 Typical diagram of a Telecom Network


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Telecommunication is the assisted transmission of signals over a distance for

the purpose of communication. It is the technology of transferring information over adistance.

Information can be of several type:

• Audio – Telephone• Text - Telegraph, email, SMS

• Pictures – Picture attachments

• Video – Clipping over internet

• Data – ATM to bank.

The same telecom technology/service cannot communicate all the types of

information.

Telecommunications transport networks are the largely unseen infrastructure

that provides local, regional, and international connections for voice, data, and video

signals. In fact, most “private” networks are implemented by leased connections

through the public transport network infrastructure. Telecommunications and datacommunications transport networks are changing rapidly with the introduction of new

technologies that address the need for new value-added services, high availability, and

integration. Equipment vendors and network providers have made considerable effort

to bridge and unify previously dedicated networks to serve the data and

telecommunications market. With the introduction of digital transmission technology,

the most appropriate multiplexing technology was Time Division Multiplexing

(TDM). Digital TDM was used both on copper cable systems and on microwave

radio.

Fig : 3 Simplified communication model


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1.2 OBJECTIVE

After reading this unit, you should be able to:

Understand the basics of telecommunication and building blocks of telecom

network.

Understand the different multiplexing techniques Describe the different types of Transport Network

Differentiate the different components of transmission network.

1.3 BUILDING BLOCK OF COMMUNICATION SYSTEM

The most general form of communication system consists of the followingcomponents:

Fig : 4 Communication System Generic Block Diagram

1.3.1 TRANSMITTER:The upper portion of the channel as a whole is called transmitter.

1.3.1.1 Information Source:

The information provided to a communication system is called baseband.

One may define as “any information signal is known as baseband signal".

1.3.1.2 Processing Unit:

The baseband signal is passed through some processing unit where necessary

operations are performed. These operations may include filtering, sampling etc.

This is helpful in separating unwanted information from baseband.

1.3.1.3 Modulation:

The process of modulation is required to make the baseband signal ready for

transmission. The modulator produces a varying signal at its output which is

proportional in some way to the signal appearing across its input terminal

(baseband signal). For example, a sinusoidal modulator may vary the amplitude,


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frequency or phase of a sinusoidal signal in direct proportion to the input baseband.

1.3.1.4 Antenna:

For wireless communication, antenna is used to send modulated information

into the channel or medium.

1.3.1.5

Channel:

The transmission medium or channel is vital link between the systems.

Without it there would be no communication problem. The transmission medium

may include wired transmission line, atmosphere (wireless) which may include the

ionosphere, the troposphere, free space etc.

It causes noise (unwanted addition to baseband signal), attenuation and

distortion in the form of electrical signal. This results in interference with our

error-free reception at receiving end.

1.3.2 RECEIVER:The portion below the channel as whole called receiver.

1.3.2.1 Demodulation:

The demodulator performs the inverse process of modulator to recover the

information signal in its original form.

1.3.2.2 RF Amplifier:

The RF amplifier is used to tune the receive to frequency of the transmitted

bandwidth.

1.3.2.3 Display Unit:

This shows us the received signal in the form by which we are familiar.

1.4 EVOLUTION OF TRANSMISSION SYSTEMS

Transmission systems interconnect communication devices by guiding signal

energy in a particular direction or directions through a transmission medium such as

copper, air, or glass. Described are the key types of transmission systems used in

modern telecommunications networks. This includes multiplexed signals on twisted

wire pairs, coax cable, fiber optic cable and radio. Several technical aspects of

transmission systems are covered including: analog transmission, digital transmission,

and transmission medium limitations

1. OPEN WIRE SYSTEM

The long distance voice communication till 1950s was almost entirely

transported over Open Wire Carrier system. The voice signals for these systems were

modulated to a higher frequency and carried through open wire systems. These open

wire systems are capable of carrying traffic of three to twelve subscribers at a time.


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Fig : 5 Open Wire Carrier system

2. COAXIAL SYSTEM

Coaxial Cable (often called coax for short) is high-capacity cable widely used

for high-frequency transmission of telephone, television, and digital audio signals.

The cable is very effective at carrying many analog signals at high frequencies.

Coaxial cables have become an essential component of our information

superhighway. They are found in a wide variety of residential, commercial and

industrial installations. From broadcast, community antenna television (CATV), localarea network (LAN), closed circuit television (CCTV) to many other applications,

coax has laid the foundation for a simple, cost effective communications

infrastructure.

With the introduction of symmetrical pair cable carrier system which was

followed by the Coxial Cable system, greatly enhanced, by the decade end, thesimultaneous voice channel carrying capacity to 960 voice channels. The first Coaxial

Cable System was commissioned between Agra and Delhi in the year 1959. Over the

years this system was improved and developed to carry 2,700 simultaneous voice

channels

Fig : 6 Co-axial cable


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3. MICROWAVE SYSTEM

Close on the heel of coaxial systems, in the mid of 60’s wireless microwavesystems were developed and inducted in the network. The first Microwave system

was installed between Calcutta and Asansole. Microwave systems with 60, 300 and

1800 voice channels capacity were inducted into the telecom network subsequently.

These systems were mostly developed and manufactured with in the country.

Fig : 7

Microwave system

4. DIGITAL TRANSMISSION SYSTEM

By mid of 1980’s Digital TAX exchanges were introduced in the network with

the aim to improve STD services. Till 1989 Coaxial cable and UHF transmission

medias were used to provide connectivity. Induction of Digital Transmission Systems

which were mainly Digital UHF, Digital Microwave, Digital Coaxial and Optical

Fiber Systems, started during 1989-90. Under ground coaxial cable was initially used

for the connectivity of large and medium cities and however, later on, it was also used

for connecting small towns. Media diversity is provided through Radio Relay (UHF

and Microwave) Systems. These Radio relay systems were very reliable and beneficial particularly for connecting hilly and backward areas where laying and

maintenance of underground cable is extremely difficult.

5. OPTICAL FIBER SYSTEM

Introduction of Optical Fiber Cable Systems started in 1989-90. These systemsare capable of carrying large no voice channels compared to the existing technologies

that were available at that time and offer the circuit at low cost per kilometer of

circuit. Department deployed these OFC system in big way for connectivity right upto

the level of Tehsils. By the year 2000 a huge network of optical fiber cable was in

place and a large number of PDH technology (Plesiocronus Digital Hierarchy) OFCwere deployed for providing backbone connectivity to switching network.


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Fig : 8 Typical Optical Fiber System

6. SATELLITE SYSTEM

Work for connecting far flung, inaccessible area and island community started

in late seventies by Department of Telecommunication. The first Domestic Satellite

Network was established by connecting Port-Blair and Car-Nicobar in Andaman & Nicobar islands, Kavaratti in Lakshadweep islands, Leh in Ladakh region and Aizwal

in North Eastern region. These station were simultaneously linked to the gateway at

Delhi and Chennai. This satellite network was commissioned in November 1980

through International Telecommunication Satellite. Satellite Communication capacity

increased with lauanch INSAT 1 and INSAT 2 series satellites.

Fig : 9 A Typical Satellite system

1.5 TRANSMISSION FUNDAMENTALS

The study of transmission units has a unique importance for communication

engineer who has to maintain and install telecommunication equipment achieving the

standards set up by international consultation committees.

In order to control the quality of wanted signal in the presence of many

undesired signals, we should be able to specify the amount of wanted and unwanted

signals at a point in the telecommunications network.


