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B.S.N.L. Training Report 11

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  1. 1. Industrial Training at B.S.N.L. Training Report’12 CAP divides the signals of the telephone line into three bands: voice, upstream channel and downstream channel. Voice conversations are carried in the 0 to 4 KHz band as they are in all POTS circuits. The upstream channel that carries data from the user to the server is between 25 and 160 KHz. The downstream channel begins at 240KHz with a maximum of 1.5 MHz which depends on a number of conditions such as distance, line noise and number of users. CAP by keeping the three channels widely separated, minimises the possibility of interference both between channels on one line and signals on different lines. DMT also operates by dividing signals into separate channels without using two quite broad channels for upstream and downstream. The modulation technique that has become standard for ADSL is called the Discrete Multitone Technique, which combines QAM and FDM. In ADSL, the available bandwidth of 1.104 MHz is divided into 256 channels. Each channel uses a bandwidth of 4.312 KHz. Each channel is 4KHz wide with a guard band of .312KHz.. Hence the name Discrete Multitone. Each sub carrier can support maximum15 number of bits. Depending on signal to noise ratio for that sub carrier, a decision is taken as to how many bits that particular sub carrier can support. Every channel is monitored and if the quality is low, the signal is shifted to another channel. DMT constantly shifts signals between different channels, looking for the best channels for transmission and reception. Moreover, some of the lower frequency channels, are used as bi- directional channels for upstream and downstream. Keeping up with the quality of all channels, monitoring and sorting the information on the bi-directionalApplied Electronics 34 Model Polytechnic College, Poonjar
  2. 2. Industrial Training at B.S.N.L. Training Report’12Wireless AccessT echnologies Wi-Fi stands for Wireless Fidelity. As the words indicates, the system seeks to do away with-wires. This system is known as wireless LAN, as with Wi-Fi we can connect different computers in a LAN using radio waves. This is as shown below in Fig-1. Figure 1 This standard is known as IEEE 802 .11. There are different versions of this standard available. They are 802.11b, 802.11a & 802.11g. A comparison of the different standards is given below. WiFi generally refers to any type of IEEE 802.11 standard. Wi-Max WiMAX is an acronym that stands for Worldwide Interoperability for Microwave Access, a certification mark for products that pass conformity and interoperability tests for the IEEE 8802.16 standards.(IEEE 802.16 is working group number 16 of IEEE 802 specializing in point-to-multipoint Broadband wireless access). WiMAX covers wider, metropolitan or rural areas. It can provide data rates up to 75 megabits per second (Mbps) per base station with typical cell sizes of 2 to 10 kilometers. This is enoughApplied Electronics 35 Model Polytechnic College, Poonjar
  3. 3. Industrial Training at B.S.N.L. Training Report’12 bandwidth to simultaneously support (through a single base station) more than 60 businesses with T1/E1-type connectivity and hundreds of homes with DSL-type connectivity. It will provide fixed, portable, and eventually mobile wireless broadband connectivity and also provides POTS services. WiMAX actually provide two forms of wireless service : 1. Non-line-of-sight, WiFi sort of service, where a small antenna on your computer connects to the tower. In this mode, WiMAX uses a lower frequency range – 2 GHz to 11 GHz 2. Line-of-sight service, where a fixed dish antenna points straight at the WiMAX tower from a rooftop or pole. Line-of-sight transmissions use higher frequencies, with ranges reaching a possible 66 GHz. IEEE 802.16 Specifications Range - 30-mile (50-km) radius from base station Speed - 70 megabits per secondApplied Electronics 36 Model Polytechnic College, Poonjar
  4. 4. Industrial Training at B.S.N.L. Training Report’12 Line-of-sight not needed between user and base station Frequency bands - 2 to 11 GHz and 10 to 66 GHz Two key features of Wi Max is the use of OFDM (Orthogonal Frequency Division Multiplexing) and Adaptive modulation techniques for achieving greater bit rates and stable connection. In OFDM, the data will be sent over narrow band carriers transmitted in parallel at different frequencies. These carrier frequencies are closely spaced and they are orthogonal. In adaptive modulation, depending on the signal to Noise ratio (SNR) value of the radio link, the modulations will the automatically changed and bit rates will be adjusted accordingly. This also ensures a stable connectivity between the subscriber station and the Wi Max base station.Applied Electronics 37 Model Polytechnic College, Poonjar
  5. 5. Industrial Training at B.S.N.L. Training Report’12 Overview of the GSM RF Interfaces Interfaces For the connection of the different nodes in the GSM network, different interfaces are defined in the GSM specifications. The GSM interfaces discussed in this lesson are: Air interface or U m –interface The Air Interface is the interface between the BTS (Base Transceiver Station) and the MS (Mobile Station). The air interface is required for supporting: — Universal use of any compatible mobile station in a GSM network — A maximum spectral efficiency A bis -interface The A bis -interface is the interface between the BSC (Base Station Controller) and the BTS. The interface comprises traffic and control channels. Functions implemented at the A bis -interface are: — Voice-data traffic exchange — Signaling exchange between the BSC and the BTS — Transporting synchronization information from the BSC to the BTS A-interface The A-interface is the interface between the BSC and the MSC.Applied Electronics 38 Model Polytechnic College, Poonjar
