PRACTICAL ASSIGNMENT FUNDAMENTALS OF ELECTRONIC COMMUNICATION 3

HISTORICAL AND PRACTICAL ACCEPTS OF TELEPHONE NETWORKSPRACTICAL ASSIGNMENT 015/07

PART 13.1

Instructor has given a bread board, a batery (power supply) wires, four resistors and two telephone instruments. The resistors were connected to the bread board as shown in the above diagram. Two telephones were connected with two wore per each. The power supply or the battery connected as it acts as a central battery. When telephone 1 and 2 off hook the handsets both can talk each other. But now dial tones. I.e. because the two telephones were not connected to an exchange.3.2The handset is the physical unit that houses the earpiece (receiver) and mouthpiece (transmitter). The earpiece contains the receiver capsule that converts electricity to sound. The mouthpiece holds the capsule for converting sound to electricity. A four-wire cord connects the handset to the base unit of the telephone.

The touch pad is a modern device for sending information about the telephone number called to the central office of the telephone company. As you press a button, tones are generated representing the number you desire.The touch pad codes your called number as tones for transmission to the central office.

Ringers serve to announce incoming calls.At idle with no current in the electromagnetic coils, the armature is in balance over the pivot. When AC current is applied to the coils, this balance is disturbed and ringing begins. Each half cycle of the current causes one electromagnetic coil to pull the armature down with magnetism. Each reversal in the current causes the other electromagnetic coil to pull. The armature now rocks back and forth with the applied current. The hammer attached to the armature first hits one gong and then the other, generating the ringing sound.

The hookswitch is the ON/OFF switch for the telephone set. It controls the circuit paths to the telephone set.When the phone is ON HOOK a circuit is closed to the telephone set ringer and the rest of the telephone set is disconnected from the telephone line. This indicates to the central office of the telephone company that the phone is idle and there is no current flow through this set.When the phone is OFF HOOK the ringer is disconnected and the rest of the set is connected. A circuit path is now connected to the central office for current to flow. Dialing and conversation takes place while the set is off hook.

The transmitter is a capsule unit that sits inside the telephone handset. This capsule serves to convert the sound of your voice into electricity for transmission over the line.The diaphragm in the capsule vibrates in response to sound waves and transfers this movement back and forth to the cell of carbon granules. The vibration of the diaphragm alternately compresses and releases pressure, changing the electrical resistance of the cell of carbon granules. As the carbon granules are compressed the resistance of the cell decreases and the current increases. As the pressure is released the resistance increases and the current through the cell decreases. This varying current flow is carried by the telephone circuit to the distant telephone and converted back to sound

The receiver is another capsule unit that inside the handset of the telephone. It can be thought of as a small speaker. It functions by converting electricity back into sound waves. During use, the varying current produced in the transmitter is directed to the receiver and flows through the two electromagnetic coils. This varying current flow generates a fluctuating magnetic field in the coils. This fluctuating magnetism either aids or opposes the permanent magnet and causes the diaphragm to vibrate in step with the varying electrical current. The vibrating diaphragm reproduces the sound waves which we hear. The electric current is thereby converted back to sound.

