DEPARTMENT OF INFORMATION TECHNOLOGY

BSNL INDUSTRIAL TRAINING REPORT

Acknowledgement

I acknowledge my gratitude and thank to all the well knowledge persons for giving me opportunity to avail all the best facilities available at this telecom centre through which I have gained knowledge thinking so as too just in the environment suitable for harmonic adjustment. I am grateful to the following persons for various help rendered by them during the training period.

Last but not the least; I thank my friends and my family members for their constant encouragement.

PREFACE

Since time immemorial, a man has tried hard to bring the world as close to himself as possible. His thirst for information is hard to quench so he has continuously tried to develop new technologies, which have helped to reach the objective.

The world we see today is a result of the continuous research in the field of communication, which started with the invention of telephone by Graham Bell to the current avatar as we see in the form INTERNET and mobile phones. All these technologies have come to existence because man continued its endeavor towards the objective.

This project report of mine, STUDY OF TRENDS TECHNOLOGIES IN COMMUNICATION AND NETWORKING has been a small effort in reviewing the trends technologies prevailing. For this purpose, no organization other than BAHRAT SANCHAR NIGAM LIMITED could have been a better choice.

ABOUT BSNL

On October 1, 2000 the Department of Telecom Operations, Government of India became a corporation and was christened Bharat Sanchar Nigam Limited (BSNL). Today, BSNL is the largest Public Sector Undertaking of India and its responsibilities include improvement of the already impeccable quality of telecom services, expansion of telecom network introduction of new telecom services in all villages and instilling confidence among its customers. At present the BSNL is the World's Seventh Largest and India's First Telecommunication Company. Responsibilities that BSNL has managed to shoulder remarkably, definitely. BSNL is the largest telecom operator in India and is known to everybody for Basic Telephony Services for over 100 years. Presently the plain old, countrywide telephone service is being provided through 32,000 electronic exchanges, 326 Digital Trunk Automatic Exchanges(TAX), Digitalized Public Switched Telephone Network (PSTN) all interlinked by over 2.4 lakhs km of Optical Fiber Cable, with a host of Phone Plus value additions to our valued Customers. BSNL's telephony network expands throughout the vast expanses of the country reaching to the remotest part of the country. Driven by the very best of telecom technology from chosen global leaders, it connects each inch of the nation to the infinite corners of the globe, to enable you to step into tomorrow. Along with its vast customer base, BSNL's financial and asset bases too are vast and strong. The telephone infrastructure along is worth about Rs. 1,00,000 crore (US $ 21.2 billion) Turnover of Rs. 22,000 crore (US $ 4.6 billion) Now from latest news BSNL records a net profit of Rs. 6,312 crore on revenues of Rs.24,300 crore for the financial year 2001-02. BSNL is working round the clock to take India into the future by providing world class telecom services for the people of India.

Services Provided by BSNL

BSNL provides almost every telecom service in India. Following are the main telecom services provided by BSNL: Universal Telecom Services: Fixed wireline services and landline in local loop (WLL) using CDMA Technology called bfone and Tarang respectively. As of June 30, 2010, BSNL had 75% marketshare of fixed lines.

BSNL Mobile

Prepaid Mobile Cellular Mobile Telephone Services: BSNL is major provider of Cellular Mobile Telephone services using GSM platform under the brand name Cellone & Excel (BSNL Mobile). As of June 30, 2010 BSNL has 13.50% share of mobile telephony in the country.[7] WLL-CDMA Telephone Services: BSNL's WLL (Wireless in Local Loop)service is a service giving both fixed line telephony & Mobile telephony.

BSNL Broadband Internet: BSNL provides Internet access services through dial-up connection (as Sancharnet through 2009[8]) as Prepaid, (NetOne) as Postpaid and ADSL broadband (BSNL Broadband). BSNL held 55.76% of the market share with reported subscriber base of 9.19 million Internet subscribers with 7.79% of growth at the end of March 2010.[citation needed] Top 12 Dial-up Service providers, based on the subscriber base, It Also Provides OnlineGames via Its Games on Demand (GOD) Intelligent Network (IN): BSNL offers value-added services, such as Free Phone Service (FPH), India Telephone Card (Prepaid card), Account Card Calling (ACC), Virtual Private Network (VPN), Tele-voting, Premium Rae Service (PRM), Universal Access Number (UAN). 3G:BSNL offers the '3G' or the'3rd Generation' services which includes facilities like video calling,mobile broadband, live TV, 3G Video portal, streaming services like online full length movies and video on demand etc. IPTV:BSNL also offers the 'Internet Protocol Television' facility which enables watch television through internet. FTTH:Fibre To The Home facility that offers a higher bandwidth for data transfer. This idea was proposed on post-December 2009. Helpdesk: BSNL's Helpdesk (Helpdesk) provide help desk support to their customers for their services. VVoIP: BSNL, along with Sai Infosystem - an Information and Communication Technologies (ICTs) provider - has launched Voice and Video Over Internet Protocol (VVoIP). This will allow to make audio as well as video calls to any landline, mobile, or IP phone anywhere in the world, provided that the requisite video phone equipment is available at both ends.[9] WiMax: BSNL has introduced India's first 4th Generation High-Speed Wireless Broadband Access Technology with the minimum speed of 256kbps. The focus of this service is mainly rural customer where the wired broadband facility is not available OPTICAL FIBER COMMUNICATION

HISTORY:

The use of visible optical carrier waves or light for communication has been common for many years. Simple systems such as signal fires, reflecting mirrors and, more recently signaling lamps have provided successful, if limited, information transfer. Moreover as early as 1880 Alexander Graham Bell reported the transmission of speech using a light beam. The photo phone proposed by Bell just for years after the invention of the telephone modulated sunlight with a diaphragm giving speech transmission over a distance of 200m. However, although some investigation of the optical communication continued in the early part of the 20th century its use was limited to mobile, low capacity communication links. This was due to both the lack of suitable light sources and the problem that light transmission in the atmosphere is restricted to line of sight and severely affected by disturbances such as rain, snow, fog dust and atmospheric turbulence. A renewed interest in optical communication was stimulated in the early 1960s with the invention of the laser. This device provided a coherent light source, together with the possibility of the modulation at high frequency. The proposals for optical communication via optical fibers fabricated from glass to avoid degradation of the optical signal by the atmosphere were made almost simultaneously in 1966 by Kao and Hock ham and Werts. Such systems were viewed as a replacement for coaxial cable system, initially the optical fibers exhibited very high attenuation and were therefore not comparable with the coaxial cable they were to replace. There were also problems involved in jointing the fiber cables in a satisfactory manner to achieve low loss and to enable the process to be performed relatively easily and repeatedly in the field.