  6. 6. Industrial Training at B.S.N.L. Training Report’12 The U m –interface Introduction One of the most important interfaces is the U m or Air interface. This interface is thoroughly specified to achieve a full compatibility between mobile stations of various manufacturers and networks of different operators. FDMA and To achieve a high spectral efficiency in the cellular network a TDMA combination of FDMA (Frequency Division Multiple Access) and methods TDMA (Time Division Multiple Access) is used. The FDMA part involves the division by frequency of the 25 MHz bandwidth into 124 carrier frequencies spaced 200 KHz for GSM-900. For GSM- 1800 the frequency spectrum of the 75 MHz bandwidth is divided into 374 carrier frequencies spaced 200 KHz. One or more frequencies are assigned to each BTS. Each of these carrier frequencies is then divided in time, using a TDMA scheme to increase the number of channels per carrier frequency. Each carrier frequency channel carries eight time-division multiplexed physical channels. A physical channel is determined by the carrier frequency (or a number of carrier frequencies and a defined hopping sequence) and the timeslot number. A mobile station can transmit speech data only during its assigned timeslot. Uplink and In the frequency range specified for the GSM-900 mobile radio downlink networks, 124 frequency channels with a bandwidth of 200 KHz are available for both the uplink and downlink direction. The uplink (mobile station to BTS) uses the frequencies between 890 MHz and 915 MHz and the downlink (BTS to mobile station) uses the frequencies between 935 MHz and 960 MHz. The duplex spacing, the spacing between the uplink and downlink channel, is 45 MHz. GSM-1800 uses a similar scheme. The difference is that for GSM- 1800 the uplink uses the frequencies between 1710 MHz and 1785 MHz and the downlink the frequencies between 1805 MHz and 1880 MHz. The duplex spacing is 95 MHz.Applied Electronics 39 Model Polytechnic College, Poonjar
  7. 7. Industrial Training at B.S.N.L. Training Report’12 BASIC TYPES OF POWER PLANTS AND SPECIFICATIONS Classification of power plants (3 piece) -Power plant comprises 3 parts -Float Rectifier -Battery Charger -Switching Cubicle. Power plants are classified based on their capacity. Feature Small exchange Medium exchange Large exchange power plants power plants power plants Capacity: 5/12A 25/50A > 50A Input Single Phase Single Phase Three Phase Paralleling Not possible Only manual Auto paralleling is of rectifier paralleling is possible possible Another classification of power plant is Single unit type Two unit Type Three unit type F.R,B.C and SC -One unit is FC/BC/SWC -FR, BC, and SC are will be in Single (Float rectifier cum in individual units. container battery charger, cum Switching cubicle ) Ex: 5/12A PP - another unit is FR Note: Nowadays mostly 2 units p/p are used with maintenance free batteries and all transmission power plant are 2-unit type only. The latest being P/P of SMPS with VRLA batteries.Applied Electronics 40 Model Polytechnic College, Poonjar
  8. 8. Industrial Training at B.S.N.L. Training Report’12 SWITCHING SYSTEMSIntroduction The telephone is a telecommunication device that is used to transmit and receiveelectronically or digitally encoded speech between two or more people conversing. It is one ofthe most common household appliances in the developed world today. Most telephones operatethrough transmission of electric signals over a complex telephone network which allows almostany phone user to communicate with almost any other user. Telecommunication networks carry information signals among entities, which aregeographically far apart. An entity may be a computer or human being, a facsimile machine, ateleprinter, a data terminal and so on. The entities are involved in the process of informationtransfer that may be in the form of a telephone conversation (telephony) or a file transferbetween two computers or message transfer between two terminals etc. With the rapidly growing traffic and untargeted growth of cyberspace,telecommunication becomes a fabric of our life. The future challenges are enormous as weanticipate rapid growth items of new services and number of users. What comes with thechallenge is a genuine need for more advanced methodology supporting analysis and design oftelecommunication architectures. Telecommunication has evaluated and growth at an explosiverate in recent years and will undoubtedly continue to do so. The communication switching system enables the universal connectivity. The universalconnectivity is realized when any entity in one part of the world can communicate with anyother entity in another part of the world. In many ways telecommunication will acts as asubstitute for the increasingly expensive physical transportation. The telecommunication links and switching were mainly designed for voicecommunication. With the appropriate attachments/equipments, they can be used to transmitdata. A modern society, therefore needs new facilities including very high bandwidth switcheddata networks, and large communication satellites with small, cheap earth antennas.Voice Signal Characteristics Telecommunication is mainly concerned with the transmission of messages betweentwo distant points. The signal that contains the messages is usually converted into electricalwaves before transmission. Our voice is an analog signal, which has amplitude and frequencycharacteristics.Applied Electronics 41 Model Polytechnic College, Poonjar