PART 23.3.1 HYSTORICAL ASPECTS OF DEVELOPMENTS IN THE PSTNThepublic switched telephone network(PSTN) is the aggregate of the world'scircuit-switchedtelephone networksthat are operated by national, regional, or localtelephonyoperators, providing infrastructure and services for publictelecommunication. The PSTN consists oftelephone lines,fiber optic cables,microwave transmissionlinks,cellular networks,communications satellites, andundersea telephone cables, all interconnected byswitching centers, thus allowing any telephone in the world to communicate with any other. Originally a network of fixed-line analogtelephone systems, the PSTN is almost entirelydigitalin its core and includesmobileas well asfixedtelephones.The technical operation of the PSTN adheres to the standards created by theITU-T. These standards allow different networks in different countries tointerconnectseamlessly. TheE.163andE.164standards provide a single globaladdress spacefor telephone numbers. The combination of the interconnected networks and the single numbering plan make it possible for any phone in the world to dial any other phone.The first telephones had no network but were in private use, wired together in pairs. Users who wanted to talk to different people had as many telephones as necessary for the purpose. A user who wished to speak whistled as loudly as possible into the transmitter until the other party heard.However, a bell was added soon for signaling, and then a switch hook and telephones took advantage of the exchange principle already employed in telegraph networks. Each telephone was wired to a localtelephone exchange, and the exchanges were wired together withtrunks. Networks were connected in a hierarchical manner until they spanned cities, countries, continents and oceans. This was the beginning of the PSTN, though the term was not used for many decades.Automation introducedpulse dialingbetween the phone and the exchange, and then among exchanges, followed by more sophisticated address signaling includingmulti-frequency, culminating in theSS7network that connected most exchanges by the end of the 20th century.The growth of the PSTN meant thattraffic engineeringtechniques needed to be deployed to deliverquality of service(QoS) guarantees for the users. The work ofA. K. Erlangestablished the mathematical foundations of methods required to determine the capacity requirements and configuration of equipment and the number of personnel required to deliver a specific level of service.In the 1970s the telecommunications industry began implementingpacket switched networkdata services using theX.25protocol transported over much of the end-to-end equipment as was already in use in the PSTN.In the 1980s the industry began planning for digital services assuming they would follow much the same pattern as voice services, and conceived a vision of end-to-end circuit switched services, known as theBroadband Integrated Services Digital Network(B-ISDN). The B-ISDN vision has been overtaken by thedisruptive technologyof the Internet.At the turn of the 21st century, the oldest parts of the telephone network still use analog technology for thelast mileloopto the end user. Digital services have been increasingly rolled out to end users using services such asDSL,ISDN,FTTx, andcable modemsystems.Several large private telephone networks are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies which are linked to the PSTN only through limitedgateways, like a largeprivate branch exchange(PBX).3.3.2 REASONS FOR THE EVALUATION IN VOCE SWITCHING TECHNOLOGYCircuit switching technology evaluationSwitches are used to build transmission path between telephones set on a flexible basis. Without switches, each telephone set would require a direct, dedicated circuit to every other telephone set in order to be able to communicate. This is a full-mesh physical topology network. Such a full mesh network clearly is resource-intensive, impractical and even impossible, as early experience proved. Circuit Switching were developed for voice communications. Contemporary circuit switches provide continuous access to logical channels over high-capacity physical circuits for the duration of the conversation.

Manual Telephone Switchboards

The first generation of telephone exchange used Manual Switchboard. These telephone exchange involved operators who manually established the desired connection at the request of the transmitting party. A unique physical and electrical connection was established, on aplug and jackbasis, for the duration of the call. When the conversation was finished and either party disconnected, the operator was alerted and was responsible for manually disconnecting the circuit, which became available for other use. The size of such switches, the complexity of interconnecting long distance calls across multiple switches, and the labor intensity of this approach all contributed to their demise.Step-by-step Switches

Second generation of telephone exchange used electromechanical step-by-step switches. Step-by-step Switches telephone exchange consists of large number ofline findersto which groups of individual subscribers are assigned for dial tone. The transmitting party dials a series of numbers, originally with a rotary telephone terminal which causes the making and breaking of an electrical circuit. Those electrical pulses cause successive mechanicalline selectorsto click across contacts to set up the conversation path as the complete number is dialed.