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1.5.1 TRANSMISSION IMPAIRMENT

With analog transmission systems using copper cable there are three major

categories of impairments. They are attenuation, noise, and distortion.

1. Attenuation: There are two commonly used processes to compensate (overcome)

for attenuation or loss:

(a) Repeaters are the most commonly used devices to compensate for "Loss."

However, repeaters amplify the noise along with the signal resulting in a poor

signal to noise ratio.

(b) Signal to Noise Ratio: The ratio of the average signal power (strength) to

the average noise power (strength) at any point in a transmission path.

2. Noise: Any random disturbance or unwanted signal on a transmission facility that

obscures the original signal. Noise is generally caused by the environment in

which the system is operating.

3. Distortion: Inaccurate reproduction of a signal caused by changes in the signal's

waveform, either amplitude or frequency, to compensate for distortion equalizers

may be used. One type of equalizer used in the analog environment is the load

coil. Load coils are used to flatten the frequency response.

Note: Generally the higher the frequency the greater the distortion. That is, the higher

voice frequencies attenuate at a higher rate than the lower voice frequencies.

Noise and distortion on a carrier facility can be separated into two types:

(a) Predictable impairments that are almost always present on our facilities.

(b) Unpredictable impairments those are transient in nature and difficult to overcome.

1.5.2 THE DECIBEL

Historically speaking ‘attenuation’ was first of all defined in terms of the

attenuation produced by a standard reference cable known as “mile of standard

cable”. It consists of 88 ohms series impedance and 0.54 µF as shunt impedance.

The fundamental objection to this unit was the fact that the attenuation of the

standard cable varied with frequency. With the introduction of systems operating over

different frequency ranges, it became necessary to define a unit which was

independent of frequency .The unit which represents the useful and convenient

concepts in connection with the transmission of signals over telephone lines has been

named and defined as “Bel”(which comes from the name Alexander Graham Bell -the

inventor of Telephone). In practice, however, a smaller and more convenient unit

called decibel (abbreviated as dB) is used.

1.5.2.1 Decibel (dB)

One tenth of the common logarithm of the ratio of relative powers, equal to

0.1 B (bel). The decibel is the conventional relative power ratio, rather than the bel,


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for expressing relative powers because the decibel is smaller and therefore more

convenient than the bel. The ratio in dB is given by

X = log P2/P1 B i.e. = 10 log P2/P1 dB

where P 1 and P 2 are the actual powers. Power ratios may be expressed in terms of

voltage and impedance, E and Z , or current and impedance, I and Z . Thus dB is alsogiven by;

X = 20 log V2/ V1 dB. (when Z 1 = Z 2 )

Note: The dB is used rather than arithmetic ratios or percentages because when

circuits are connected in tandem, expressions of power level, in dB, may be

arithmetically added and subtracted. For example, in an optical link if a known

amount of optical power, in dBm, is launched into a fiber, and the losses, in dB, of

each component (e.g., connectors, splices, and lengths of fiber) are known, the overall

link loss may be quickly calculated with simple addition and subtraction.

Example 1

Let us look at the following network:

The input is 1W and its output 2W, therefore,

Gain = 10 log (output)/(input) dB.

= 10 log 2/1 dB= 10 (0.3010) dB=3.101 dB

= 3dB approximately

1.5.3 dBm

Till now decibel has referred to ratios or relative units. We cannot say that the

output of an amplifier is 33 dB. We can say that an amplifier has a gain of 33 dB or

that a certain attenuator has a 6 dB loss. These figures or units don't give any idea

whatsoever of absolute level. Whereas, several derived decibels units do.

Perhaps the dBm is the most common of these. By definition dBm is a power level

related to 1 mw. The most important relationship to remember is:

0 dBm = 1mW.

The dBm formula may then be written as:

Power (in dBm) = 10 log Power (mW)/(1mW)

Example

An amplifier has an output of 20 W; what is its output in dBm?

Net Work1 W 2 W


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Power (dBm) = 10 log 20 W/1 mW = 10 log 20x103 mW/1mW = +43 dBm.

(The plus sign indicates that the quantity is above the level of reference, 0 dBm.)

1.5.4 SIGNAL-TO-NOISE RATIO

In Analogue Transmission system, the quality of communication is mainly

assessed by the value of Signal to noise ratio.

It is popularly known as SNR. SNR is the ratio of signal power to the noise

power at any point in a circuit. This ratio is usually expressed in Decibels (dB). For

satisfactory operation of a channel the value of SNR should be sufficiently high i.e.,

the signal power should be sufficiently higher than the noise power.

SNR at any point in a circuit is given as SNR = S/N = Signal Power / Noise Power

Both powers are expressed in watts.

Expressing dBs: SNR = 10 log10 (S/N) dB.

Example: Signal voltage Vs = 0.923 µV; Noise voltage Vn = 0.267 µV, then

calculate the signal-to-noise ratio.

S/N = Vs2 / Vn2 = 0.923/0.267)2 = 11.95

In decibels, S/N = 10 log10 (11.95) = 10.77 dB.

1.5.5 DIGITAL PERFORMANCE PARAMETERS

In Digital Transmission system, the quality of communication is mainly

assessed by two factors.

1. BER (Bit Error Ratio)

2.

Jitter

These two factors can be taken as Quality Factors as they are used for judging the

quality of Digital Transmission.

1.5.5.1 BIT ERRORS

In the digital transmission, the bits transmitted at the transmitting end (1 or 0 )are not always detected as 1 or 0 at the receiving end. When the transmitted bit 1 or 0

is not identified as 1 or 0 at the receiver, the bit is counted as an error bit.

For assessing the real error performance, the bit error ratio (BER) is to be

calculated instead of actual error bits.

1.5.5.2 Bit Error Rate (BER)

The BER is the measure of error bits with respect to the total number of bits

transmitted in a given time. The total number of bits transmitted can be known from

the bit rate of the digital signal. The bit rate is the number of bits transmitted in one


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second and is specified for each transmission system. Hence, the total number of bits

transmitted in a given time can be counted. In the measurement of BER, generally the

measuring instrument measures the number of bits transmitted in a given time.

The time setting can be from a few seconds to a few hours, depending on the

feasibility. The standards are set by ITU (International Telecommunication Union).

The time set for the measurement of BER, is called gating time. Larger the gatingtime better is the assessment of BER. But for the measurement of BER, the Digital

Equipment has to be taken off-line.

Digital communication can just run with one error bit in one thousand bits

received. For more than one error bit, in one thousand bits received, communication

gets affected. For good quality communication, the requirement is, not more than one

error bit in one million bits.

1.5.5.3 Jitter

Abrupt and unwanted variations of one or more signal characteristics, such asthe interval between successive pulses, the amplitude of successive cycles, or the

frequency or phase of successive cycles. Jitter must be specified in qualitative terms

(e.g., amplitude, phase, pulse width or pulse position) and in quantitative terms ( e.g.,

average, RMS, or peak-to-peak). The low-frequency cut-off for jitter is usuallyspecified at 1 Hz. Contrast with drift, wander.

Short term variations of the significant instances of a digital signal from their

reference position in time.( Short term frequency equal to or greater than 10 Hz.).

Long term variations of significant instances of a digital signal from their ideal

positions in time, are called wander. (Long-term variations – frequency less than 10

Hz).

Jitter, like BER, is transmission impairment. It is not very significant in the

case of voice signal transmission but it has a great impact in the transmission of data

signals, especially with high-speed digital transmission. The present bit rates are as

high as 565 Mb/s and (140 x 16) Mb/s. Today Jitter is considered as a performance

parameter of any digital transmission system.