  9. 9. Industrial Training at B.S.N.L. Training Report’12 Voice frequencies: - The range of frequencies used by a communication devicedetermines the communication channel, communicating devices, and bandwidth or informationcarrying capacity. The most commonly used parameter that characterizes an electrical signal isits bandwidth of analog signal or bit rate if it is a digital signal. In telephone system, thefrequencies it passes are restricted to between 300 to 3400 Hz. In the field of telecommunications, a Telephone exchange or a Telephone switch is asystem of electronic components that connects telephone calls. A central office is the physicalbuilding used to house inside plant equipment including telephone switches, which maketelephone calls "work" in the sense of making connections and relaying the speech information.Switching system fundamentals Telecommunications switching systems generally perform three basic functions: theytransmit signals over the connection or over separate channels to convey the identity of thecalled (and sometimes the calling) address (for example, the telephone number), and alert (ring)the called station; they establish connections through a switching network for conversationaluse during the entire call; and they process the signal information to control and supervise theestablishment and disconnection of the switching network connection. In some data or message switching when real-time communication is not needed, theswitching network is replaced by a temporary memory for the storage of messages. This type ofswitching is known as store-and-forward switching.Signaling and control The control of circuit switching systems is accomplished remotely by a specific form ofdata communication known as signaling. Switching systems are connected with one another bytelecommunication channels known as trunks. They are connected with the served stations orterminals by lines. In some switching systems the signals for a call directly control the switching devicesover the same path for which transmission is established. For most modern switching systemsthe signals for identifying or addressing the called station are received by a central control thatprocesses calls on a time-shared basis. Central controls receive and interpret signals, select andestablish communication paths, and prepare signals for transmission. These signals includeaddresses for use at succeeding nodes or for alerting (ringing) the called station. Most electronic controls are designed to process calls not only by complex logic butalso by logic tables or a program of instructions stored in bulk electronic memory. The tabularApplied Electronics 42 Model Polytechnic College, Poonjar
  10. 10. Industrial Training at B.S.N.L. Training Report’12technique is known as translator. The electronic memory is now the most accepted techniqueand is known as stored program control (SPC). Either type of control may be distributed amongthe switching devices rather than residing centrally. Microprocessors on integrated circuit chipsare a popular form of distributed stored program control.Switching fabrics Space and time division are the two basic techniques used in establishing connections.When an individual conductor path is established through a switch for the duration of a call, thesystem is known as space division. When the transmitted speech signals are sampled and thesamples multiplexed in time so that high-speed electronic devices may be used simultaneouslyby several calls, the switch is known as time division. In the early stages of development in telecommunication, manual switching methodswere deployed. But later on to overcome the limitations of manual switching; automaticexchanges, having Electro-mechanical components, were developed. Strowger exchange, thefirst automatic exchange having direct control feature, appeared in 1892 in La Porte (Indiana).Though it improved upon the performance of a manual exchange it still had a number ofdisadvantages, viz., a large number of mechanical parts, limited availability, inflexibility, bulkyin size etc. As a result of further research and development, Crossbar exchanges,having anindirect control system, appeared in 1926 in Sweden. The Crossbar exchange improved upon many short- comings of the Strowger system.However, much more improvement was expected and the revolutionary change in field ofelectronics provided it. A large number of moving parts in Register, marker, Translator, etc.,were replaced en-block by a single computer. This made the exchange smaller in size, volumeand weight, faster and reliable, highly flexible, noise-free, easily manageable with nopreventive maintenance etc.Influence of Electronics in Exchange Design. When electronic devices were introduced in the switching systems, a new concept ofswitching evolved as a consequence of their extremely high operating speed compared to theirformer counter-parts, i.e., the Electro-mechanical systems, where relays, the logic elements inthe electromechanical systems, have to operate and release several times which is roughly equalto the duration of telephone signals to maintain required accuracy. Research on electronic switching started soon after the Second World War, butcommercial fully electronic exchange began to emerge only about 30 years later. However,Applied Electronics 43 Model Polytechnic College, Poonjar