Common Control Crossbar Switches

Third generation of telephone exchange used Common Control Crossbar Switches. A request for dial tone is recognized by amarker, which directs asenderto store the dialed digits to a Register. A translatoris then directed to route the call, reserving a path through aswitching matrix. Once the call is connected, these various components are released and are available to serve other calls. Compared to the Step-by-step Switch, the Crossbar Switch had relatively few moving parts. Crossbar Switch offered the advantages of increased intelligence, common control, and greater speed of connection, smaller footprint, lower maintenance, and greater capacity.Electronic Common Control (ECC) Switches

Forth generation of telephone exchange used Electronic Common Control (ECC) switches. It reflected the marriage of telephony and computer technology. Voice conversations are digitized and switched over high-speed digital circuits, with all processes accomplished through programmed logic. ECC switches are microprocessor controlled, with the total processing power of such a switch often rivaling that of a general purpose, mainframe computer.ECC Switches offer lower maintenance costs and can, in fact, be monitored and managed from a remote location.Advantages of ECC Switches ECC Switches increases intelligence, greater speed of call setup and overall call processing, and a still smaller footprint. ECC Switches offer lower maintenance costs and can, in fact, be monitored and managed from a remote location. Many contemporary ECC switches are unmanned in favor of control from a centralized As specialized, software controlled software systems, their functionality and feature content often can be upgraded through additional software and/or firmware. Such switches generally process the ability to switch data and video , as well as voice ECC switches can be interface with various application processors to further increase of the range of services provide. Such application processors might include voice processors and fax servers.

3.4 BASIC SWITCHING TECHNOLOGIES3.4.1 STROWGERTheStrowger switchwas the first example of an electromechanicalstepping switchtelephone exchange system. It was invented byAlmon Brown Strowger, and first patented in 1891. Because of its operational characteristics it is also known as astep-by-step(SXS) switch.The Strowger switch used two telegraph type keys on a telephone set for dialing. Each key required a separate wire to the exchange. The keys were tapped to step the switch in two stages. The first set of incoming pulses raises the armature of an electromagnet to move a shaft which selects the desired level of contacts, by engaging a pawl with the upper ratchet. Another pawl, pivoting on the frame, holds the shaft at that height as it rotates. The second set of pulses, from the second key, operates another electromagnet. Its pawl engages the (hidden) vertical teeth in the lower ratchet to rotate the shaft to the required position. It is kept there against spring tension by a pawl pivoted on the frame. When the switch returns to its home position, typically when a call is complete, a release magnet disengages the pawls that hold the shaft in position. An interlock ensures that the spring on the shaft rotates it to angular home position before it drops to its home position by gravity.The commercial version of the Strowger switch, as developed by the Strowger Automatic Telephone Exchange Company, used arotary dialfor signaling to the exchange3.4.2 CROSSBARInelectronics, acrossbar switch(also known ascross-point switch,cross point switch, ormatrix switch) is aswitchconnecting multiple inputs to multiple outputs in a matrix manner. Originally the term was used literally, for a matrix switch controlled by a grid of crossing metal bars, and later was broadened to matrix switches in general. It is one of the principal switch architectures, together with arotary switch, memory switch and acrossover switchA crossbar switch is an assembly of individual switches between multiple inputs and multiple outputs. The switches are arranged in a matrix. If the crossbar switch has M inputs and N outputs, then a crossbar has a matrix withMNcross-points or places where the "bars" cross. At each cross point is a switch; when closed, it connects one ofMinputs to one ofNoutputs. A given crossbar is a single layer, non-blocking switch. "Non-blocking" means that other concurrent connections do not prevent connecting an arbitrary input to any arbitrary output. Collections of crossbars can be used to implement multiple layer and/or blocking switches. A crossbar switching system is also called a co-ordinate switching system.3.4.3 ELECTRONICAn electronic device in which one or more input signals can be routed to one or more outputs by the application of the appropriate electrical control signals. The term is most often applied when analog signals are involved, but the terminology is occasionally used when digital signals are involved.Conceptually, an electronic switch can be visualized as a group of one or more mechanical electrical switches (such as light switches used in commercial wiring or toggle switches used in many electronic control panels) in which, instead of mechanically opening or closing the contacts, the physical opening and closing is achieved by applying appropriate electrical control signals to separate terminals on the switch in much the same way that a relay performs. The electronic switch does not contain mechanical contacts but semiconductor devices such as bipolar junction transistors or field-effect transistors. The basic electronic switch is depicted in illus.a.AandBare the terminals of the switch. When a control signal is applied toVC, the switch closes. When the electronic switch is closed, a smallresidual resistanceRsremains between the terminals as depicted in the simple model of illus.b. The value of this resistance is termed the on resistance. In most applications the nonzero on resistance does not prove problematic, but the user needs to be aware of this limitation. The electronic switch is typicallybidirectionalin the sense that the terminalsAandBare interchangeable.See alsoElectric switch;Relay;Transistor.