In microwave system if we double the distance the loss will be increased by 6db. For the shorter distance the loss is higher. In ofc system Optical wire is small size, light weight, high strength and flexibility. Its transmission benefits includes wide band width, low loss and low cost. They are suitable for both analog and digital transmission. It is not suffered by digging, electrical interference etc. problems.

THE GENERAL SYSTEM

An optical fiber communication system is similar in basic concept to any type of communication system. A block diagram of a general communication system in fig.a. The function of which is to convey the signal from the information source over the transmission medium to the destination. In electrical communication, the information source provides an electrical signal, usually derived from a message signal which is not electrical, to a transmitter comprising electrical and electronic components which converts the signal into a suitable form for propagation over the transmission medium. The transmission medium can consists of a pair of wires, a coaxial cable or a radio link through free space down which the signal is transmitted to the receiver where it is transformed into original electrical information signal before being passed to the destination.

Information Source, transmitter(modulator), Transmission medium, Receiver (demodulator), Destination, Communication system, Information Source, electrical transmit, optical source, optical fiber cable, optical Detector, electrical receive Destination

Optical fiber communication system

For optical fiber communication system shown in fig b. information source provides an electrical signal to a transmitter comprising an electrical stage which drives an optical source to give modulation of the light wave carrier. The optical source which provides the electrical, optical conversion may be either a semiconductor laser or light emitting diode (LED). The transmission medium consist of an optical fiber and the receiver consist of an optical detector which drives a further electrical stage hence provide demodulation of optical carrier. Photodiodes and in some instances, photransistor and photoconductors are utilized for the detection of the optical signal and the opticalelectrical conversion. The optical carrier may be modulated either an analog or digital information signal. In the system shown in fig b. analog modulation involves the variation of the light emitted from the optical source in a continuous manner. With digital modulation, however, discrete changes in the light intensity are obtained (on/off pulses). Although often simpler to implement, analog modulation with an optical fiber communication system is less efficient, requiring a far higher s/n ratio at than digital modulation. Also, the linearity needed for analog modulation is not always provided by semiconductor optical sources, especially at high modulation frequencies. For this reasons, analog optical fiber communication links are generally limited to shorter distances and lower bandwidths than digital links.

GENERAL IDEA OF OFC

The idea of fiber optics is use to light, instead of current or voltage, as the energy which carries the signal, with the light as a carrier that is turned on and off, with binary amplitude modulation. The problem is to direct the light from the transmitter to the receiver. The solution is to use a hair-thin fiber of glass as a light pipe. If a light source is put at one end, any light that enters the fiber stay in that fiber end travels through the fiber to the other end. The light does not pass out of the walls of fiber as it travels. This is because of a property called total internal reflection. If a light wave is traveling through a material with a high refractive index compared to an adjacent material, and it hits the interface between them at certain low angles, the light does not cross the boundary but completely reflects back (Fig.1).

At the receiving end of fiber, a light detector senses the light. Thus, the communication medium is the fiber, and the energy used is light energy.

FEATURES:

The fiber optics has become a preferred medium due to its some important features like: The bandwidth of the fiber and light beam is extremely wide. It is possible to handle signals which turn on and off at gigabit per second rates (1 gigabit, gbit =1000 Mbitts). The fiber itself is very thin and not expensive. The thinness means that it is easy to handle, and many fibers can be put in the trenches or narrow conduits. The light signal is absolutely immune to electrical noise from any sources. Even if there are sources of electrical noise directly touching the cable, the electric fields of the noise source cannot affect the light beam in the fiber. The signal in the cable is secure from unauthorized listeners. It is relatively hard to tap into the cable without being noticed, and the entire light signal is confined within the fiber. No light escapes to the outside where someone else could see it. Since there is no electricity or electrical energy in the fiber, it can be run in hazardous atmospheres where the danger of explosion from spark may exist. Also, the fiber itself is immune to many types of poisonous gases, chemicals, and water.

ESSENTIAL FEATURES OF AN OPTICAL FIBER

1. Optical fibers may be produced with good stable transmission characteristics in long lengths at a minimum cost and with maximum reproducibility. 2. A range of optical fiber types with regard to size, refractive indices and index profiles, operating wavelengths, materials etc. be available in order to fulfill many different system applications.

3. The fibers may be converted into practical cables which can be handled in a similar manner to conventional electrical transmission cables without problems associated with the degradation of their characteristics or damage.

4. The fibers and fiber cables may be terminated and connected together without excessive [practical difficulties and in ways which limit the effect of this process on the fiber transmission characteristics to keep them within acceptable operating levels. It is important that these jointing techniques may be applied with ease in the field location where cable connection takes place.

JOINT OF FIBER

Optical fiber links, in common with any line communication system, have a requirement for both jointing and termination of the transmission medium. The number of intermediate fiber connections or joints is dependent upon the link length, the continuous length of the fiber cable that may be produced by the preparation methods and the length of the fiber cable that may be practically installed as a continuous section on the link. It is therefore apparent that fiber to fiber connection with low loss and minimum distortion (i.e. modal noise) remains an important aspect of optical fiber communication system. Before optical fibers splicing and joining are done certain preparations are made with fiber or fiber cables as case may be to achieve best results at the end surface. First of all the protective plastic that covers the glass cladding is stripped from each fiber end, which is then cleaved with a special tool, producing a smooth and flat end. 1. Fiber splices: these are semipermanent or permanent joints which find major use in most optical fiber telecommunication system (analogous to electrical soldered joints). 2. Demountable fiber connectors or simple connectors: these are removable joints which allow easy, fast, manual coupling and uncoupling of fibers (analogous to electrical plugs and sockets). The above fiber to fiber joints are designed ideally to couple all the light propagating in one fiber into the adjoining fiber. By contrast fiber couplers are branching devices that split all the light from main fiber into two or more fibers or, alternatively, couple a proportion of the light propagating in the main fiber into main fiber.