For example, Jitter due to unwanted phase change is called Phase Jitter. The

amount of change of phase, converted into time, is generally expressed in milli-

seconds or nano-seconds.

BER and Jitter are the unwanted by products of any transmission system and

they get associated with the transmission path and affect the quality of transmission.

Bit Errors beyond a limit, affect the communication and Jitter in the digital

transmission system, is a source of generation of errors.

Digital Transmission Analyzer (DTA) is used for the measurement of both BER and

Jitter.

1.5.5.4 Digital TRANSMISSION - Performance Criteria ( General)

1 in 106 (1X E – 6) : Better


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1 in 105 (1XE – 5) : Good

1 in 104 (1XE – 4) : Reasonably good

1 in 103 (1XE – 3) : Just Acceptable

More than 1 in 103

: Unacceptable

Bit errors greatly affect data service.

For data channels 1 in 109 (1.OE – 9) is normally realizable.

1.5.6 QUALITY PARAMETERS

To pin point the exact number of seconds or minutes, in which the bit errors

take place and up to what extent, the quality parameters are defined.

The quality parameters are:

1. Error Seconds (ES)

2. Severely Error Seconds (SES)

3. Non Severely Error Seconds (NSES)

4.

Degraded Minutes (DM).

1.5.6.1 Error Seconds (ES): Number of one-second intervals with one or more

errors.

1.5.6.2 Severely Error Seconds (SES): Number of one-second intervals with an

error rate, worse than 1.OE-3

1.5.6.3 Non-Severely Error Seconds (NSES): Number of one-second intervals

with an error rate, better than or equal to 1.OE-3.

1.5.6.4 Degraded Minutes (DM): Number of one-second intervals with a bit error

rates worse than 1.OE-6.

1.5.6.5 Available and non-available time

A period of available time begins with a period of ten consecutive seconds

each of which has a BER better than 1.0E-3. These 10 seconds are considered to be

available time.

A period of unavailable time begins when the bit error rate in each second is

worse than 1.0E-3 for a period of 10 consecutive seconds. These 10 consecutive

seconds are considered to be unavailable time.


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1.6 TYPES OF MULTIPLEXING

In telecommunications and computer networks, multiplexing (also known as

muxing) is a method by which multiple analog message signals or digital data streams

are combined into one signal over a shared medium. The aim is to share an expensive

resource. For example, in telecommunications, several telephone calls may be carried

using one wire. Multiplexing originated in telegraphy, and is now widely applied in

communications. George Owen Squier is credited with the development of

multiplexing in 1910.

The multiplexed signal is transmitted over a communication channel, which

may be a physical transmission medium. The multiplexing divides the capacity of the

high-level communication channel into several low-level logical channels, one for

each message signal or data stream to be transferred. A reverse process, known as

demultiplexing, can extract the original channels on the receiver side.

A device that performs the multiplexing is called a multiplexer (MUX), and a

device that performs the reverse process is called a demultiplexer (DEMUX). Inversemultiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data

stream into several streams, transfer them simultaneously over several communication

channels, and recreate the original data stream.

Multiplexing technologies may be divided into several types, all of which have

significant variations: space-division multiplexing (SDM), frequency-division

multiplexing (FDM), time-division multiplexing (TDM), and code division

multiplexing (CDM). Variable bit rate digital bit streams may be transferred

efficiently over a fixed bandwidth channel by means of statistical multiplexing, for

example packet mode communication. Packet mode communication is an

asynchronous mode time-domain multiplexing which resembles time-division

multiplexing.

Digital bit streams can be transferred over an analog channel by means of

code-division multiplexing (CDM) techniques such as frequency-hopping spread

spectrum (FHSS) and direct-sequence spread spectrum (DSSS).

In wireless communications, multiplexing can also be accomplished through

alternating polarization (horizontal/vertical or clockwise/counterclockwise) on each

adjacent channel and satellite, or through phased multi-antenna array combined with a

multiple-input multiple-output communications (MIMO) scheme.


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Fig : 10

General multiplex scheme: the ν input lines-channels are multiplexed intoa single fast line. The demultiplexer receives the multiplexed data stream and

extracts the original channels to be transferred.

1.6.1 SPACE-DIVISION MULTIPLEXING

In wired communication, space-division multiplexing simply implies different

point-to-point wires for different channels. Examples include an analogue stereo

audio cable, with one pair of wires for the left channel and another for the right

channel, and a multipair telephone cable. Another example is a switched star network

such as the analog telephone access network (although inside the telephone exchange

or between the exchanges, other multiplexing techniques are typically employed) or aswitched Ethernet network. A third example is a mesh network. Wired space-divisionmultiplexing is typically not considered as multiplexing.

In wireless communication, space-division multiplexing is achieved by

multiple antenna elements forming a phased array antenna. Examples are multiple-

input and multiple-output (MIMO), single-input and multiple-output (SIMO) and

multiple-input and single-output (MISO) multiplexing. For example, a IEEE 802.11n

wireless router with N antennas makes it possible to communicate with N multiplexed

channels, each with a peak bit rate of 54 Mbit/s, thus increasing the total peak bit rate

with a factor N. Different antennas would give different multi-path propagation (echo)

signatures, making it possible for digital signal processing techniques to separatedifferent signals from each other. These techniques may also be utilized for space

diversity (improved robustness to fading) or beam forming (improved selectivity)

rather than multiplexing.


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Fig : 11 Frequency-division multiplexing (FDM): The spectrum of each input

signal is shifted to a distinct frequency range.

1.6.2 FREQUENCY-DIVISION MULTIPLEXING

Frequency-division multiplexing (FDM): The spectrum of each input signal is

shifted to a distinct frequency range.

Frequency-division multiplexing (FDM) is inherently an analog technology.

FDM achieves the combining of several digital signals into one medium by sending

signals in several distinct frequency ranges over that medium.

One of FDM's most common applications is cable television. Only one cable

reaches a customer's home but the service provider can send multiple television

channels or signals simultaneously over that cable to all subscribers. Receivers must

tune to the appropriate frequency (channel) to access the desired signal.[1]

A variant technology, called wavelength-division multiplexing (WDM) is used

in optical communications.

1.6.3 TIME-DIVISION MULTIPLEXING

Time-division multiplexing (TDM) is a digital (or in rare cases, analog)

technology. TDM involves sequencing groups of a few bits or bytes from each

individual input stream, one after the other, and in such a way that they can be

associated with the appropriate receiver. If done sufficiently quickly, the receiving

devices will not detect that some of the circuit time was used to serve another logicalcommunication path.

Consider an application requiring four terminals at an airport to reach a central

computer. Each terminal communicated at 2400 bit/s, so rather than acquire four

individual circuits to carry such a low-speed transmission, the airline has installed a

pair of multiplexers. A pair of 9600 bit/s modems and one dedicated analog

communications circuit from the airport ticket desk back to the airline data center are

also installed.


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Fig : 12 Time-division multiplexing (TDM).

1.6.4 CODE-DIVISION MULTIPLEXING

Code division multiplexing (CDM) or spread spectrum is a class of techniques

where several channels simultaneously share the same frequency spectrum, and this

spectral bandwidth is much higher than the bit rate or symbol rate. One form is

frequency hopping, another is direct sequence spread spectrum. In the latter case, each

channel transmits its bits as a coded channel-specific sequence of pulses called chips. Number of chips per bit, or chips per symbol, is the spreading factor. This coded

transmission typically is accomplished by transmitting a unique time-dependent series

of short pulses, which are placed within chip times within the larger bit time. All

channels, each with a different code, can be transmitted on the same fiber or radio

channel or other medium, and asynchronously demultiplexed. Advantages over

conventional techniques are that variable bandwidth is possible (just as in statistical

multiplexing), that the wide bandwidth allows poor signal-to-noise ratio according to

Shannon-Hartley theorem, and that multi-path propagation in wireless communication

can be combated by rake receivers.