Switching devices. (a) Symbol for electronic switch. (b) Simple model of electronic switch.

Electronic switches can be very small, allowing a large number of these devices to be placed in a small area they can be very fast, with on and off response times which are orders of magnitude faster than can be achieved with mechanical counterparts; and they are considerably more reliable over a large number of cycles than their mechanical counterparts.3.4.4 DIGITALA digital switch is a device that handles digital signals generated at or passed through a telephone companycentral officeand forwards them across the company'sbackbone network. It receives the digital signals from the office'schannel banks that have been converted from users' analog signals and switches them with other incoming signals out to the wide area network.Digital switches are described in terms of classes based on the number of lines and features that are provided. A private branch exchange (PBX) is a digital switch owned by a private company. ACentrexis a digital switch at the central office that manages switching for the private company from the central office.

PART 3Routing in the PSTNis the process used to routetelephone callsacross thepublic switched telephone network. This process is the same whether the call is made between two phones in the same locality, or across two different continents.Each time a call is placed for routing, thedestination number(also known as the called party) is entered by the calling party into their terminal. The destination number generally has two parts, aprefixwhich generally identifies the geographical location of the destination telephone, and a number unique within that prefix that determines the specific destination terminal. Sometimes if the call is between two terminals in the same local area (that is, both terminals are on the sametelephone exchange), then the prefix may be omitted.When a call is received by an exchange, there are two treatments that may be applied: Either the destination terminal is directly connected to that exchange, in which case the call is placed down that connection or the destination terminal rings. Or the call must be placed to one of the neighboring exchanges through a connecting trunk for onward routing.Each exchange in the chain uses pre-computed routing tables to determine which connected exchange the onward call should be routed to. There may be several alternative routes to any given destination, and the exchange can select dynamically between these in the event of link failure orcongestion.The routing tables are generated centrally based on the knowntopology of the network, thenumbering plan, and analysis oftraffic data. These are then downloaded to each exchange in the telephonenetwork. Because of the hierarchical nature of the numbering plan, and its geographical basis, most calls can be routed based only on their prefix using these routing tables.Some calls however cannot be routed on the basis of prefix alone, for examplenon-geographic numbers, such astoll-free or free phone calling. In these cases theIntelligent Networkis used to route the call instead of using the pre-computed routing tables.In determining routing plans, special attention is paid for example to ensure that two routes do not mutually overflow to each other; otherwise congestion will cause a destination to be completely blocked.According toBraess' paradox, the addition of a new, shorter, and lower cost route can lead to an increase overall congestion.[1]The network planner must take this into account when designing routing paths.Telephone switch hierarchyIn order to organizeDirect Distance Dialing(DDD)American Telephone & Telegraphdivided the various switches in the U.S. public switched telephone network (PSTN) into anoffice classificationhierarchy containing five levels (classes). Class 1exchanges were international gateways - handing off and receiving traffic from outside the USA and Canadian networks. Class 2exchanges were tandem exchanges which interconnected whole regions of the AT&T network. Class 3exchanges were tandem exchanges connecting major population centers within a particular region of the AT&T network. Class 4exchanges were tandem exchanges connecting the various areas of a city or towns in a region. Class 5exchanges were those to which subscribers and end-users telephone lines would connect.In modern times only the terms Class 4 and Class 5 are much used, as any tandem office is referred to as a Class 4. This change was prompted by changes in the power of switches and the relative cost of transmission, both of which tended to flatten the switch hierarchy.Routing calls requires multiple switching offices. The phone number itself is a coded map for routing the call. In the United States, for example, we have 10-digit phone numbers. The first three digits are the area code or national destination code (NDC), which helps route the call to the right regional switching station. The next three digits are the exchange, which represents the smallest amount of circuits that can be bundled on the same switch. In other words, when you make a call to another user in your same exchange -- maybe a neighbor around the corner -- the call doesn't have to be routed onto another switch. The last four digits of the phone number represent the subscriber number, which is tied to your specific address and phone lines.Within a company or larger organization, each employee or department might have its own extension. Extensions from the main phone number are routed through something called a private branch exchange (PBX) that operates on the premises.To make an international call requires further instructions. The call needs to be routed through your long-distance phone carrier to another country's long-distance phone carrier. To signal such a switch, you have to dial two separate numbers, your country's exit code (or international access code) and the corresponding country code of the place you're calling.3.5.1 LOCAL EXCHANGE (CENTRAL OFFICE/CLASS 5 OFFFICE)AClass 5 telephone switchis atelephone switchortelephone exchangein thepublic switched telephone networklocated at thelocal telephone company's central office, directly servingsubscribers. Class 5 switch services include basicdial-tone,calling features, and additional digital and data services to subscribers using thelocal loop. Class 5 switches were slower to convert fromswitching technologies totime division multiplexingthan the other switch classes.Typically a Class 5 switch covers an area of a city, an individual town, or several villages and could serve from several hundred to 100,000 subscribers.3.5.2 LOCAL TENDAM EXCHANGE (CLASS 4 OFFICE)AClass 4, orTandem,telephone switchis a U.S.telephone companycentral officetelephone exchangeused to interconnectlocal exchange carrieroffices forlong distancecommunications in thepublic switched telephone network.A Class 4 switch doesn't connect directly to any telephones; instead, it connects to other Class 4 switches and toClass 5 telephone switches. The telephones of servicesubscribersare wired to Class 5 switches. When a call is placed to a telephone that is not on the same Class 5 switch as the subscriber, the call may be routed through one or more Class 4 switches to reach its destination.In the past, most of theaccounting, billing management, andcall record-keepingwas handled by the tandem switches. During the last third of the 20th century, these tasks were performed by the Class 5 end-office switches.