FIBER SPLICES A permanent joint formed between two individual optical fibers in the field or factory is known as a fiber splice. Fiber splicing is frequently used to establish long haul optical fiber links where smaller fiber lengths need to be joined, and there is no requirement for repeated connection and disconnection. Splices may be divided into two broad categories depending upon the splicing technique utilized. These are fusion splicing or welding and mechanical splicing. Fusion splicing is accomplished by applying localized heating(e.g. by a flame or an electric are ) at the interface between two butted, prealigned fiber ends causing them to soften and fuse. Mechanical splicing, in which the fibers are held in alignment by some mechanical means, may be achieved by various methods including the use of tubes around the fiber ends (groove splices).

A requirement with fibers intended for splicing is that they have smooth and square end faces. In general this end preparation may be achieved using a suitable tool which cleaves the fiber as illustrated.

FUSION SPLICES The fusion splicing of single fibers involves the heating of the two prepared fiber ends to their fusing point with the application of sufficient axial pressure between the two optical fibers. It is therefore essential that the stripped (of cabling and buffer coating) fiber ends are adequately positioned and aligned in order to achieve good continuity of the transmission medium at the junction point. Hence the fiber are usually positioned and clamped with the aid of an inspection microscope. Flame heating sources such as micro plasma torches (argon and hydrogen) and oxhydric microburners (oxygen, hydrogen and alcohol vapour) have been utilized with some success. However, the most widely used heating source is an electric arc. This technique offers advantages of consistent, easily controlled heat with adaptability for use under field conditions. A schematic diagram of the basic two fibers are welded together. Shows a development of the basic are fusion process which involves the rounding of the fiber ends with a low energy discharge before pressing the fibers together and fusing with a stronger arc. This technique, known as perfusion, removes the requirement for fiber end preparation which has a distinct advantage in the field environment. A possible drawback with fusion splicing is that the heat necessary to fuse the fibers may weaken the fiber in the vicinity of the splice. It has been found that even with careful handling; the tensile strength of the fused fiber may be as low as 30 % of that of the uncoated fiber before fusion.

EQUIPMENT REQUIRED FOR OFC JOINT1) Optical fiber fusion splicer specification ( spicer machine ) AC input 100 to 240v, frequency 50/60Hz DC input 12v/aA 2) Fiber cutter It converts irregular shaped fiber end into smooth & flat end. 3) Chemicals used in OFC joint HAXENE : To remove jelly from the fiber ACETONE : For cleaning the OFC ISO PROPENOT: For smoothness of optical glass. 4) Sleeve: - To enclose fiber joint. 5) Tool Kit 6) Joint kit. Joint encloser Buffer Adhesive tap. 7) Generator /12V Battery 8) Cotton clothes for fiber cleaning.

OPTICAL TIME DOMAIN REFLECTOMETRY (OTDR) A measurement technique which is far more sophisticated and which finds wide application in both the laboratory and the field is the of optical time domain reflectometry (OTDR). This technique is often called the backscatter measurement method. It provides measurement of the attenuation on an optical link down its entire length giving information on the length dependence of the link loss. OTDR also allows splice and connector losses to be evaluated as well as the rotation of any faults on the link. It relies upon the measurement and analysis of the fraction of light which is reflected back within the fibers numerical aperture due to Rayleigh scattering. Coupler Pulsed Laser fiber

Photo detector APD

Box car Integrator

log amplifier

chart recorder

A block schematic of the backscatter measurement method. A light pulse is launched into the fiber in the forward direction from an injection laser using either a directional coupler or a system of external lenses with a beam splitter (usually only in the laboratory). The backscattered light is detected using an avalanche photodiode receiver which drives an integrator in order to improve the received signal to noise ratio by giving an arithmetic average over a number of measurements taken at one point within the fiber. This provides location dependent attenuation values which give an overall picture of the optical loss down the link.

ADVANTAGES OF OPTICAL FIBER COMMUNICATION

Enormous Potential Bandwidth: - The optical carrier frequency in the range 1013 to 1016 Hz (generally in the near infrared around 1014 Hz or 105 GHz) yields a far grater potential transmission bandwidth than metallic cable systems. (i.e. coaxial cable bandwidth up to around 500 MHz) or even millimetre wave radio systems (i.e. systems currently operating with modulation bandwidths of 700 MHz ). At present, the bandwidth available to fiber systems is not fully utilized but modulation at several gigahertz over a hundred kilometers and hundreds of megahertz over three hundred kilometers without intervening electronics (repeaters) is possible. Therefore, the information carrying capacity of optical fiber systems has proved far superior to the best copper cable systems. By comparison the losses in wideband coaxial cable systems restrict the transmission distance to only a few kilometers at bandwidths over one hundred megahertz. Although the usable fiber bandwidth will be extended further towards the optical carrier frequency, it is clear that this parameter is limited by the use of a signal optical carrier signal. Hence a much enhanced bandwidth utilization for an optical fiber can be achieved by transmitting several optical signals, each at different centre wavelengths, in parallel on the same fiber. This wavelength division multiplexed operation, particularly with dense packing of the optical wavelengths ( or, essentially, fine frequency spacing ), offers the potential for a fiber information carrying capacity which is many orders of magnitude in excess of that obtained using copper cables or a wideband radio system. Small Size and Weight: - Optical fibers have very small diameters which are often no grater than the diameter of a human hair. Hence, even when such fibers are covered with protective coatings they are far smaller and much lighter than corresponding copper cables. This is a tremendous boon towards the alleviation of duct congestion in cities, as well as allowing for an expansion of signal transmission within mobiles such as aircraft, satellites and even ships. Electrical Isolation: - Optical fibers which are fabricated from glass, or sometimes a plastic polymer, are electrical insulators and therefore, unlike their metallic counterparts, they do not exhibit earth loop and interface problems. Furthermore, this property makes optical fiber transmission ideally suited for communication in electrically hazardous environments as the fibers create no arching or spark hazard at abrasions or short circuits. Immunity To Interference And Crosstalk :- Optical fibers form a dielectric waveguide and are therefore free from electromagnetic interference (EMI), radiofrequency interference (RFI), or switching transients giving electromagnetic pulses (EMP). Hence the operation of an optical fiber communication system is unaffected by transmission through an electrically noisy environment and the fiber cable requires no shielding from EMI. The fiber cable is also not susceptible to lightning strikes if used overhead rather than underground. Moreover, it is fairly easy