Code Division Multiplex techniques are used as an channel access scheme,namely Code Division Multiple Access (CDMA), e.g. for mobile phone service and in

wireless networks, with the advantage of spreading intercell interference among many

users. Confusingly, the generic term Code Division Multiple access sometimes refers

to a specific CDMA based cellular system defined by Qualcomm.

Another important application of CDMA is the Global Positioning System

(GPS).

1.7 PULSE CODE MODULATION

It was only in 1938, Mr. A.M. Reaves (USA) developed a Pulse CodeModulation (PCM) system to transmit the spoken word in digital form. Since then

digital speech transmission has become an alternative to the analogue systems. Pulse-

code modulation (PCM) is a method used to digitally represent sampled analog

signals. It is the standard form of digital audio in computers, Compact Discs, digital

telephony and other digital audio applications. In a PCM stream, the amplitude of the

analog signal is sampled regularly at uniform intervals, and each sample is quantized

to the nearest value within a range of digital steps

To develop a PCM signal from several analogue signals, the following

processing steps are required

• Filtering


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• Sampling

• Quantization

• Encoding

• Line Coding

Fig : 13 Basic operating steps in PCM

1.7.1 FILTERINGFilters are used to limit the speech signal to the frequency band 300-3400 Hz.

1.7.2 SAMPLINGThe process of generating pulses of zero width and of amplitude equal to the

instantaneous amplitude of the analog signal. The no. of pulses per second is called

“sampling rate”.

1.7.3 QUANTIZATIONIt is the process of dividing the maximum value of the analog signal into a

fixed no. of levels in order to convert the PAM into a Binary Code. The levels


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obtained are called “quantization levels”. The sample is quantified in 256 levels in 30

channel PCM.

A digital signal is described by its ‘bit rate’ whereas analog signal is described

by its ‘frequency range’.

*Bit rate = sampling rate x no. of bits / sample.

1.7.4 ENCODINGConversion of quantized analogue levels to binary signal is called encoding.

To represent 256 steps, 8 level code is required. The eight bit code is also called an

eight bit "word".

The 8 bit word appears in the form

P ABC WXYZ

Polarity bit ‘1’ Segment Code Linear encoding

for + ve 'O' for - ve. in the segment

The first bit gives the sign of the voltage to be coded. Next 3 bits gives the

segment number. There are 8 segments for the positive voltages and 8 for negative

voltages. Last 4 bits give the position in the segment. Each segment contains 16

positions. Referring to Fig.8, voltage Vc will be encoded as 1 111 0101.

Fig : 14 Encoding Curve with Compression 8 Bit Code

1.8 EVOLUTION OF DIGITAL MULTIPLEXING

The evolution of transport technology with the increase in bandwidth demand

is shown in Figure.


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The analog voice was digitized and the Plesiochronous Digital (PDH)

techniques were discovered for the transportation of information. Though, these

techniques were popular in the old days, the increasing demand for bandwidth proved

that these techniques have many drawbacks. The highest data rate available in PDH is

140 Mbps and the hardware required for multiplexing and demultiplexing of the

signal is much more than that of in SDH/SONET due to the Plesiochronous signals.

All these drawbacks of the PDH techniques carved the way for today’s SDH/SONETtechniques for information transportation over the telecom networks. Both SDH and

SONET techniques are widely used due to their efficiency and reliability. Today’s

metro area networks (MANs) are built on legacy SONET/SDH ring infrastructure and

both the SDH & SONET are used to transmit data over voice-optimized SDH/SONET

network resulting in the wastage of bandwidth. The SDH/SONET networks lack the

dynamic functionality and rapid scalability needed to cope-up with the increasing

volumes and unpredictable bandwidth demands. Also, due to the rigid multiplexing

hierarchies in the SDH/SONET standards, the customer cannot avail the flexible data

rates and has to pay more. The next available bandwidth in a SDH network after 10

Gbps is 40 Gbps. e.g. - A customer, who requires, says 20 Mbps, actually has tosubscribe to a 45 Mbps service because of the rigidity in the multiplexing hierarchy,

resulting in the wastage of bandwidth and ending up paying bill for 45 Mbps link.

Fig : 15 Transport Technologies Evolution

Also, customer may demand extra bandwidth for a limited period of time and

may again switch back to a low bandwidth service. The service activation and service

provisioning in both the cases should be quick enough to satisfy the customer’s

demands. The ports of SDH/SONET network elements are not programmable and the

bandwidth offered by these ports cannot be changed dynamically. If a subscriber

changes his bandwidth demand, the port from which he is getting the service needs to

be changed physically. This is very time-consuming. e.g. – An enterprise customer is

having a STM-4 connection initially and he needs to upgrade it to STM-16 for one


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month only. The service provisioning and service activation for this requirement

should be quick enough to fulfill the requirement of the customer in minimum time so

that his business is not affected and the customer enjoys the flexibility in the service.

Also, the time required to revert back to the original low bandwidth requirement

should be very less. Time required for designing, deploying and maintaining a

separate voice and data network is very high. To isolate and diagnose the faults

through a complex hierarchical network is a cumbersome task and the operationalexpenses to maintain these separate voice and data networks are very high as it needs

a larger workforce.

Considering the limitations of SDH/SONET, what is needed are ways to

manage data-service bandwidth dynamically in small increments, to provide a range

of service guarantees, and to engineer traffic flows more efficiently. So to improve

SDH/SONET into a new generation, while keeping its essential virtues, the main

technological focus is on devising new client-service encapsulations and scrapping the

traditional multiplexing/mapping scheme, replacing it with a more flexible alternative

within the basic SDH/SONET framing.

1.8.1 PLESIOCHRONOUS DIGITAL MULTIPLEXING

With the introduction of PCM technology in the 1960s, communications

networks were gradually converted to digital technology over the next few years. To

cope with the demand for ever higher bit rates, a multiplex hierarchy called the

Plesiochronous digital hierarchy (PDH) evolved. The bit rates start with the basic

multiplex rate of 2 Mbit/s with further stages of 8, 34 and 140 Mbit/s. In North

America and Japan, the primary rate is 1.5 Mbit/s. Hierarchy stages of 6 and 44

Mbit/s developed from this. Because of these very different developments, gateways

between one network and another were very difficult and expensive to realize. PCMallows multiple use of a single line by means of digital time-domain multiplexing.

The analog telephone signal is sampled at a bandwidth of 3.1 kHz, quantized and

encoded and then transmitted at a bit rate of 64kbit/s. A transmission rate of 2048

kbit/s results, when 30 such coded channels are collected together into a frame along

with the necessary signaling information. This so-called primary rate is used

throughout the world. Only the USA, Canada and Japan use a primary rate of 1544

kbit/s, formed by combining 24 channels instead of 30. The growing demand for more

bandwidth meant that more stages of multiplexing were needed throughout the world.

A practically synchronous (or, to give it its proper name: plesiochronous) digital

hierarchy is the result. Slight differences in timing signals mean that justification or

stuffing is necessary when forming the multiplexed signals. Inserting or dropping anindividual 64 kbit/s channel to or from a higher digital hierarchy requires a

considerable amount of complex multiplexer equipment.

