TELEPHONE EXCHANGE

INTRODUCTION

The main function of an exchange is to process call from a calling subscriber and makethe 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.

LOCAL EXCHANGE:

Local exchanges service the subscribers within a particular area e.g. a district of a city or

a locality. They switch incoming traffic to and outgoing traffic from the connected

subscriber. The number of subscribers connected to an EWSD exchange can be as

low as a few hundred or as high as 250000

TRANSIT EXCHANGE:

At node points in the telephone network transit exchanges connect together trunks to and

from other exchanges up to 60,000 incoming outgoing or both ay trunks can be connected

to EWSD transit or long distance exchanges.

LOCAL/TRANSIT EXCHANGE:

These exchanges handle transit or long distance traffic as well as incoming and

outgoing local traffic. Any number of subscriber lines and trunks can be

combined within the maximum traffic handling capabilities of the exchange. Within

the above-mentioned local and transit exchange capacity limits, any combination of

subscriber lines and trunks can be connected, as well as the limit of 25,200 erlangs is not

exceeded.

CAL L PROCESSING ARCHIT ECT URE

The main function of the exchange is to process subscriber calls. The exchange does thisby 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. Thesecategories 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 originatedin 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 ADDRESS SIGNAL RECEIVED

IDLE RINGING

DISCONNECT ANSWER SIGNAL SIGNAL

TALKING

FIG: BASIC CALL SAGES INTRA EXCHANGE CALL

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

different.

DIGITRECEPTION

Establish Dialing Establish RingingConnection Connection

IDLE RINGING

TAKE DOWN ESTABLISH TALKING

CONNECTION CONNECTION

TALKING

SERVI CE CAT EGORY

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, out going 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 memberin 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.

DESCRIP TION 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 solderingat 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 internalD.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.

PillarPillar 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. Onthe 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. MDF is separately explained in another section.

ExchangeCard is a basic functional unit of the exchange. Various cards are utilized for variouspurposes e.g. Subscriber cards are utilized for termination of subscriber's cables coming from MDF.

PCMVarious 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 iscombination 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 equipmentand 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 DI STRIBUTION FRAME (M.D.F.)

The subscriber’s line enter an exchange through a number of large capacity cables, eachof 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 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 withinthe 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

CONSTRUCTION OF M.D.F.SUBSCRIBER

Main distribution frame is mainly divided in two parts.

(1) Vertical Side or Line side(2) LEN side or Exchange side

VERTICAL SIDE

VERTICAL VERTICAL

NO. 1

TAG BLOCK

1 2 23 24

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.

This 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.

LEN S I DE

0 COPY 1 COPY 0 COPY 1COPY

10TH TAG BLOCK

0 1 2 3 0 1 2 3

1STTAG BLOCK

1 2 14 15

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 LENside there is 128 tags on each LEN block. The reason for this difference is that thereis 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 withexchange.

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 affectedby 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.

LOCATIO N 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 upto the LEN side then fault is in the outdoor side and if tone is received from the subscriber only upto the vertical side then fault is in the indoor side.

This faults are also identify by either subscriber line tester or by using the computerized programme.

PROTECT IVE DE VI CE US ED 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 mixture 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 voltageson the lines, it offers extremely high impedance and thus does not introduce any transmission loss.

APPLICAT IONS

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.

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:- FILTERING

Frequencies below 300Hz and above 3400Hz (Voice Frequency range) are

filtered from the analog signal.. The lower frequencies are filtered out to remove

electrical noise induced from the power lines. The upper frequencies are filtered

out because they require additional bits and add to the cost of a

digital transmission 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 analog

signal is sampled is related to the highest frequency present in the signal. This is

based on Nyquist Sampling Theorem. In his calculations, Nyquist used a

voice frequency range of 4000Hz (which represents the voice frequency

range that contains “intelligent” speech). Thus, the standard became a

sampling rate of

8000Hz, or twice the bandwidth. The signal that is the result of the

sampling process contains sufficient information to accurately represent the

information contained in the original signal. The output of this sampling

procedure is a Pulse Amplitude Modulated, or, PAM signal.