to ensure that there is no optical interference between fibers and hence, unlike communication using electrical conductors, crosstalk is negligible, even when many fibers are cabled together. Signal Security: - The light from optical fibers does not radiate significantly and therefore they provide a high degree of signal security. Unlike the situation with copper cables, a transmitted optical signal cannot be obtained from a fiber in a noninvasive manner (i.e. without drawing optical power from the fiber). Therefore, in theory, any attempt to acquire a message signal transmitted optically may be detected. This feature is obviously attractive for military, banking and general data transmission (i.e. computer network) application. Low Transmission Loss :- The development of optical fibers over the last twenty years has resulted in the production of optical fiber cables which exhibit very low attenuation or transmission loss in comparison with the best copper conductors. Fibers have been fabricated with losses as low as 0.2 dB km-1 (see Section 3.3.2) and this feature has become a major advantage of optical fiber communications. It facilitates the implementation of communication links with extremely wide repeater spacing ( long transmission distances without intermediate electronics), thus reducing both system cost and complexity. Together with the already proven modulation bandwidth capability of fiber cable this property provides a totally compelling case for the adoption of optical fiber communication in the majority of long-haul telecommunication applications. Ruggedness and Flexibility :- Although protective coatings are essential, optical fibers may be manufactured with very high tensile strengths. Perhaps surprisingly for a glassy substance, the fibers may also be bent to quite small radii or twisted without damage. Furthermore cable structures have been developed which have proved flexible, compact and extremely rugged. Taking the size and weight advantage into account, these optical fiber cables are generally superior in terms of storage, transportation, handling and installation to corresponding copper cables, whilst exhibiting at least comparable strength and durability. System Reliability And Ease Of Maintenance :- These features primarily stem from the low loss property of optical fiber cables which reduces the requirement for intermediate repeaters or line amplifiers to boost the transmitted signal strength. Hence with fewer repeaters, system furthermore, the reliability of the optical components is no longer a problem with predicted lifetimes of 20 to 30 years now quite common. Both these factors also tend to reduce maintenance time and costs.

Potential Low Cost :- The glass which generally provides the optical fiber transmission medium is made from sand not a scarce resource. So, in comparison with copper conductors, optical fibers offer the potential for low cost line communication. Although over recent years this potential has largely been realized in the costs of the optical fiber transmission medium which for bulk purchases is now becoming competitive with copper wires (i.e. twisted pairs), it has not yet been achieved in all the other component areas associated with optical fiber communication. For example, the costs of high performance semiconductor lasers and detector photodiodes are still relatively high, as well as some of those concerned with the connection technology ( demountable connectors, couplers, etc. ).

DRAWBACKS OF OPTICAL FIBER COMMUNICATION

The use of fibers for optical communication does have some drawbacks in practice. Hence to provide a balance picture these disadvantages must be considered. They are The fragility of the bare fibers; The small size of fibers and cables which creates some difficulties with splicing and forming connectors; Some problems involved with forming low loss T- couplers; Some doubts in relations to the long term reliability of optical fibers in the presence of moisture; An independent electrical power feed is required for any electronic repeaters; New equipment and field practice are required; Testing procedures tend to be more complex.

APPLICATION OF THE OPTICAL FIBER COMMUNICATION

TRUNK NETWORK: The trunk or toll network is used for carrying telephone traffic between major conurbations. Hence there is generally a requirement for the use of transmission systems which have a high capacity in order to minimize costs per circuit. The transmission distance for trunk systems can very enormously from under 20 km to over 300 km, and occasionally to as much as 1000 km. Therefore transmission systems which exhibit low attenuation and hence give a maximum distance of unrepeatered operation are the most economically viable. In this context optical fiber systems with their increased bandwidth and repeater spacing offer a distinct advantage.

JUNCTION NETWORK: The junction or interoffice network usually consists of routes within major conurbations over distances of typically 5 to 20 km. However, the distribution of distances between switching centers (telephone exchanges ) or offices in the junction network of large urban areas varies considerably for various countries.

MILITARY APPLICATION:

In these applications, although economics are important, there are usually other, possibly overriding, considerations such as size, weight, deployability, survivability (in both conventional and nuclear attack and security. The special attributes of optical fiber communication system therefore often lend themselves to military use. MOBILES

One of the most promising areas of milita5ry application for optical fiber communication is within military mobiles such as aircraft, ships and tanks. The small size and weight of optical fibers provide and attractive solution to space problems in these mobiles which are increasingly equipped with sophisticated electronics. Also the wideband nature of optical fiber transmission will allow the multiplexing of a number of signals on to a common bus. Furthermore, the immunity of optical transmission to electromagnetic interference (EMI) in the often noisy environment of military mobiles is a tremendous advantage. This also applies to the immunity of optical fiber to lighting and electromagnetic pulses (EMP) especially within avionics. The electrical isolation, and therefore safety, aspect of optical fiber communication also proves invaluable in these applications, allowing routing through both fuel tanks and magazines.

COMMUNICATION LINKS:

The other major area for the application of optical fiber communication in the military sphere includes both short and long distance communication links. Short distance optical fiber systems may be utilized to connect closely spaced items of electronics equipment in such areas as operations rooms and computer installations. A large number of this system have already been installed in military installations in the united kingdom. These operate over distances from several centimeters to a few hundred meters at transmission rates between 50 bauds and 4.8 kbits-1. In addition a small number of 7 MHz video links operating over distances of up to 10 m are in operation. There is also a requirement for long distance communication between military installations which could benefit from the use of optical fibers. In both these advantages may be gained in terms of bandwidth, security and immunity to electrical interference and earth loop problems over conventional copper systems.