Traditionally, transmission systems have been asynchronous, with each

terminal in the network running on its own clock. In digital systems, clocking (timing)

is one of the most important considerations. Timing means using a series of repetitive

pulses to keep the bit rate of the data stream constant and to indicate where the ones

and zeros are located in a data stream. Because these clocks are free running and not

synchronized, large variations occur in the clock rate and thus the signal bit rate.


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Fig : 16 Plesiochronous Digital Hierarchies (PDH)

The Plesiochronous Digital Hierarchy (PDH) signals have the essential

characteristics of time scales or signals such that their corresponding significant

instants occur at nominally the same rate. The prefix plesio, which is of Greek origin,

means “almost equal but not exactly,” meaning that the higher levels in the CCITT

(ITU today) hierarchy are not an exact multiple of the lower level. Any variation in

rate is constrained within specified limits. The PDH systems belong to the first

generation of digital terrestrial telecommunication systems in commercial use.Before SDH transmission networks were based on the PDH hierarchy. 2

Mbit/s service signals are multiplexed to 140 Mbit/s for transmission over optical

fiber or radio. Multiplexing of 2 Mbit/s to 140 Mbit/s requires two intermediate

multiplexing stages of 8 Mbit/s and 34 Mbit/s. Multiplexing of 2 Mbit/s to 140 Mbit/s

requires multiplex equipment known as 2nd, 3rd and 4th order multiplexer.

1.8.2 SYNCHRONOUS DIGITAL HIERARCHY

SDH is the international version of the standard published by the International

Telecommunications Union (ITU). The basic unit of framing in SDH is a STM-1

(Synchronous Transport Module, level 1), which operates at 155.520 megabits persecond (Mbit/s). Different SDH rates are given below:

STM-1 = 155.52 Mbit/s

STM-4 = 622.08 Mbit/s

STM-16 = 2588.32 Mbit/s

STM-64 = 9953.28 Mbit/s

SDH in detail will be discussed in subsequent chapters.


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1.9 CLASSIFICATION OF TRANSPORT NETWORK BY

GEOGRAPHY

One traditional approach to classifying transport networks is in relation to

their geographic scope. These classifications are illustrated in Figure. The access

network is that portion of the network that connects the end users (subscribers) to the

edge switching elements in the network. The metropolitan (metro) transport network

is the network that interconnects central offices (COs) within an urban/suburban

region. COs within a metro network are typically directly connected to both access

networks and core long distance networks. These metro COs are typically owned by

the same carrier, and in many cases either allow the carrier to centralize specialized

services (e.g. ISDN or Ethernet routing) in just one CO, or to use different COs for

back-up redundancy for each other (e.g., to take over switching functions in the event

of a failure of the primary CO for that subscriber). The span lengths between metro

COs are typically relatively short. The long distance core transport network provides

the interconnection between metro networks, smaller community COs, service

providers (e.g., Internet), and regional or international gateways. Higher bandwidthtechnology typically sees its first deployment in the core network since the longer

facility lengths necessitate more efficient utilization of the facilities. The technology

used in the core networks, however, typically eventually finds its way into the metro

network as the cost of technology decreases and the bandwidth needs of metro

networks increase. From the management, craft training, and equipment inventory

perspectives, it is desirable to have as much commonality as possible between core

and metro networks when they exist within the same carrier. LECs typically have both

metro networks and core networks to provide interconnection within their region.

IECs also typically have both metro and core networks since they often deploy metro

networks in order to more efficiently reach their business/corporate subscribers.

As shown in Figure, both metro and core transport networks can consist of

ring and mesh topologies. Rings have become increasingly popular since they provide

inherent route diversity that can be exploited for protection switching. (See City 1 and

upper portion of the core network.) Rings have also become increasingly popular in

access networks (e.g., City 3). Traffic routing on rings is also more straightforward

than in arbitrary mesh networks.

Ring topologies are not always convenient, however, due to such constraints

as geography or having to use pre-existing right of ways. Arbitrary mesh networks are

constructed in order to use convenient cable routings or, in some cases, allow more

bandwidth-efficient protection schemes. Transport networks often consist of a mix ofring and mesh sub-networks, including interconnected rings.


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Fig : 17 Illustrations of a telecommunications network

Traditionally, a sharp distinction was drawn between transmission and switching

equipment. For the purposes of this white paper, however, transmission and switching

are both considered as part of the transport network. The switches provide the

automatic routing of voice (or data) traffic, while the transmission equipment handled

the multiplexing and facility connections to carry the traffic between the switches. Forexample, a voice switch is the equipment to which a subscriber’s telephone is

connected that does the digit collection when the subscriber dials, and routes the call

according to the number that was dialed. Typical transmission equipment includes

SONET/SDH terminals. The distinction between transmission and switching has

continued to blur over the past 20 years. Transmission networks have increasingly

deployed digital cross connect systems (DCSs) that switch subscribers’ traffic

between the various DCS interfaces according to a provisioned route. DCS-type

cross-connect capability has increasingly been integrated into add-drop multiplexers

(ADMs).


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1.10 TRANSPORT NETWORK AND THE ROAD ANALOGY

The transport network is analogous to any road network of a country as shown

in Figure. The national highway of a country has a greater capacity for vehicle traffic

than the state highway and the city roads. The state highway has less vehicle traffic-

carrying capacity than the city road network. Analogous to this, the access part of a

transport network has less capacity than the metro network and the metro part has less

capacity than the core part of the transport network. The transport networks are

deployed using different technologies in the different parts of the network. DSL is a

popular technology deployed in the access network today. SDH/SONET technologies

are widely being used for the deployment of the Metro network and DWDM is used

for the core network.

Fig : 18 Transport Network and Road Analogy

1.11 COMPONENTS OF TRANSMISSION NETWORK

1.11.1 DIGITAL DISTRIBUTION FRAME

A Digital Distribution Frame (DDF) is the interface when coaxial cable has to be terminated, organized or cross-connected in long-distant transport networks, or in

access networks close to subscribers.

In fixed networks, a DDF is installed between the exchange and transmission

equipment, to mention one example. In mobile networks, DDFs can also serve as the

interface between an MSC (Mobile Services Switching Centre) or BSC (Base Station

Controller) and the transmission equipment.

75 ohm Digital Distribution Frames are used to terminate, cross-connect and

inter-connect 75 ohm coaxial cables, and to supervise digital transmission equipment.

In the DDF, signals can be extracted from the desired level to measure incoming and

outgoing signals, allowing the rearrangement or disconnection of traffic.


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Fig : 19 DDF

1.11.2

FIBER DISTRIBUTION FRAMEFiber distribution frame (FDF) provides efficient cable connections between

outside plant cable and equipment in the buildings and communication facilities. FDF

integrates fiber splicing, storage, and cable connections together in single unit.

Fig : 20

FDF


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1.11.3 TRANSMISSION EQUIPMENTSVarieties of transmission equipment are used for connecting the exchanges

(switches), routers etc. in telecom network. It includes-

Transmission lines

Optical fiber, Co-axial cable etc.

Microwave stations

Multiplexers

Communications satellites

1.11.4 CONNECTORSConnectors are vital elements in the Fibre Optics Technology. Connectors can

be defined as a remittable means of arranging transfer of optical energy from one fibre

optic component to another in an optical fibre system. The connector is a mechanical

device mounted on the end of a fiber optic cable, light source, receiver, or housing. It

allows it to be mated to a similar device. The transmitter provides the information- bearing light to the fiber optic cable through a connector. The receiver gets the

information-bearing light from the fiber optic cable through a connector. The

connector must direct light and collect light. It must also be easily attached and

detached from equipment.