  11. 11. Industrial Training at B.S.N.L. Training Report’12electronic techniques proved economic for common control systems much earlier. Inelectromechanical exchanges, common control systems mainly used switches and relays, whichwere originally designed for use in switching networks. In common controls, they are operatedfrequently and so wear out earlier. In contrast, the life of an electronic device is almostindependent of its frequency of operation. This gave a motivation for developing electroniccommon controls and resulted in electronic replacements for registers, markers, translators etc.having much greater reliability than their electromechanical predecessors. In electromechanical switching, the various functions of the exchange are achieved bythe operation and release of relays and switch (rotary or crossbar) contacts, under the directionof a Control Sub-System. These contracts are hard - wired in a predetermined way. Theexchange dependent data, such as subscriber’s class of service, translation and routing,combination signaling characteristics are achieved by hard-ware and logic, by a of relay sets,grouping of same type of lines, strapping on Main or Intermediate Distribution Frame ortranslation fields, etc. When the data is to be modified, for introduction of a new service, orchange in services already available to a subscriber, the hardware change ranging frominconvenient to near impossible, are involved. In an SPC exchange, a processor similar to a general-purpose computer is used tocontrol the functions of the exchange. All the control functions, represented by a series ofvarious instructions, are stored in the memory. Therefore the processor memories hold allexchange dependent data. such as subscriber date, translation tables, routing and charginginformation and call records. For each call processing step. e.g. for taking a decision accordingto class of service, the stored data is referred to, Hence, this concept of switching. Thememories are modifiable and the control program can always be rewritten if the behavior or theuse of system is to be modified. This imparts and enormous flexibility in overall working of theexchange. Digital computers have the capability of handling many tens of thousands ofinstructions every second, Hence, in addition to controlling the switching functions the sameprocessor can handle other functions also. The immediate effect of holding both the controlprogramme and the exchange data, in easily alterable memories, is that the administration canbecome much more responsive to subscriber requirements. both in terms of introducing newservices and modifying general services, or in responding to the demands of individualsubscriber. For example, to restore service on payment of an overdue bill or to permit changefrom a dial instrument to a multi frequency sender, simply the appropriate entries in thesubscriber data-file are to be amended. This can be done by typing- in simple instructions froma teletypewriter or visual display unit. The ability of the administration to respond rapidly andeffectively to subscriber requirements is likely to become increasingly important in the future.Applied Electronics 44 Model Polytechnic College, Poonjar
  12. 12. Industrial Training at B.S.N.L. Training Report’12 The modifications and changes in services which were previously impossible beachieved very simply in SPC exchange, by modifying the stored data suitably. In some cases,the subscribers can also be given the facility to modify their own data entries for supplementaryservices, such as on-demand call transfer, short code (abbreviated) dialing, etc. The use of a central processor also makes possible the connection of local and remoteterminals to carry out man-machine dialogue with each exchange. Thus, the maintenance andadministrative operations of all the SPC exchanges in a network can be performed from a singlecentralized place. The processor sends the information on the performance of the network, suchas, traffic flow, billing information, faults, to the centre, which carries out remedial measureswith the help of commands. Similarly, other modifications in services can also be carried outfrom the remote centre. This allows a better control on the overall performance of the network. As the processor is capable of performing operations at a very high speed, it has gotsufficient time to run routine test programmes to detect faults, automatically. Hence, there is noneed to carry out time consuming manual routine tests. In an SPC exchange, all control equipment can be replaced by a single processor. Theprocessor must therefore be quite powerful, typically it must process hundreds of calls persecond, in addition to performing other administrative and maintenance tasks. However, totallycentralized control has drawbacks. The software for such a central processor will bevoluminous, complex, and difficult to develop reliably. Moreover, it is not a good arrangementfrom the point of view of system security, as the entire system will collapse with the failure ofthe processor. These difficulties can be overcome by decentralizing the control. Some routinefunctions such as scanning, signal distributing, marking, which are independent of callprocessing, can be delegated to auxiliary or peripheral processors. Stored program control (SPC) has become the principal type of control for all types ofnew switching systems throughout the world, including private branch exchanges, data andTelex systems. Two types of data are stored in the memories of electronic switching systems.One type is the data associated with the progress of the call, such as the dialed address of thecalled line. Another type, known as the translation data, contains infrequently changinginformation, such as the type of service subscribed to by the calling line and the informationrequired for routing calls to called numbers. These translation data, like the program, are storedin a memory, which is easily read but protected to avoid accidental erasure. This informationmay be readily changed, however, to meet service needs. The flexibility of a stored programApplied Electronics 45 Model Polytechnic College, Poonjar