TELEPHONE EXCHANGE

INTRODUCTION The main function of an exchange is to process call from a calling subscriber and make the connection to the called subscriber. This connection can be direct or via another exchange. This requires all parts of the exchange to work as a unit to ensure the call is properly handled. CALL PROCESSING ARCHITECTURE The main function of the exchange is to process subscriber calls. The exchange does this by connecting an incoming line or trunk to another line or trunk. However call processing involves much more than simply connecting subscribers. In order to process the calls the exchange must perform four basic switching function. Supervision: Detects and reports service requests, acknowledgements and requests to terminate service. Signaling: Transmits information about lines and trunks and information about other aspects of call handling to control switching equipment. Routing: Converts address information to the location of the corresponding call line or to the location of a trunk on the way to that line. Alerting. Notifies a subscriber of incoming calls.

BASIC CALL TYPES Subscriber calls are grouped in to categories that distinguish one call from another. These categories are referred to as call types. The basic call types are Intra exchange calls: - these are calls between two subscribers served by the same exchange. These calls are normally line to line calls. Inter exchange calls: - these are calls that involve two or more exchanges. Within a given exchange there are different types of inter exchange calls.

An outgoing call is a call that goes out of the exchange via a trunk. If the call originated in the same exchange, it is called an originating outgoing call. An incoming call is a call that comes into the exchange via a trunk.

A tandem call is a call that comes into the exchange on one trunk and leaves the exchange on another trunk. Thus a tandem call is both incoming and out going. CALL PROCESSING STAGES An intra exchange call which is the simplest of the call types mentioned above, progresses through four basic stages :

DIGIT RECEPTION SEIZURE SIGNAL ADDRESS RECEIVED

IDLE

RINGING

DISCONNECT SIGNAL TALKING

ANSWER SIGNAL

Idle Digit reception and analysis Ringing talking Inter exchange calls are more complex, and their call processing stages are somewhat different.

DIGIT RECEPTION

Establish Dialing Connection

Establish Ringing Connection

IDLE

RINGING

TAKE DOWN CONNECTION

ESTABLISH TALKING CONNECTION

TALKING

SERVICE CATEGORY Residence and business subscriber services: - Example of this category are individual, 2-party and multiparty lines, abbreviated dialing, call waiting, 3-way calling all diversion, call barring and multi line hunting. Extended business services: - Examples of these services are PBX, indirect inward dialing and toll diversion. Public safety services: - Examples of this category are basic emergency service, outgoing call trace, and in-progress call trace and in progress call trace. Miscellaneous local system services: - it like loop-range services, integrated and universal pair gain interface and line signaling. Inter exchange services: - Various inter exchange signaling types. Call processing services: - Generalized screening, digit interpretation timing, routing and remote switching modules. Toll services: - Toll exchange trunks, auxiliary service trunks, and operator trunks. Of termination: trunk and line. The trunk termination involves selecting an idle member in the trunk group and out pulsing the received digits. For a trunk, the particular selected trunk group, the no. of members in the trunk group and the digits to be out pulsed and the way the trunk group is selected, are of utmost importance. The line termination involves checking to find whether the line is busy and applying rin2ging to the line.

DESCRIPTION OF VARIOUS BLOCKS

DP (Distribution Panel) Distribution point box commonly know as D.P. box is a terminal arrangement where under ground cable pairs are connected to overhead wires or drop wires for providing connections at subscribers premises. It is a cast iron box with a facility for termination of distribution cable on pins fitted on an insulating plate. The distribution cable pairs can be connected to these pins by soldering at the rear. The overhead wires are connected by means of screwing nuts provided on the front side of insulating plate. Types of D.Ps. There are two types of D.Ps. suitable for external/internal use. These are called internal D.P. and external D.Ps. and are generally available in 10 or 20 pair sizes. Location of D.Ps. External D.Ps. are fitted on posts by means of suitable size of U backs. Internal D.Ps. are fitted in side buildings on the wall at suitable location. In case of multi storied buildings where the telephone demand is very high, the distribution cables or some times even the primary cabled are terminated on distribution frames at suitable location, from where the distribution cables of 20 pairs or 10 pairs sizes are taken to different floors or block and terminated on 10 or 20 pair subs D.Ps. Individual wire are further provided from the subs D.Ps. to the location of the telephone. Pillar Pillar is fabricated from steel or cast from casings enclosing a frame-work on which cable terminal boxes are mounted. The term "pillar" is used with reference to a flexibility point where MDF's cables and DP's cables are interconnected. MDF (Main Distribution frame) The Subscriber's lines enter an exchange through a number of large capacity U/G cables, each of which serves a different part of the exchange area. The numbers given to the subscriber's lines do not bear any relationship to the geographical location of the subscriber. Hence, the exchange numbers included in any one cable are entirely haphazard. Moreover, as subscribers cease to have telephones and new subscribers are connected, the exchange numbering of the external cable pairs is constantly changing. On the other hand, all lines within the exchange are in strict numerical order. It is, therefore, necessary that some means must be provided for temporary connection between the two.

Exchange Card is a basic functional unit of the exchange. Various cards are utilized for various purposes e.g. Subscriber cards are utilized for termination of subscriber's cables coming from MDF. PCM Various subscriber's cables coming out from the subscriber's cards (After processing) are terminated into the DDF (Digital Distribution Frame) located inside PCM in between these two PCM tag block is there, which provide connectivity between these two. Various DDF's cables combine together and terminated into the OFC module (which is combination of electrical to light converter (Multiplexer and Demultiplexer). PCM is separately explained in another section.

MAIN DISTRIBUTION FRAME

INTRODUCTION

To obtain flexibility in interconnecting, external line plants and the exchange equipment and between different circuits in the exchange itself, certain arrangements is made by the use of iron frames. These iron frames are called main distribution frames, intermediate distributions frames or combined main and intermediate distribution frames, depending upon their functions.

MAIN DISTRIBUTION FRAME (M.D.F.)

The subscribers line enter an exchange through a number of large capacity cables, each of which serves a different part of the exchange area. The numbers given to the subscribers lines do not bear any relationship to the geographical location of the subscriber. Hence the exchange numbers include in any one cable are entirely haphazard. Moreover, as subscribers cease to have telephones and new subscribers are connected, the exchange numbering of the external cable pairs is constantly changing. On the other hand, all lines within the exchange are in strict numerical order. It is, therefore, necessary that some means must be provided for temporary connection between the two. This conversion from the geographical order of the external pairs to the numerical order within the exchange is carried out on a main distribution frame.