STEP THREE:- QUANTIZING

In the third step of the A/D conversion process, we quantize the amplitude of the

incoming samples to one of 225 amplitudes on quantizing scale (figure

3.13). Thus, in this step the sampled signal is matchrd to the segmented

scale. The purpose of step three is to measure the amplitude (or height) of the

PAM signal

and assign a decimal value that defines the amplitude. Based on the

quantizing scale, each sampled signal is assigned a number between 0 and +127

to define its amplitude.

STEP FOUR:- ENCODING

In the fourth step of A/D conversion process, the quantized samples are encoded

into a digital bit stream (series of electrical pulses).

A DIGITAL ENCODER-

It recognizes the 255 different voltage levels of the quantized samples. Converts

each into a specific string of 8 bits (1s and 0s) that represent a particular voltage

value. Fig.3.14 is helpful for understanding the binary code used in the encoding

step. Each bit position in the 8-bit word (byte) iis given a decimal weight (2 to

some power ), except for the first bit position. Using this coding scheme, we can

code any number between +127 and –127 and zero.

For example:- If the PAM signal measures +45 on the quantizing scale, the output

of the encoding step is 10101101 (fig 3.15). This binary number (or 8 bit word) is

transmitted over the network as a series of electrical or optical pulses. This series

of pulses is called a digital bit stream. The PCM process requires a

64000bps channel to encode a 4kHz audio input signal because

8000samples/sec.*8

bits/word=64000bps. This is known as the DS0 (Digital Signal 0) or VF (Voice

Frequency) in the digital hierarchy. It is the basic building block of the

digital network.

DIGITAL-TO-ANALOG CONVERSION-

At the receive end of the transmission, the digital signal may need to be

converted back into its analog form. The digital-to-analog (D/A) conversion

consists of two steps .

Each 8-bit word (byte) that enters the decoder results in one PAM signal value.

The decoder:

Reads 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 smoothes out the stream of 8000 pulses per second into an analog waveform that closely resembles the waveform that was input into the A/D converter at the originating end. The filter stores a part of each pulse’s energy and slowly releases it until the next pulse arrives. The filter thus reconstructs the analog signal at a rate of 8000 times per second.

POWER PLANT:

The power plant gives the supply to the whole exchange so there equipment such as the

transformer there is work to learn how this work there is also a set of the invertors. There may be

failure of the power supply due to the any fault in the SMPS. Main problem occurs due to the

power supply failure in the exchange then the battery gets the automatically starts but these

battery can give the power supply only the 5-6 hours then voltage goes on decreasing. The voltage

should remain the –48v if it is not then the exchange will stop working so when such happens

the generators get started.

POWER PLANT IN EXCHANGE

INTRODUCTION

The POWER PLANT of any telecommunication system is usually referred as the “Heart” of

installation since the communication system can function only as long as power supply is available.

The importance of power plant for any system need not be stressed.

Therefore, utmost care has to be taken for their proper handling to ensure uninterrupted and fault

free working of power plant.

POWER PLANT REQUIREMENT

The requirement of power plant for main exchange and are different because of the types

of equipments. In main exchange equipment there is very little variation in peak hour and slack

hour load of power plant contrary to electromechanical exchange where the load varies with

traffic. Therefore, power plants are designed for continuous handling of constant load-both at RLU

and main exchange.

Any Power Plant for communication should have two basic characteristics:

a) Reliability of components of Power Plant and continuity of power supply.

b) The power fed the exchange equipments should be free from hum, noises.

ARRANGEMENT AT MAIN EXCHANGE

The main exchange requires two types of supplies. The first one is –48 v dc supply mainly

for switch room and second one is 220 v ac supply for OMC room, exploitation room and switch

room mainly for computer peripheral devices. The –48 v dc supply requirement is met by a

power plant similar to all other types of exchanges. This has essentially rectifiers

on load sharing basis, two battery sets and a switching cubicle. The operation and

maintenance features are same as in other types of exchanges.

The requirement of 220 v ac power supply is met through direct main supply and invertors which

converts –48 v dc supplies into 220 v ac supply. The –48 v dc power is taken from rectifiers and fed to

invertors. The invertors convert the dc supply into 220 v ac supply. The reasons for using invertors are,

firstly uninterrupted power supply due to battery available

as standby and secondly availability of stabilized power supply, free from variations in

voltage and frequency. This ensures safety of both the equipment and the programs stored

in various storage devices.