  13. 13. Industrial Training at B.S.N.L. Training Report’12also aids in the administration and maintenance of the service so that system faults may belocated quickly. SPC exchanges can offer a wider range of facilities than earlier systems. In addition,the facilities provided to an individual customer can be readily altered by changing thecustomer’s class-of-service data stored in memory. Moreover, since the processor’s stored datacan be altered electronically,some of these facilities can be controlled by customers. Examplesinclude:-1. Call barring (outgoing or incoming): The customer can prevent unauthorized callsbeing made and can prevent incoming calls when wishing to be left in peace.2. Call waiting: The ‘Call waiting’ service notifies the already busy subscriber of a thirdparty calling him.3. Alarm calls: The exchange can be instructed to call the customer at a pre-arranged time(e.g. morning alarm).4. Call Forwarding: The subscriber having such a feature can enable the incoming callscoming to his telephone to be transferred to another number during his absence.5. Conference calls: Subscriber can set up connections to more than one subscriber andconduct telephone conferences under the provision of this facility.6. Dynamic Barring Facility: Subscriber having STD/ISD facilities can dynamically locksuch features in their telephone to avoid misuse. Registering and dialing a secret code willextend such such a facility.7. Abbreviated Dialing: Most subscribers very often call only limited group of telephonenumbers. By dialing only prefix digit followed by two selection digits, subscribers can call upto 100 predetermined subscribers connected to any automatic exchange. This shortens theprocess of dialing all the digits.8. Malicious call Identification: Malicious call identification is done immediately and theinformation is obtained in the print out form either automatically or by dialing an identificationcode.9. Do Not Disturb: This facility enables the subscriber to free himself from attending hisincoming calls. Using this facility the calls coming to the subscriber can be routed to anoperator position or to an answering machine. The operator position or the machine can informthe calling subscriber that the called subscriber is temporarily inaccessible. Today SPC is astandard feature in all the electronic exchanges.Applied Electronics 46 Model Polytechnic College, Poonjar
  14. 14. Industrial Training at B.S.N.L. Training Report’12Implementation of Switching Network. In an electronic exchange, the switching network is one of the largest sub-system interms of size of the equipment. Its main functions are Switching (setting up temporaryconnection between two or more exchange terminations), Transmission of speech and signalsbetween these terminations, with reliable accuracy. There are two types of electronic switching system. viz. Space division and TimeDivision.Space Division switching System In a space Division Switching system, a continuous physical path is set up betweeninput and output terminations. This path is separate for each connection and is held for theentire duration of the call. Path for different connections is independent of each other. Once acontinuous path has been established., Signals are interchanged between the two terminations.Such a switching network can employ either metallic or electronic cross points. Previously,usage of metallic cross-points using reed relays and all were favored. They have the advantageof compatibility with the existing line and trunk signaling conditions in the network.Time Division Switching System In Time Division Switching, a number of calls share the same path on time divisionsharing basis. The path is not separate for each connection, rather, is shared sequentially for afraction of a time by different calls. This process is repeated periodically at a suitable high rate.The repetition rate is 8 KHz, i.e. once every 125 microseconds for transmitting speech ontelephone network, without any appreciable distortion. These samples are time multiplexedwith staggered samples of other speech channels, to enable sharing of one path by many calls.The Time Division Switching was initially accomplished by Pulse Amplitude Modulation (PAM) Switching. However, it still could not overcome the performancelimitations of signal distortion noise, cross-talk etc. With the advent of Pulse Code Modulation(PCM), the PAM signals were converted into a digital format overcoming the limitations ofanalog and PAM signals. PCM signals are suitable for both transmission and switching. ThePCM switching is popularly called Digital Switching.Applied Electronics 47 Model Polytechnic College, Poonjar
  15. 15. Industrial Training at B.S.N.L. Training Report’12Digital Switching Systems A Digital switching system, in general, is one in which signals are switched in digitalform. These signals may represent speech or data. The digital signals of several speech samplesare time multiplexed on a common media before being switched through the system. To connect any two subscribers, it is necessary to interconnect the time-slots of the twospeech samples, which may be on same or different PCM highways. The digitalized speechsamples are switched in two modes, viz., Time Switching and Space Switching. This TimeDivision Multiplex Digital Switching System is popularly known as Digital Switching System.The general architecture of a Digital Switching System is depicted in Fig2.Applied Electronics 48 Model Polytechnic College, Poonjar
  16. 16. Industrial Training at B.S.N.L. Training Report’12General architecture of Digital Switching System Subs interface Digital Switch Trunks interface Otherexchanges CONTROL PROCESSOR Other auxiliary inter faces Such as, (a) Tone generator (b) Frequency receives (c) Conference call facility (d) CCS# 7 Protocol Manager Operation & (e) V 5.2 access manager Maintenance Figure-2 The ESS No.1 system was the first fully electronic switching system but not digital.But later came ESS No.4 system which was digital for trunk portion only. When designed, thecost of A/D conversion (CODEC) on each subscriber line was seen as prohibitive. So the ESSNo.4 system was acting as a Trunk/Tandem exchange but not as a local exchange. So the maindifficulty for implementing a digital local exchange was the implementation of the subscriberline interface. This was solved by the introduction of Integrated Circuits, which made thedigital local exchange economically feasible. This implementation handles the followingfunctions:B-Battery feedO-Over-voltage protection (from lightning and accidental power line contact)Applied Electronics 49 Model Polytechnic College, Poonjar