FACILITIES PROVIDED BY M.D.F.

The M.D.F. provides for the following requirements: A means for permanently terminating the external cables. For mounting the protective devices connected to the incoming circuits. Providing the connection between the exchange side and the line side by the jumpers. An interception point for use in connection with fault locating tests.

EXCHANGE

LEN

CABINET

PILLAR

DP BOX

MDF VERTICAL SUBSCRIBER

CONSTRUCTION OF M.D.F.

Main distribution frame is mainly divided in two parts. (1) Vertical Side or Line side (2) LEN side or Exchange side

All the part from vertical side to the subscriber are generally called outdoor section. 1 vertical has 10 tag block. Each tag block has 10 rows and each row has 10 tags. So each tag block has 100 tags. Connection between vertical side & subscribers are provided by jelly filled cables. These wires are first terminated in cabinet box, then according to requirements the group of the wires (e.g. 200 wires, 100 wires etc.) are terminated in pillar box & from here connections are given to the subscribers via DP box.

All the parts from LEN to the exchange is called indoor section. The connection of subscriber from exchange is terminated on this side of MDF. In 1 tag block there are 128 tags. Each tag block is divided in 4 segments. That is 0, 1, 2 & 3 and in each segments. There are sixteen tags. On the vertical side there is 100 tag in one vertical tag block where as on the LEN side there is 128 tags on each LEN block. The reason for this difference is that there is always a reserve of spare capacity in the external cables to cover fluctuations in the distribution of the subscribers lines as between the different localities served by the cables.

DIFFERENT TYPES OF FAULTS

The faults are given below which are established in communication of subscriber with exchange. LOOP FAULT:-If two wires are joined together because of improper connection, storming air etc. then this type of fault occur.

EARTH FAULT:-If two wires get scrape at some places and if this wire comes in contact with tree, pillar or any metal objects then this type of fault occurs.

CABLE FAULT:-For outdoor connections, jelly filled wires are used which are affected by natural causes such as rain, earthquake etc. At such time this fault occurs.

DISCONNECT FAULT:-This type of fault occurs due to the breaking of wires between the vertical side & LEN side.

LOCATION OF FAULTS :This can be determined by putting pack up. If pack up is put in one of the tag of LEN side and if dial tone is received only up to the LEN side then fault is in the outdoor side and if tone is received from the subscriber only up to the vertical side then fault is in the indoor side. These faults are also identify by either subscriber line tester or by using the computerized programs.

PROTECTIVE DEVICE USED ON M.D.F.

Fuses

These are the devices used to protect apparatus and wiring from excessive currents. A fuse is a small length of thin wire which melts if there is an excess of current and disconnect the equipment before possible damage. The rated current of fuse is the maximum current which it can carry without melting or fusing.

The types of fuses used for connecting line to equipment are:(1) Glass type (2) Gate type GAS DISCHARGE TUBE (GD TUBE)

In case of heavy lighting discharges or induction of high voltages, gas discharge protectors are used as protective device to protect the communication lines and equipments from damages due to high voltages. The gas discharge protector essentially consists of two of three tungsten electrodes sealed in a special glass envelope or ceramic envelop[e containing a mixure of inert gases , mainly neon. In case of three pin G.D. tube, Two of the electrodes are for connections to the lines and third is the earth electrode and in case of two pin device, one electrode is connected to a limb of a line and other electrode is connected to earth. If the potential difference across the electrodes rises to a certain critical value, the gas is ionized and becomes conducting. This condition will continue till the potential difference across the electrodes falls to the extinction voltage value. For voltages less than striking value, it will not conduct. For normal operating voltages on the lines, it offers extremely high impedance and thus does not introduce any transmission loss.

APPLICATIONS :

MDF mainly provide connection between outdoor and indoor. MDF is basically the protection system for exchange. It uses Fuse as a protection device which prevents to reach the high current from outside to exchange. It uses Gas Discharge Tube (GD Tube) which provide protection against high excessive voltage.

C-DOT EXCHANGE

INTRODUCTION

Center development of telematics was formed in year 1985 by an act of parliament under ministry of telecommunication with prime objective to develop indigenous state of art electronics switch suitable for Indian network condition. Various products proposed to be developed by C-DOT include small RAX exchange of 128 ports for rural network to big C-DOT MAX-XL exchange for metropolitan applications. C-DOT DSS MAX is a universal switch and can be used as local, transmit or integrated local and transmit switch. It can have minimum capacity of 512 ports. And can grow up to 16000 ports without concentration. It has digital switching based on basic 64kpbs basic rate and 2mbps primary rate multiplexing structure. The development of C-DOT DSS MAX has taken place in family concept. It is fully integrated switch starting from smaller switches, bigger switches and can be built in a modular fashion by configuring hardware and software modules in variety of ways. The C-DOT DSS system can be used in telecommunication network at the various switching nodes for different type of services. Some of them are

C-DOT DSS as MAX:

This can be used as main automatic exchange which is expandable to large capacity of order of 2000 lines or beyond. The MAX may be here remote module(RM) and remote line concentrators(RLC) connected to it. C-DOT DSS as RAX This can be used as rural automatic exchange and is expandable upto 2000 lines capacity. Single base module configuration comes under the RAX category. Thus it is universal switch which can be configured as local, transit and integrated local and transit switch. It provides both local and centralized operation and maintenance.

SYSTEM ARCHITECTURE :It can serve metropolitan, urban, rural environments. Its architecture is such that it is possible to upgrade a working C-DOT SBM( single base module) or MBM ( multi base module) exchange to provide ISDN service through RSU ( remote switching unit). RSU can provide switching facility locally even in case of failure of communication path to parent exchange. In uses TST(time space switching).