3.5.3 REGIONAL TENDEM EXCHANGE (CLASS 3 OFFICE)The class 3 office was the Primary Center (PC). Calls being made beyond the limits of a small geographical area where circuits are not connected directly between class 4 toll offices would be passed from the toll center to the primary center. These locations use high usage trunks to complete connection between toll centers. The primary center never served dial tone to the user. The number of primary centers in the network fluctuated from time to time, ranging between 150 and 230.3.5.4 NATIONAL TENDEM EXCHANGE (CLASS 2 OFFICE)The class 2 office was the Sectional Center (SC). The sectional center typically connected major toll centers within one or two states or provinces, or a significant portion of a large state or province, to provide interstate or interprovincial connections for long-distance calls. At various times, there were between 50 and 75 active class two offices in the network.3.5.5 INTERNATIONAL GATEWAYIntelephony, aninternational gateway exchangeis atelephone switchthat forms the gateway between a national telephone network and one or more other international gateway exchanges, thus providing cross-border connectivity.RequirementsWhereas international gateway exchanges are commonly implemented using hardware that could also serve to build aClass 4(national transit) switch, some of the differences between an international gateway exchange and a Class 4 switch include: International variants of signaling protocols, such as InternationalISUPand#5, in addition to the relevant national signaling protocols. Support forecho cancellers. Support forDCME Support for international accounting and settlement agreements. Support forA-law/mu-lawtranscoding High capacity (some of the largest telephone exchanges in the world are international gateway exchanges). Support for thenumbering plansof each of the countries that may be dialed. Advancedtraffic routingcapabilities, in order to take advantage of the best available tariffs for each destination.