There are many different connector types. Table 1 illustrates some types of

optical connectors and lists some specifications. Each connector type has strong

points. For example, ST connectors are a good choice for easy field installations; the

FC connector has a floating ferrule that provides good mechanical isolation; the SC

connector offers excellent packing density, and its push-pull design resists fiber end

face contact damage during unmating and remating cycles.

Table 1: Common Types of Fiber Optic Connectors

Connector Insertion Loss Fiber

Type

Applications

0.5--1.0 dB SM, MM Datacom, telecom

FC

0.15 db (SM)0.10 dB (MM)

SM, MM High-densityinterconnection, datacom,

telecomLC

0.3-1.0 dB SM, MM High-density

interconnection

MT Array


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0.2-0.45 dB SM, MM Datacom, telecom

SC

Type. 0.4 dB(SM)

Type. 0.5 dB

(MM)

SM, MM Inter-/intra-building,security

ST

1.12 DEVICES USED IN TRANSMISSION SYSTEMS

1.12.1 FILTERSIt is sometimes desirable to have circuits capable of selectively filtering one

frequency or range of frequencies out of a mix of different frequencies in a circuit.

Electronic filters are electronic circuits which perform signal processing functions,

specifically to remove unwanted frequency components from the signal, to enhance

wanted ones, or both.

A circuit designed to perform this frequency selection is called a filter circuit,

or simply a filter. Different types of filters by technology are as given below:

1.12.1.1 Passive Filters

Passive filters are based on combinations of resistors (R), inductors (L) and

capacitors (C) and they do not depend upon an external power supply and/or

they do not contain active components (amplifying elements) such as transistors,

operational amplifiers etc.).

Since they are not restricted by the bandwidth limitations of op amps, they canwork well at very high frequencies. They can be used in applications involving

larger current or voltage levels than can be handled by active devices.

Since they use no active elements, they cannot provide signal gain.

With respect of passed frequency in the filter, we have four categories of filters:

1.12.1.1.1 Low Pass Filter

By definition, a low-pass filter is a circuit offering easy passage to low-frequency

signals and difficult passage to high-frequency signals.

One use of Low Pass Filters (LPF) is to reduce high-frequency noise on signals.

The LPF is also used in the audio amplifiers to limit the maximum inputfrequency, usually to 20 kHz.


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1.12.1.1.2 High-pass filters

A high-pass filter's task is just the opposite of a low-pass filter: to offer easy passage of a high-frequency signal and difficult passage to a low-frequency signal.

High-pass filters have many uses, such as blocking DC from circuitry sensitive to

non-zero average voltages or RF devices; another use is to set the lowest usable

input signal for amplifiers, which is often 20 Hz for audio amps. They can also beused in conjunction with a low-pass filter to make a band-pass filter.

1.12.1.1.3 Band-pass filter

There are applications where a particular band, or spread, or frequencies need to

be filtered from a wider range of mixed signals.

Filter circuits can be designed to accomplish this task by combining the properties

of low-pass and high-pass into a single filter. The result is called a band-passfilter. Creating a band-pass filter from a low-pass and high-pass filter can be

illustrated using block diagrams:

It has got application in Microwave Systems, Microwave duplexers etc.

1.12.1.1.4 Notch or band-reject filter (Wien-Robinson bridge)

A filter with effectively the opposite function of the band-pass is the band-

reject or notch filter. Notch filters are used to remove an unwanted frequency

from a signal, while affecting all other frequencies as little as possible.

Band reject filters have been widely used in many microwave circuits and

systems.


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1.12.1.2 Active Filter

Active filters use amplifying elements, especially op amps, with resistors and

capacitors in their feedback loops, to synthesize the desired filter characteristics.

Active filters can have high input impedance, low output impedance, and virtually any

arbitrary gain.

Performance at high frequencies is limited by the gain-bandwidth product of

the amplifying elements, but within the amplifier's operating frequency range, the op

amp-based active filter can achieve very good accuracy, provided that low-tolerance

resistors and capacitors are used.

Active filters will generate noise due to the amplifying circuitry, but this can

be minimized by the use of low-noise amplifiers and careful circuit design.

1.12.1.3 Digital filters

A digital filter is programmable, i.e. its operation is determined by a program

stored in the processor's memory. This means the digital filter can easily be

changed without affecting the circuitry (hardware). An analog filter can only

be changed by redesigning the filter circuit. All digital filters utilize one or more previous inputs and/or outputs.

The current output is the average of the current input and the previous input.

A “moving average” filter, it has a low pass characteristic and a FiniteImpulse Response

The characteristics of analog filter circuits (particularly those containing active

components) are subject to drift and are dependent on temperature. Digital

filters do not suffer from these problems, and so are extremely stable with

respect both to time and temperature.

Unlike their analog counterparts, digital filters can handle low frequencysignals accurately. As the speed of DSP technology continues to increase,

digital filters are being applied to high frequency signals in the RF (radio

frequency) domain, which in the past was the exclusive preserve of analog

technology.

Digital filters are very much more versatile in their ability to process signals in

a variety of ways; this includes the ability of some types of digital filter to

adapt to changes in the characteristics of the signal.

1.12.1.4 Quartz filters and piezoelectrics

The biggest advantage of quartz is that it is piezoelectric. This means that quartz

resonators can directly convert their own mechanical motion into electrical

signals.


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Quartz also has a very low coefficient of thermal expansion which means that

quartz resonators can produce stable frequencies over a wide temperature range.

Quartz crystal filters have much higher quality factors than LCR filters.

1.12.1.5 SAW filters

SAW (surface acoustic wave) filters are electromechanical devices commonlyused in radio frequency applications.

Electrical signals are converted to a mechanical wave in a device constructed of a

piezoelectric crystal or ceramic; this wave is delayed as it propagates across the

device, before being converted back to an electrical signal by further electrodes.

SAW filters are limited to frequencies up to 3 GHz.

1.12.1.6 BAW filters

BAW (Bulk Acoustic Wave) filters are electromechanical devices. BAW filters

can implement ladder or lattice filters.

BAW filters typically operate at frequencies from around 2 to around 16 GHz.

1.12.1.7 Atomic filters

For even higher frequencies and greater precision, the vibrations of atoms must beused.

Atomic clocks use cesium masers as ultra-high Q filters to stabilize their primary

oscillators.

1.12.1.8 Optical filters

Optical filters selectively transmit light in a particular range of wavelengths, thatis, colors, while blocking the remainder.

They can usually pass long wavelengths only (long-pass), short wavelengths only(short-pass), or a band of wavelengths, blocking both longer and shorter

wavelengths (band-pass). The pass-band may be narrower or wider; the transition

or cutoff between maximal and minimal transmission can be sharp or gradual.

These types of filters are used in Optical Communication systems for narrowingdown the LASER.

1.12.2 WAVEGUIDES AND TRANSMISSION LINESWave guide and transmission line is important, not only for its loss

characteristics, which enter into the path loss calculation, but also for the degree of

impedance matching attainable, because of the effect on echo distortion noise. The

later becomes important with high-density systems having long waveguide runs.

1.12.2.1 Coaxial Transmission Lines

In bands up to 2 GHz, coaxial cable is usually used, and except for very short

runs, it is usually of the air dielectric type.