C-DOT DSS MAX exchange can be configured from the following basic modules: 1. 2. 3. 4. 5. Base Module (BM) Central Module (CM) Administrative Module (AM) Input Output Processor (IOP) Alarm Display Panel (ADP) BASE MODULE (BM)

Depending upon the capacity of the exchange either single BM or more BMs are used. In case of 512 ports, only one BM is used. However under low traffic condition, the capacity of the ports can be increased up to 2048 ports by using two line modules and adopting concentration mode. In multi module working of BMs will range from 2 to 32. Frames of BM One BM consists of 6frames inside it. The top four frames are called terminal units (TU). TU houses various types of cards in them e.g. Power Supply Card (PSU), Subscriber Line Card (SLC), Trunk Cards (TWT), Announcement Card (ANNC), Conference Card (CONF), Terminal Test Controller Card (TTC).The fifth frame of BM is called Base Processor Unit (BPU) which is the heart of the system. It consists controller and memory cards. The sixth frame of the BM is called Time Switch Unit (TSU). Power Supply To energize various cards in the BM, different DC Voltages are required. They are 48V, +5V, +12V, -12V and 9V.for this purpose two types of power supply cards are employed: a) PSU-1: It caters various supplies to the four terminal units of the BM.In each TU, two PSU-1cards are there which work in load sharing mode i.e. in case of PSU-1card goes faulty, the other is capable of taking the full load of the terminal unit. b) PSU-2: Two cards are used each in BPU and TSU. PSU-1differs from PSU-2 in one respect i.e. PSU-1besides supplying various D.C. supplies, generates 75V AC ringing current. CENTRAL MODULE (CM)

When in the system the number of BMs exceeds one ,the CM is used. The purpose of CM is to provide setting up of calls between a subscriber of one BM and a subscriber of the other BM. For this purpose one CM is sufficient up to 32 BMs.

ADMINISTRATIVE MOFULE (AM)

Like CM, AM also used when the number of BMs in the system exceeds one. The AM is housed along with CM in one cabinet called CM.AM performs administrative and maintenance function.

INPUT OUTPUT PROCESSOR (IOP) :The IOP performs following function: 1) Serves as a media for man machine communication. 2) Keeps the data concerning system. 3) Does system initialization. IOP is connected to the following peripheral units for the purpose noted against each, Visual Display Unit: This is used for giving command to establish communication with the system. VDU gives the display of the reports as a result of the command execution. Printer: For the printed reports.

ALARM DISPLAY UNIT (ADP)

This is basically used for displaying the alarms raised in the system. The alarm gives both audio and visual indication. However, the audio alarm can be stopped by pressing acknowledge button on the ADP. The alarms raised are of three types, Critical: indicated by red LED. Urgent: indicated by orange LED. Non-urgent: indicated by green LED.

SYSTEM FEATURES

The C-DOT is a fully digital system with stored program control. In non-concentration mode of working, the switch is non-blocking i.e. calling party gets the called party if it is free. System environment a) Temperature = 17C to 27C Relative Humidity = 45% to 65% The application of C-DOT can be either new exchange or a replacement of the existing exchange. The C-DOT can be used as a local or transmit exchange. The system has a modular growth i.e. for expanding the capacity of the exchange additional modules are to be installed. The system provides for detailed billing in case of STD and ISD calls. However billing for local calls can also be arranged. The C-DOT employs distributed control system.

DIGITAL SWICHING

In Digital switching system signals are switched in digital form. These signals can be speech or data. For this Time Division multiplexing and pulse code modulation (PCM) techniques are used. Time Division multiplexer involves sharing of same transmission medium by a number of circuits or channels during a sequence of time periods. Thus the medium is periodically available to each channel

The channels are connected to individual gates which are opened one by one in fixed sequence. At the receiving end also. Similar gates are opened in unison with the transmitting g end. Before transmitting these samples of individual channel signals are coded in binary form and pulses corresponding to the digits are transmitted. This is called pulse code modulation. These pulses are decoded at receiving end and combines to reproduce the original signal. In Digital switching the digital signals of several speech signals are multiplexed on common media. Therefore same path is shared by different calls for fraction of time. This process is repeated periodically at a suitably high rate. This type of path is called PCM highway. To connect any two subscriber, it is necessary to interconnect the time slots of the two speech samples which may be same or different PCM highway.

The in connection of time slots i.e. switching of digital signal is normally achieved using a combination of two different modes of operation. These modes are (1) Space switching (2) Time switching In space switching mode, corresponding time slots of I/C and O/G PCM highways are interconnected. A sample in given time slot in an incoming highway is connected (switched) to same time slot of an outgoing highway. For e.g. in fig.2 the time slot 5 of incoming highway 2 is switched to time slot 5 of outgoing highway say highway 3. In this case there is no delay in switching of the sample from one highway to another highway. Since the sample transfer takes place in the same time slot of the PCM frame. In Time switching different time slots on the incoming and outgoing highways are interconnected by re-assigning the channel sequence. For e.g in fig,2 the time slot TS5 of incoming Highway2 can be connected to a different time slot TS6 of the O/G Highway2. In other words, a time switch is basically a time slot changer. TWO DIMENSIONAL SWITCHING Using one type of switch for large network is uneconomical. Hence to build a large network a number of stages are employed using small switches as building blocks. Such a network requires changing both the time slot and highway. Therefore the switching network usually employs both types of switches. Viz.space and time switch. This type of network is known as two dimensional networks. These networks can have various combinations of the two types of switches and denoted as TS. TST. TSST etc. TST Network As the name suggests. In a TST network there are two time stages separated by a space stage. The function of time slot changing where as the latter performs highway jumping. Let us consider a network having 4 input and 4 output highways. Each of the input and output time stages will have n time switches and the space stage will consist of an nxn crospoint matrix. Thus it is possible to connect any time slot in incoming PCM to any time slot in outgoing PCM. For example let us take only four I/C and outgoing PCM highways as shown below. There will be four time switches in each T-stages and space switch will consist of 4x4 matrix. let us consider connecting two subscribers through this network. Let us assume central processor assigns TS4 on Hwy0 to the calling party and TS6 on Hwy3 to the called party. The central controller establishes the paths in 3 steps. To introduce greater flexibility it uses an intermediate time slot. TSX which is also known as internal timeslot.

As the message can be conveyed only in one direction through this path another independent path to carry the message in other direction is also established as shown in fig.3. The internal time slot for other direction is Tsy. CONCLUSION Digital switching has become a synonym for time division multiplex digital switching system. Digital signals are switched in two modes. Viz. time switching and space switching. In a practical digital switch a combination of both is employed to increase the traffic handling capacity of the switch at minimum cost. Among the various possible combinations TST structure is most commonly used.