  17. 17. Industrial Training at B.S.N.L. Training Report’12R-RingingS-Supervisory SignalingC-Coding (A/D inter conversion & low pass filtering)H-Hybrid (2W to 4W conversion)T-Testing the connectivity of SubscriberExamples of digital exchanges (switching systems) include CDOT, OCB, AXE, EWSD, 5ESSetc. The next evolutionary step was to move the PCM codec from the exchange endof the customer’s line to the customer’s end. This provides digital transmission over thecustomer’s line, which can have a number of advantages. Consider data transmission. If thereis an analog customer’s line, a modem must be added and data can only be transmitted atrelatively slow speeds. If the line is digital, data can be transmitted by removing the codec(instead of adding a modem). Moreover, data can be transmitted at 64 kbit/s instead of at, say,2.4 kbit/s. Indeed, any form of digital signal can be transmitted whose rate does not exceed 64kbit/s. This can include high-speed fax, in addition to speech and data. This concept had led to the evolution of Integrated services digital network(ISDN), in which the customer’s terminal equipment and the local digital exchange can be usedto provide many different services, all using 64 kbit/s digital streams. In simple terms, we cansay ISDN provides end-to-end digital connectivity.Access to an ISDN is provided in two forms:1. Basic-Rate Access (BRA) The customer’s line carries two 64 kbit/s “B” channels plus a 16 kbit/s “D”channel (a common signaling channel) in each direction.2. Primary Rate Access (PRA) The line carries a complete PCM frame at 2 Mbit/s in each direction. Thisgives the customer 30 circuits at 64 kbit/s plus a common signaling channel, also at 64 kbit/s.Control of switching systemsApplied Electronics 50 Model Polytechnic College, Poonjar
  18. 18. Industrial Training at B.S.N.L. Training Report’12 Switching systems have evolved from being manually controlled to being controlled byrelays and then electronically. The change from the manual system to the Strowger step-by-step system brought about a change from centralized to distributed control. However, assystems developed and offered more services to customers, it became economic to performparticular functions in specialized equipments that were associated with connections only whenrequired, thus, common control was introduced. Later, the development of digital computer technology enabled different functions to beperformed by the same hardware by using different programs; thus switching system enteredthe era of stored-program control (SPC). There are basically two approaches to organizing stored program control: centralizedand distributed. Early electronic switching systems (ESS) developed during the period 1970-75almost invariably used centralized control. Although many present day exchange designscontinue to use centralized SPC, with the advent of low cost powerful microprocessors and verylarge scale integration (VLSI) chips such as programmable logic arrays (PLA) andprogrammable logic controllers (PLC), distributed SPC is gaining popularity.Applied Electronics 51 Model Polytechnic College, Poonjar
  19. 19. Industrial Training at B.S.N.L. Training Report’12 Development of exchanges year Figure 3 The figure above shows the evolution of electronic switching systems from the manualswitching systems. The figure also depicts the changing scenario from digital switching toBroadband where the focus will be for high bit rate data transmissions.Signaling In Telecommunication A telecommunication network establishes and releases temporary connections, inaccordance with the instructions and information received from subscriber lines and interexchange trunks, in form of various signals. Therefore, it is necessary to interchangeinformation between an exchange and it external environment i.e. between subscriber lines andexchange, and between different exchanges. Though these signals may differ widely in theirimplementation they are collectively known as telephone signals. A signaling system uses a language, which enables two switching equipments toconverse for the purpose of setting up calls. Like any other language. it possesses a vocabularyof varying size and varying precision, ie. a list of signals which may also vary in size and asyntax in the form of a complex set of rules governing the assembly of these signals. Thishandout discusses the growth of signaling and various type of signaling codes used in IndianTelecommunication.Applied Electronics 52 Model Polytechnic College, Poonjar
  20. 20. Industrial Training at B.S.N.L. Training Report’12Types of Signaling 1. Subscriber Line Signaling 2. Inter exchange Signaling The signaling information exchanged between a subscriber and an exchange for theestablishment and release of a call is termed as Subscriber line signaling. Example forSubscriber line signaling –Calling subscriber going off-hook, feeding of dial tone to thesubscriber by the exchange, feeding of Ringing current, digits dialed by the subscriber(Address information) in the form of pulses or tones etc. The signaling information exchanged between different exchanges via inter exchangetrunks for the routing of calls is termed as Inter exchange Signaling. Earlier in band /out ofband frequencies were used for transmitting signaling information. Later on, with theemergence of PCM systems, it was possible to segregate the signaling from the speech channel.Applied Electronics 53 Model Polytechnic College, Poonjar