PROVISION OF STAND BY POWER SUPPLY

The power plant essentially consist of two units:

1. Float Rectifiers

2. Batteries

FLOAT RECTIFIER:

It converts A.C. voltage into D.C. voltage (+48V). There may be several Float Rectifiers in the

exchange depending upon requirement of load.

The rectifying element consists of silicon diodes of appropriate voltage & current ratings so

as to suit the charging current required for charging of batteries under different conditions. The

battery charger can also be used as a float charger in the event of failure of float

rectifier

by utilizing the filter element associated with later. Under these circumstances, o/p voltage can be

adjusted between 50 and 52 volts under varying conditions of main voltage and

load.

The float rectifier has fully automatic stabilization of voltage (51.5 +- 0.5 volts.).

BATTERIES:

There may and must be two or three set of batteries in an exchange. Each set contains 24

cells, each having voltage to be 2v which turns to be approximately 2.5 amperes in the form

of current. Both of the cells trace voltage of 48v by getting combined as illustrated below:-

Voltage of one cell (24v)+ voltage of other cell (24v)=

48v (total voltage)

if the main supply is cut-off then we use these batteries for some time until their charge

gets finished. Batteries are connected in parallel to the power supply so that if main supply

is functioning then batteries are cut-off otherwise batteries will handle the entire load of the exchange

just after the failure of main power supply.

Batteries are also called to be the secondary voltage sours.

BATTERY CHARGER:

It charges the batteries automatically when the voltage of batteries goes down to a certain

level with the passage of time or due to their extensive and long use. It keeps on charging the

batteries until they are fully charged.

Both MAIN EXCHANGE and RLU exchanges in many parts of the country are subjected to

prolonged power cuts. Therefore, battery capacity is not at all sufficient for maintaining

uninterrupted supply to various units in the exchange. Also to maintain proper

environmental conditions inside main exchang air

conditioning plants have to be run continuously for which continuous mains power supply is

required. Therefore, to take care of all possibilities and ensure continuous supply engine-

alternator must be provided at both the places i.e. Main exchange and RLU exchange.

Preferably a standby engine alternator should also be provided. Sufficient quantities of diesel

oil should also be kept into stock taking into account the supply of oil is erratic, minimum

72 hours storage should be maintained.

The capacity of engine alternator provided at main exchange and RLU depends on their load

requirements of the equipment and a/c plant. At RLU, the load is comparatively very small due to

very few types of equipment and only window type ac units are required there.

CONCLUSION

The power plant is a very vital organ of main and RLU exchanges. The main exchange

power plant requires all the units of regular power plant used in other exchanges for –48 dc supply

and in addition it requires 220 v ac supplies for OMC and peripheral devices. The provision of

engine alternator is a must at main and RLU exchanges and it should be capable of taking

combined load of exchange equipments and a/c plant.

SWITCHING ROOM:

The main work in the switching room is only to install the modules that are discussed

before. First is to install the racks in the room and then to install the modules in these racks and

then connect them with the MDF room through the transmission room. After the installation

of the switch room, there is the work to operate them by the software and maintain them

through the OMC room and other fault occurs in the cards that are replaced when there is fault in

them and send for the repairing. The fault occurs due to any fault in

the electronic equipment install in that card.

THE RISK FACTOR OF EXCHANGES’ WORK

The work in the few parts of the exchange is very risky. These parts are given below:

Transformer room

Power plant

Battery room

Switching room

TRANSFORMER ROOM:

In this the voltage is very high i.e. is 11000v coming from the high-tension supply of the

P.S.E.B. Therefore, the single touch to the any naked wire can burn. There is too much risk

so there should be awareness about the equipment in this room.

POWER PLANT:

In the power plant there is the battery room as well as the invertors, which also work on the

AC voltage to touch the any equipment is also dangerous in this room. In the battery room there is

the acid filled in these batteries that is changed time by the time so there is also dangerous

because the single touch to this acid can burn your skin.

SWITCHING ROOM:

There is the high dc voltage in this room so there is also risk to touch these cards of this switching

room. One thing should remember that we should done any process in this room when we can do that

comfort other wise it can damage our body or the any module in the frame.