PULSE CODE MODULATION (PCM) PROCESS

Pulse Code Modulation (PCM) converts analog signals to a digital format(signal). This process has four major steps.

STEP ONE:- FILTERINGFrequencies below 300Hz and above 3400Hz (Voice Frequency range) arefiltered from the analog signal.. The lower frequencies are filtered out to removeelectrical noise induced from the power lines. The upper frequencies are filteredout because they require additional bits and add to the cost of a digitaltransmission system. The actual bandwidth of the filtered signal is 3100Hz (3400-300). It is often referred to as 4kHz.

STEP TWO:- SAMPLING

The analog signal is sampled 8000 times per second. The rate at which the analogsignal is sampled is related to the highest frequency present in the signal. This isbased on Nyquist Sampling Theorem. In his calculations, Nyquist used a voicefrequency range of 4000Hz (which represents the voice frequency range thatcontains intelligent speech). Thus, the standard became a sampling rate of8000Hz, or twice the bandwidth. The signal that is the result of the samplingprocess contains sufficient information to accurately represent the informationcontained in the original signal. The output of this sampling procedure is a PulseAmplitude Modulated, or, PAM signal.

STEP THREE:- QUANTIZING

In the third step of the A/D conversion process, we quantize the amplitude of theincoming samples to one of 225 amplitudes on quantizing scale (figure 3.13).Thus, in this step the sampled signal is matched to the segmented scale. Thepurpose of step three is to measure the amplitude (or height) of the PAM signal35and assign a decimal value that defines the amplitude. Based on the quantizingscale, each sampled signal is assigned a number between 0 and +127 to define itsamplitude.

STEP FOUR:- ENCODING

In the fourth step of A/D conversion process, the quantized samples are encodedinto a digital bit stream (series of electrical pulses).

A DIGITAL ENCODER

It recognizes the 255 different voltage levels of the quantized samples. Convertseach into a specific string of 8 bits (1s and 0s) that represent a particular voltagevalue. Fig.3.14 is helpful for understanding the binary code used in the encodingstep. Each bit position in the 8-bit word (byte) iis given a decimal weight (2 tosome power ), except for the first bit position. Using this coding scheme, we cancode any number between +127 and 127 and zero.For example:- If the PAM signal measures +45 on the quantizing scale, the outputof the encoding step is 10101101 (fig 3.15). This binary number (or 8 bit word) istransmitted over the network as a series of electrical or optical pulses. This seriesof pulses is called a digital bit stream. The PCM process requires a 64000bpschannel to encode a 4kHz audio input signal because 8000samples/sec.*8bits/word=64000bps. This is known as the DS0 (Digital Signal 0) or VF (VoiceFrequency) in the digital hierarchy. It is the basic building block of the digitalnetwork.

DIGITAL-TO-ANALOG CONVERSION

At the receive end of the transmission, the digital signal may need to beconverted back into its analog form. The digital-to-analog (D/A) conversionconsists of two steps .Each 8-bit word (byte) that enters the decoder results in one PAM signal value.The decoder: 36Reads the 8-bit binary word inputs ,creates a sream of 8000 pulses per second.These pulses have an amplitude of +127 to 127. The filtering process smoothesout the stream of 8000 pulses per second into an analog waveform that closelyresembles the waveform that was input into the A/D converter at the originatingend. The filter stores a part of each pulses energy and slowly releases it until thenext pulse arrives. The filter thus reconstructs the analog signal at a rate of 8000times per second.

WIRELESS IN LOCAL LOOP (WLL)

WLL is a communication system that connects customers to the Public Switch Telephone Network (PSTN) using radio frequency signals as substitutes of conventional wires for all part of connection between the subscribers and the telephone exchange. It works on CDMA technique. The local loop is access part of telecommunication network i.e. the part between PSTN switch and subscribers. WLL network application involves uses of radio to replace of the wire link between PSTN switch and subscriber. The radio technology is able to provide same quality of services as that provided by the wires line. Application of wireless loop technology has just been started in the worldwide. There is no standard for this so far. However, a number of national and international air interface standards for digital cellular mobile telephone system are available.

TECHNICAL ASPECTS:

WLL is based on CDMA technique and is entirely different from GSM. The system for WLL services can be divided in to following parts:-

BSC (Base Switching Centre):- It provides links between BTS & BSM; it consists of different processors, in BSNL it is of SUN Polaris of LG Company. In LG 1 BSC can have 48 BTS? In BSNL we have two types of BSC:-

V-5.2:- This type of BSC cannot switch by itself so it is dependent on local exchange / PSTN for switching and keeping records of billing etc. BSNL uses this type of BSC for rural areas.

CCS-7 / R2:- These types of BSC are totally automatic it doesnt depend on local exchange for its functions, it is complete in itself. BSNL uses this type of BSC for urban areas.

BTS (Base Transreceiver System):- As it is clear from its name it transmits as well as receive signal, it works as an amplifier (router) to overcome the loss in signal in transmission.

BSM (Base Station Management):- It controls and manages the WLL services. It can troubleshoot the problem; add new users as well as capable to block service given to user. It is basically a computer system, which manages the whole process of WLL service. In BSNL BSM are two UNIX based computer system.

ADVANTAGES OF WLL:

1.Country wide induction of WLL underway of areas than are non-feasible for the normal network.

2. Helping relieves congestion of connections in the normal cable / wire based network in urban areas.

3. Limits the mobility without any airtime charges.

4. It has improved signal and reduces the interference.

5. Greater capacity than mobile.

6. Provides ease of operation, administration & maintenance at lower cost.

The telecommunication is the biggest factor in influencing the speed of life in the modern age. Today we can get connection with any corner of world through the push button of computer; with the small mobile phone we can send not only the messages but also the secret document. As we know that there is positive view behind any mention that it should be helpful in the development of society. But humans have diverted mentality some of them of positive view and some of them of negative view. Where use any invention for the welfare of society but some uses for the satisfaction their disturbed mentality and to earn more and more money whether it may be harmful for the society. They infringe the norms of society and their behavior is condemned as antisocial, immoral and sinful.