Typical sizes are: 2.2 cm. diameter.

Cables are flexible enough to provide direct connection at the rear of the antenna

provided that the mount allows direct access in horizontal plane. If the vertical run

of the coaxial cable is down the side of the tower away from the antenna, this can

be easily accomplished.


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1.12.2.2 Wave guides

Bands higher than 2 GHz require the use of waveguides almost exclusively andone of three basic types may be used rigid rectangular, rigid circular, and flexible

elliptical. The latter is of continuous construction, having the advantages of

minimizing the number of flange connection usually of two, one at the antenna

end, one at the equipment end.

1.12.2.3 Rectangular Guide

Rigid rectangular waveguide is the most commonly used, with oxygen-free, high

conductivity copper (OFHC), the recommended material. The types and

approximate characteristics are as follows:

4 GHz band: WR 229 is standard for most installations. It has a lossof approximately 2.79 dB per 100 meters.

6 GHz band: WR 137 is normally used. It has a loss of approximately

6.6 dB per 100 meters. In cases where, due to high towers, a reduced

transmission loss is required, transitions can be supplied for use with

WR 159, which has a loss of about 4.6 dB per 100 meters. 7-8 GHz: WR 112 is normally used. Attenuation is approximately 8.8

dB per 100 meters.

11 GHz: WR 90 is normally used. Attenuation is approximately 11.5

dB per 100 meters.

12-13 Ghz: WR 75 is normally used. Attenuation is approximately14.7 dB per 100 meters.

This wave guide is recommended, where extremely low VSWR is required.

1.12.2.4 Circular Guide

Circular waveguide has the lowest loss of all, and in addition, it can supporttwo orthogonal polarizations within the single guide.

It is also capable of carrying more than one frequency band in the same guide.

For example, WC 281 circular, guide is normally used with horn reflector

antennas to provide two polarizations at 6 GHz.

But circular guide has certain disadvantages. It is practical only for straight

runs, requires rather complicated and extremely critical networks to make the

transitions from rectangular to circular and can have significant moding

problems, when the guide is large enough to support more than one mode for

the frequency range in use.

1.12.2.5

Elliptical Guide Semi-flexible elliptical waveguide is available in sizes comparable to most of

the standard rectangular guides, with attenuations differing very little from the

rectangular equivalents.

The distinctive features of elliptical guide are that it can be provided andinstalled as a single continuous run, with no intermediate flanges. When

carefully transported and installed it can provide good VSWR performance but

relatively small deformations can introduce enough impedance mismatch to

produce severe echo distortion noise. However, usually the effect of small

deformations can be 'tuned' out.

The most commonly used types and their approximate characteristics are as follows:


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4 GHz band : EW - 37 Approximately 2.8 dB per 100 meters

6 GHz band : EW – 56 Approximately 5.7 dB per 100 meters

7-8 GHz band : EW – 71 Approximately 8.2 dB per 100 meters

11 GHz band : EW – 107 Approximately 12.1 dB per 100 meters

12-13 GHz band : EW – 122 Approximately 14.7 dB per 100 meters

All attenuation figures given at mid band.

1.12.3 ANTENNAS:

The words antenna (plural: antennas) and aerial are used interchangeably; but

usually a rigid metallic structure is termed an antenna and a wire format is called an

aerial. Antennas have practical uses for the transmission and reception of radio

frequency signals such as radio and television. In air, those signals travel very quickly

and with a very low transmission loss. The signals are absorbed when moving through

more conductive materials, such as concrete walls or rock. When encountering an

interface, the waves are partially reflected and partially transmitted through. Different

types of antenna and their application are as given in table:

Sl.

No.

Type of Antenna Applications

1.

Monopole antennas used for Broadcasting, car

radio and mobile communications commonly rely

on the body of the vehicle to provide the ground

plane.

2.

A common example of a dipole is the "rabbit ears"

television antenna found on broadcast television

sets

3. Loop antennas are frequently used for receiving

applications such as pagers, radio direction finding

and as field strength probes used in wireless

measurements.


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4. This antenna is suitable for vehicular

communication when the antenna is deployed on

the roof top of the vehicle and as a RFID (Radio-

frequency identification) tag antenna.

5. The helix antenna is a travelling wave antenna,

which means the current travels along the antenna

and the phase varies continuously. It has a wide

bandwidth. The antenna produces radio waveswith circular polarisation. In the axial mode, the

helix dimensions are at or above the wavelength of

operation. It is suitable for applications in

broadband satellite communications.

6.

The radiation field of this mode is elliptically polarized in all directions. The normal mode or

broadside helix, the dimensions of the helix (the

diameter and the pitch) are small compared with

the wavelength. It is used for GPS.

7.

The horn antenna is used in the transmission and

reception of RF microwave signals, and the

antenna is normally used in conjunction with

waveguide feeds. One particular use of horn

antennas themselves is for short range radar

systems.

8.

A Yagi-Uda array, commonly known simply as a

Yagi antenna, is a directional antenna. A Yagi-Uda

antenna is familiar as the commonest kind of

terrestrial TV antenna to be found on the rooftops

of houses. It is usually used at frequencies between

about 30MHz and 3GHz.

9. Parabolic antennas are used as high-gain antennas

for point-to-point communications, in applications

such as microwave relay links that carry telephone

and television signals between nearby cities,

wireless WAN/LAN links for data

communications, satellite communications and

spacecraft communication antennas.


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10 Cassegrain: In this antenna the feed is located on

or behind the dish. Advantage of this configuration

is that the feed, with it’s and front end electronic

does not have to be suspended in front of this, so it

is used for antenna with bulky feed such as large

satellite communication antenna and radio

telescope. Aperture efficiency is on the order of

65-70%.

Gregorian: Similar to the Cassesgrain design

except that the secondary reflector is concave in

the shape. Extensive terrestrial microwave links,

such as those between cell phone base stations,

and wireless WAN/LAN applications have also

proliferated this antenna type. Earlier applications

included ground-based and airborne radar and

radio astronomy. The aperture efficiency over 70%

can be achieved.1.12.4 AMPLIFIERSAn electronic amplifier, amplifier, or (informally) amp is an electronic device

that increases the power of a signal. It does this by taking energy from a power supply

and controlling the output to match the input signal shape but with larger amplitude.

In this sense, an amplifier modulates the output of the power supply.

There are many types of electronic amplifiers, commonly used in radio and

television transmitters and receivers, high-fidelity ("hi-fi") stereo equipment,

microcomputers and other electronic digital equipment, and guitar and other

instrument amplifiers. Critical components include active devices, such as vacuum

tubes or transistors. A brief introduction to the many types of electronic amplifier

follows.

1.12.4.1 Valve amplifier

According to Symons, while semiconductor amplifiers have largely displaced

valve amplifiers for low power applications, valve amplifiers are much more cost

effective in high power applications such as "radar, countermeasures equipment, or

communications equipment". Many microwave amplifiers are specially designed

valves, such as the klystron, traveling wave tube, and these microwave valves provide

much greater single-device power output at microwave frequencies than solid-state

devices

1.12.4.2

Transistor amplifiers

The essential role of this active element is to magnify an input signal to yield a

significantly larger output signal. The amount of magnification (the "forward gain") is

determined by the external circuit design as well as the active device. Many common

active devices in transistor amplifiers are bipolar junction transistors (BJTs) and metal

oxide semiconductor field-effect transistors (MOSFETs). Applications are numerous;

some common examples are audio amplifiers in a home stereo or PA system, RF high

power generation for semiconductor equipment, to RF and Microwave applications

such as radio transmitters.