  21. 21. Industrial Training at B.S.N.L. Training Report’12 TRANSMISSION SYSTEMS Telephonic conversation is possible on a single wire connected between two telephonesbut distance is limited, approximately 5 to 10 Kms. If two wires are used which run parallelbetween two telephones, long distance communication can be provided. However, this is acostly affair therefore, many different systems of transmission are designed.What is a Transmission System? A transmission system consists of two terminals Transmitter and Receiver, with mediafor transmission between the two. There may be few repeaters (for amplification) atintermediate stations if required. There are different types of Transmission systems.Carrier Systems This system is installed between two cities and both systems areconnected by two wires called line. It provides three telephonic subscribers conversationsimultaneously. The distance between two cities is 50Km. to 100 Km. These systems work on230 V AC.Different Carrier systems are,  3 Channel system  8 Channel system  12 Channel system  24 Channel systemCo-axial cable systems In the initial stages of Tele –Communication two wires were used asline (as explained above). The disadvantage is that, it cannot handle more traffic. With theincreasing demand of more simultaneous telephone- calls different measures were adopted. Co-axial cable is one of them. The systems working on Co-Axial Cables are called Coaxial Cablesystems. After multiplexing telephone calls are transmitted by different coaxial cable systems.The description is as follows. A. 4 MHz SystemApplied Electronics 54 Model Polytechnic College, Poonjar
  22. 22. Industrial Training at B.S.N.L. Training Report’12 960 telephone calls are multiplexed by multiplexing equipment and it comes inthis system. This system amplifies the power and transmits on coaxial cable to distant station. B. 12 MHz System 2700 multiplexed signals are fed in this system for distant end. This systemamplifies the power and makes it suitable for transmission to other stations.Micro Wave SystemsMicrowave working is resorted to provide reliable communication especially in difficultterrains where communication by coaxial cable and other means cannot be provided. It has thefollowing advantages. It can provide a very large bulk of speech circuits. It can be provided over rough and in-accessible terrain where other types of communication cannot be economically arranged.Applied Electronics 55 Model Polytechnic College, Poonjar
  23. 23. Industrial Training at B.S.N.L. Training Report’12 The fault liability in M/W system is very low. Provision of remote fault localization and standby microwave channel almost provides uninterrupted communication. The annual maintenance cost is very low as compared with other transmission system.Pulse Code Modulation System (PCM System) PCM systems use Time Division Multiplexing technique to provide a numberof circuits on the same transmission medium viz-open wire or underground cable pair of achannel provided by carrier, coaxial, microwave or satellite systemTime Division Multiplexing Basically, time division multiplexing involves nothing more than sharing atransmission medium by a number of circuits in time domain by establishing a sequence of timeslots during which individual channels (circuits) can be transmitted. Each channel is sampled at a specified rate and transmitted for a fixedduration. All channels are sampled one by one and transmitted one by one, the cycle isrepeated again and again. The channels are connected to individual gates, which are openedone by one in a fixed sequence. At the receiving end also similar gates are opened in unisonwith the gates at the transmitting end. The signal received at the receiving end will be in the form of discretesamples and these are combined to reproduce the original signal. Thus at a given instant oftime, only one channel is transmitted through the medium, and by sequential sampling anumber of channels can be staggered in time as opposed to transmitting all the channel at thesame time as in FDM systems. This staggering of channels in time sequence for transmissionover a common medium is called Time Division Multiplexing (TDM). To develop a PCM signal from several analogue signals, the followingprocessing steps are required: Filtering Sampling Quantization EncodingApplied Electronics 56 Model Polytechnic College, Poonjar
  24. 24. Industrial Training at B.S.N.L. Training Report’12 Line CodingOptical Fiber Systems Optical fibre is a medium, in which information (voice, data or video) istransmitted through a glass or plastic fibre, in the form of light, following the transmissionsequence given below: Information is encoded into electrical signals. Electrical signals are converted into light signals. Light travels down the fibre. A detector changes it into electrical signals at receiver. Electrical signals are decoded into information. Recei Tra Gat ve Ga ns CH 1 ( ) e ) ( te 1 CH 1 1 CH 2 ( ) ) ( 2 CH 2 2 ~~ Mediu CH 3 ( ) m ) ( 3 CH 3 3 CH n ( ) ) ( n CH n n Time Division Multiplexing MultiplexingApplied Electronics 57 Model Polytechnic College, Poonjar
  25. 25. Industrial Training at B.S.N.L. Training Report’12Advantages Of Optical Fibre Optical Fibre is non conductive (Dielectric) Electromagnetic immunity Large Bandwidth Low Loss Small, light weight cables Available in Long lengths Security Safety Universal mediumPDH Fiber Optic Transmission Systems: - Present telecom technology provides both PDH (Plesiochronous DigitalHierarchy) and SDH (Synchronous Digital Hierarchy) optical fiber equipments. The PDHsystems are basically used for the Point-to-Point transmission (carrying a signal between twoend points) e.g. for connecting two cities. The different PDH Fiber Optic Transmission Systemswhich are used in the network of BSNL, capacity wise, are given below: - 8 Mbps system 120 channels capacity 34 mbps system 480 channels capacity 140 mbps system 1920 channels capacity 565 mbps system 7680 channels capacity The above PDH Fiber Optic Transmission Systems available either inseparate OLTE (Optical Line Terminating Equipment) and MUX (Multiplexer) or in integratedOLTE + MUX (OPTIMUX) version housed in a single rack. The application of these systemsdepends upon the traffic of that particular route. These PDH Fiber Optic Transmission Systemsare supplied by different manufacturers to BSNLMain parts of PDH Fiber Optic Transmission Systems: - Multiplexing Equipments Line EquipmentsApplied Electronics 58 Model Polytechnic College, Poonjar