3.6 SOFTWARE AND HARDWARE SYSTEMS USED TO SWITCH AND CONTROL CALL CONNECTIONS IN DIGITAL EXCHANGES3.6.1 DIGITAL SWITCHING SUBSYSTEMThe Digital Local Exchange (DLE) connects to the concentrator and routes calls to different DLEs or DMSUs depending on the destination of the call. The heart of the DLE is the Digital Switching Subsystem (DSS) which consists of Time Switches and a Space Switch. Incoming traffic on the 30 channel PCM highways from the Concentrator Units is connected to Time Switches. The purpose of these is to take any incoming individual Time Slot and connect it to an outgoing Time Slot and so perform a switching and routing function. To allow access to a large range of outgoing routes, individual Time Switches are connected to each other by a Space Switch. The Time Slot inter-connections are held in Switch Maps which are updated by Software running on the Processor Utility Subsystem (PUS).3.6.2 MAN MACHINE INTERFACE SUBSYSTEMMMI Man-Machine Interface MMIS Man-Machine Interface Subsystem. The MMIS allows engineers to control and manage the exchange through VDT's etc.MSS Management Statistic Subsystem. Keeps records of the junction routes in the local exchange and keeps traffic records for DS3s and RCUs under its control and also for the AAS. MTS S-X Message Transmission Subsystem. Terminal part of CCS (not to be confused with CSS) link. It transmits and receives messages sent between distance CPSs (Call Processing Subsystems) and carriers out error checks on messages and re-transmits if necessary. MTS (MsgFor) the MTS message format is: - Error Check, Message, Routing label, header. The header contains the message number in case a resend is needed; the routing label includes the OPC (Originating Point Code) which identifies the originating exchange and the Destination Point Code (DPE) which identifies the destination exchange. The message tells CPS what it has to do and the error check obviously checks the message for errors.

3.6.3 MAINTENANCE CONTROL SUBSYSTEMThere are various activities and tests involved to maintain a switching system. Some of them are Supervision of the proper functioning of the exchange equipment, trunks and subscriber lines. Monitoring the database of line and trunk assignments. Efforts for the system recovery in case of failure. Automatic line tests, which permits maintenance persons to attend several exchanges from one control location. Effective diagnostics programs and maintenance strategies used to reduce the maintenance cost. The root cause of failure of any digital switching system is related to the software bugs which affects the memory and program loops, hardware failures, failure to identify the exact problem of failure and at least but not least the human error. Thus, the first step in software build is to select the appropriate program modules which are suitable for the switching system, signaling systems, availability of skilled person or the level of diagnoisation.Preventive maintenance programs are activated during the normal traffic. if a fault occurs, the OS activates the maintenance program to recover the system. Effective preventive and maintenance programs and strategies help in proper maintenance of digital switching system with reduced maintenance cost. 3.6.4 PROCESSOR UTILITY SUBSYSTEMThe Processor Utility Subsystem (PUS) controls the switching operations and is the brain of the DLE or DMSU. It hosts the Call Processing, Billing, Switching and Maintenance applications Software amongst others. The PUS is divided into up to 8 'clusters' depending on the amount of telephony traffic dealt with by the DLE/DMSU. Each cluster of processors contains 4 Central Processing Units (CPUs), the main memory stores (STRs) and the backing store memory. The PUS was coded with a version of the CORAL66 programming language known as PO CORAL (Post Office CORAL) later known as BTCORAL.3.6.5 TEST NETWORK SUBSYSTEM Functional Tests- Verify that DUT does what the specification says it should or must do. Negative Tests-Verify that the DUT behaves appropriately if another device on the network is not functioning according to specifications. Stress Tests Verify the DUT can perform correctly on busy networks with many devices and a high volume of network traffic. Performance/Reliability Test Refer to IETF RFC-2330:Framework for IP Performance Metric about 15 test itemsAdvanced Switch test: Max throughput, HOLblocking, XstreamIxia Communications: Mesh peak load testSelf-Developed:BX-Stream, EBX-Stream, MCAST X- Stream Multivendor Interoperability TestIUse different vendors router equipment to build real-world (tree or star topology) operation

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3.7 REFERENCES INTERNET