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1.12.4.3 Operational amplifiers (op-amps)

An operational amplifier is an amplifier circuit with very high open loop gain

and differential inputs that employs external feedback to control its transfer function,

or gain. Though the term today commonly applies to integrated circuits, the original

operational amplifier design used valves.

1.12.4.4

Fully differential amplifiers

A fully differential amplifier is a solid state integrated circuit amplifier that

uses external feedback to control of its transfer function or gain. It is similar to the

operational amplifier, but also has differential output pins. These are usually

constructed using BJTs or FETs.

1.12.4.5 Video amplifiers

These deal with video signals and have varying bandwidths depending on

whether the video signal is for SDTV, EDTV, HDTV 720p or 1080i/p etc.. The

specification of the bandwidth itself depends on what kind of filter is used — and at

which point (-1 dB or -3 dB for example) the bandwidth is measured. Certainrequirements for step response and overshoot are necessary for an acceptable TV

image.

1.12.4.6 Oscilloscope vertical amplifiers

These deal with video signals that drive an oscilloscope display tube, and can

have bandwidths of about 500 MHz. The specifications on step response, rise time,

overshoot, and aberrations can make designing these amplifiers difficult. One of the

pioneers in high bandwidth vertical amplifiers was the Tektronix Company.

1.12.4.7 Distributed amplifiers

These use transmission lines to temporally split the signal and amplify each

portion separately to achieve higher bandwidth than possible from a single amplifier.

The outputs of each stage are combined in the output transmission line. This type of

amplifier was commonly used on oscilloscopes as the final vertical amplifier. The

transmission lines were often housed inside the display tube glass envelope.

1.12.4.8 Switched mode amplifiers

These nonlinear amplifiers have much higher efficiencies than linear amps,

and are used where the power saving justifies the extra complexity.

1.12.4.9 Microwave amplifiers

1.12.4.9.1 Travelling wave tube amplifiers

Traveling wave tube amplifiers (TWTAs) are used for high power

amplification at low microwave frequencies. They typically can amplify across a

broad spectrum of frequencies; however, they are usually not as tunable as klystrons.

1.12.4.9.2 Klystrons

Klystrons are are specialized linear-beam vacuum-devices, designed to

provide high power, widely tunable amplification of millimetre and sub-millimetre

waves. Klystrons are designed for large scale operations and despite having a

narrower bandwidth than TWTAs, they have the advantage of coherently amplifying a


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reference signal so its output may be precisely controlled in amplitude, frequency and

phase.

1.12.4.10 Musical instrument amplifiers

An audio power amplifier is usually used to amplify signals such as music or

speech. Several factors are especially important in the selection of musical instrument

amplifiers (such as guitar amplifiers) and other audio amplifiers (although the whole

of the sound system – components such as microphones to loudspeakers.

1.12.4.11 Optical Amplifier

An optical amplifier is a device which amplifies the optical signal directly

without ever changing it to electricity. The light itself is amplified. There are various

types of optical amplifiers are used in optical communication:

1.12.4.11.1 Semiconductor optical amplifiers

Semiconductor optical amplifiers are similar in construction to semiconductor

lasers. Optical gain occurs as excited electrons in the semiconductor material arestimulated by incoming light signals. The gain is usually sufficient for single channel

operation but in a WDM system you usually want up to a few mW per channel.

1.12.4.11.2 Erbium-doped fiber amplifier (EDFA)

Erbium-doped fiber amplifier (EDFA) most common commercially available

optical amplifier since the early 1990’s and works best in the range 1530 to 1565 nm

And the gain is up to 30dB.

Amplification is achieved by stimulated emission of photons from dopant ions

in the doped fibre.

1.12.4.11.3

Raman amplifierIn a Raman amplifier, the signal is intensified by Raman amplification. Unlike

the EDFA and SOA the amplification effect is achieved by a nonlinear interaction

between the signal and a pump laser within an optical fibre. There are two types of

Raman amplifier: distributed and lumped. A distributed Raman amplifier is one in

which the transmission fibre is utilised as the gain medium by multiplexing a pump

wavelength with signal wavelength, while a lumped Raman amplifier utilises a

dedicated, shorter length of fibre to provide amplification.

1.12.5 SOURCES AND DETECTORSIn optical communication sources are used at the transmitter end to convert

electrical signal to optical and the detectors are used to convert optical signal tooptical signal at the receiving end of optical fiber system. Typical sources are LED

and LASER diode. APD and PIN diodes are used as a detector.

1.13 GENERAL INSTALLATION PRACTICE FOR

TRANSMISSION EQUIPMENT

The following chronological order is generally used for installation of any equipment

work.

1.

Receipt of equipment and safety keeping in stores.

2. Opening of packing cases and check of items as per packing list.


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3. Safety transportation of equipment to each site and with proper equipment

configuration.

4.

Physical installation of steel iron structure as per approved lay out plan

(Location, Alignment & Rigidity). The distance between two suits may taken

1.5 meter. Keep the proper distance from wall to handle the eqpt. and to keep

testing instruments/LCT near eqpt. The equipment room should be provided

with antistatic facilities and the floor supports properly grounded, with thegrounding resistance and antistatic facilities meeting relevant requirements.

Make sure that the grounding cables are laid as designed, the air-conditioner

has been installed and is in good condition, and the corridors of the

equipment room are clean.

5. The room should have sufficient space for the equipment and its maintenance

paths. There should be more than 800mm space between two rows of racks

and between wall and racks for operators to open the racks.

6. Main earthing work along runways, Main DC box and suit DC box

installation.

7.

Physical installation of FDF, Eqpt racks, DDF.8.

Mounting of Power supply S/R, Equipment Subrack.

9. Earthing and station DC power supply cabling to each Rack and Subrack.

10. Insertion of modules/ cards in Proper slots/ positions with proper tool.

11. EOW installation and its cabling.

12. Station Alarm wiring to Main Alarm Panel.

13. External Clock wiring for synchronization of Equipment.

14. 2 Mbps Signal from Equipment subrack 2 mb interface to DDF rack 120

ohms.

15. Powering of equipment.

16.

Configuration/ provisioning of equipment.( Node name, Node IP/NSAP

address, modules configuration, cross connections, Synchronization, Nodemap, Protection etc.)

17. Local and through testing.

18.

Offering for A/T.

19. Loading of Traffic.

1.14 ROUTINE MAINTENANCE

Initiatives to be taken by maintenance personnel in the best interest of the

system’s health

1.14.1 ROLE OF MAINTENANCE PERSONNELKeeping a watch on the system's health, trouble fixing and programming

periodic routining strategy in advance form the major functions to be performed by

the maintenance personnel. In addition to above, following functions also require

human attention:

1. Co-ordination with remote station for trunk testing.

2. Providing the necessary feedback to the support centre.

3. Day to day logging of important observations and maintenance actions.

1.14.2 WATCH ON SYSTEM'S HEALTHThis involves ensuring periodic dump of desired information, scanning

reports generated by the system and verifying systems integrity with a view to


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uncover any abnormalities in system's behavior, and being vigilant towards the

audio-visual alarms raised by the system

System, on its own initiative, keeps generating various reports regarding

system’s health as and when significant events take place. Maintenance personnel too

can programme the system in advance, for generating various periodic reports

including the following. Such reports are to be scanned daily to enable them to track

system's health on a day-to-day basis.• Scanning Spontaneously Generated Reports

• Verifying S

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Digital Transmission Technology an Overview

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