  26. 26. Industrial Training at B.S.N.L. Training Report’12  OLTE (Optical Line Terminating Equipment)  Repeater (Regenerator)  Transmission Media (Optical Fiber) Block diagram of FOTS O R O M E MTelephon Telephonee calls U L calls G Lfrom/to from/toExchange X N U Exchange T E T E R X A E T SDH Optical Fiber Systems O R It is an international standard networking principle and a multiplexing method. The name of hierarchy has been taken from the multiplexing method, which is synchronous, by nature. The evolution of this will assist in improving the economy of operability and reliability of a digital network SDH was very quickly perceived to be a better way of deploying optical networks. SDH played a crucial role in the fast and efficient deployment of high speed backbone connecting routers. SDH starts its hierarchy at 155.52 mbps and available in different capacity systems like: - Name of system Speed Capacity  STM-1 155.52 Mbps 1890 channels  STM –4 622.08 Mbps 7560 channels  STM –16 2.50 Gbps 30,240 channels  STM - 64 10.0 Gbps 1,20,960 channels Applied Electronics 59 Model Polytechnic College, Poonjar
  27. 27. Industrial Training at B.S.N.L. Training Report’12 The first attempt to formulate standards for optical transmission started inUSA as SONET (Synchronous Optical Network) The aim of these standards was to simplifyinterconnection between network operator by allowing inter connection of equipment fromdifferent venders to the extent that compatibility could be achieved. When in year 1986 USA reported about SONET, which was developed byAT&T USA .UK and Japan took the interest in the technique and it was discussed in ITU (T)[International Telecommunication Union (telecommunication standardization Sector)] which isa world standard in telecommunication field , and SONET was renamed as SDH (SynchronousDigital Hierarchy ), after making some modifications in SONET .Thereafter SDH become theglobal of transmission system.Merits of SDH1. Simplified Multiplexing and De-multiplexing process2. Direct access to lower speed channel without need to demultiplex the entire high- speed signal.3. Enhanced OAM & P: - Due to enhanced Operation, Administration, Maintenance and Provisioning capabilities user can control the whole network from a central location i.e. remote supervision and control is very easy.4. Easy Growth: - Easy growth to high capacity systems.5. Capable of transporting existing PDH signals.6. Capable of transporting future broadband signals like, interactive multimedia & video conferencing.7. Capable of operating in a multivender and multi-operator environment: - Before SDH optical solutions for the long distance transmission were intensely vendor specific but SDH were firm standards for vendor inter operability.8. Synchronous networking: - SDH supports multi-point, Hub and Ring Configuration whereas PDH networking only supports Point-to-Point Configuration.SDH Network Elements: - Terminal Multiplexer (TM) – TM an end point device of SDH network, it is used at terminals of point to point SDH network.Applied Electronics 60 Model Polytechnic College, Poonjar
  28. 28. Industrial Training at B.S.N.L. Training Report’12 Add/Drop Multiplexer (ADM) – ADM is network element, which allows configurable add/drop of a subset of traffic channels from higher rate data stream. Regenerator (REG): -The most basic element is regenerator. They terminate and regenerate the optical signal. These are not simple regenerator but have alarm and performance monitoring capability. Synchronous Digital Cross Connect (SDXC): - This device will form the cornerstone of new SDH. They can function as semipermanent switch for transmission channels and can switch at any level from 64 kbps to STM-1 under software control .The previous systems like analogue transmission systems and PDHbased digital transmission systems are only point to point or in bus configuration. In busconfiguration repeaters are having drop and insert facility of channels and in point to pointconfiguration repeaters do not have drop and insert facility. But in both of these configuration ifmedia breaks or repeater fails, the full system goes out of order. EXCHANGE ADM “A” E E X X C ADM ADM C H H A A N N “B” “D” G G E E ADM “C” EXCHANGEApplied Electronics 61 Model Polytechnic College, Poonjar
  29. 29. Industrial Training at B.S.N.L. Training Report’12 In SDH this problem is overcome by using other type of configurationsspecially RING TYPE as shown in fig. In the fig. Four nodes (A, B, C, D) are shown, when media between say nodeA and B breaks, the traffic interrupted is automatically rerouted between nodes A&B via nodesD&C.Applied Electronics 62 Model Polytechnic College, Poonjar
  30. 30. Industrial Training at B.S.N.L. Training Report’12Satellite Systems Long distance communications, particularly to remote locations, usingconventional terrestrial media is both uneconomic and unreliable. A geo-stationarycommunication satellite which acts as a repeater hung in the sky can cover a very largearea and provide a reliable and cost effective alternative. Although satellite communication would seem to be a straightforward extensionof Conventional radio system, the use of satellites for communications brings in newoperational features not found in terrestrial systems. In this hand out, some of thefeatures of satellite communication are discussed. Basic knowledge of terrestrial radiosystems is assumed.Structure of a Satellite LinkApplied Electronics 63 Model Polytechnic College, Poonjar