This document provides an industrial training report on Bharat Sanchar Nigam Limited (BSNL) submitted by Pramod Kumar Maurya. It includes an acknowledgment, introduction to the telecom industry and BSNL in India. It also covers various topics related to telephone exchanges like types of exchanges, switching equipment, telephone instruments, power plants and other abbreviations used in telephone exchanges.

1. G.L. Bajaj Institute of
Technology & Management
Computer Science & Engineering Department
(Session: 2012-13)
Industrial Training Report
On BSNL
_____________________
Submitted by:
PRAMOD KUMAR MAURYA
0919210069
CS 4th
year
1

2. ACKNOWLEDGEMENT
It is a pleasure to acknowledge many people who knowingly and
unwittingly helped us, to complete our training . First of all let us thank
God for all the blessings, which carried us through all these years.
We extend our utmost gratitude to Mr. Atul Jain J.T.O who has always
stood by our side and guided, appreciated and encouraged us to get
into more and more ventures. Continuing the same, he enlightened us
in the various stages during the training and provided us with many
insights and useful examples, which proved to be of immense help in
successful completion of this course.
Name- Pramod Kumar Maurya
. Roll.no-0919210069
INDUSTRY PROFILE
2

3. NDUSTRY PROFILE The telecom industry is one of the meteoric
growing industries in India. India has nearly 200 million earpiece
lines making it the third largest in the world after China and USA.
With a growth rate of 45%, Indian telecom industry has the
highest growth rate in the world. fib of Indian
Telecommunications started in 1851 when the original
operational land lines were position by the government near
Calcutta (seat of British power). Telephone function were
introduced in India in 1881. In 1883 telephone helps were
coordinated with the postal system. Indian Radio Telegraph
Company (IRT) was make in 1923. After independence in 1947,
all the unlike telecommunication companies were nationalized to
form the Posts, Telephone and Telegraph (PTT), a monopoly
electric discharge by the governments Ministry of
Communications. Telecom sector was considered as a strategic
profit and the government considered it best to bring chthonic
states control. The first wind of reforms in telecommunications
sector began to tend in 1980s when the private sector was
allowed in telecommunications equipment manufacturing. In
1985, Department of Telecommunications (DOT) was
established.
It was an exclusive provider of domestic and long- distance
service that would be its own regulator (separate from the postal
system). In 1986, two wholly government-owned companies were
created: the Videsh Sanchar Nigam Limited (VSNL) for
international telecommunications and Mahanagar Telephone
Nigam Limited (MTNL) for service in metropolitan areas. In
1990s, telecommunications sector benefited from the general
commencement up of the economy.
3

4. Contents
Topics Page No.
INTRODUCTION………………………………….…………………………………….6
E-10B exchange
OCB exchange
5ESS exchange
TELEPHONE EXCHANGE……………………………………………………………7
Manual Exchange
Automatic Exchange
TELEPHONE INSTRUMENT.................................................9
PMBX..............................................................................................................................12
PRIVATE MANUAL BRANCH EXCHANGE .............................................................12
EPABX............................................................................................................................12
ELECTRONIC PRIVATE AUTO BRANCH EXCHANGE .....................................12
ALU.................................................................................................................................12
ARITHMETIC LOGIC UNIT ..........................................................................................12
LDCA..............................................................................................................................12
LONG DISTANCE CHARGING AREA..........................................................................12
SDCA..............................................................................................................................12
SHORT DISTANCE CHARGING AREA .......................................................................12
LAN.................................................................................................................................12
LOCAL AREA NETWORK.............................................................................................12
E-MAIL............................................................................................................................12
ELECTRONIC MAIL.......................................................................................................12
I-NET...............................................................................................................................12
4

5. COMPUTER NETWORK WITHIN LAND.......................................................................12
BATTERY CHARGER- IT IS A PART OF POWER PLANT IN AN EXCHANGE AND
PROVIDES INITIAL, REFRESHER AND BOOST CHARGING FOR THE BATTERIES.
THE 24 LEAD ACID CELL BATTERY CAN BE CHARGED FROM 1.8V PER CELL
TO 2.7 VOLT PER CELL...............................................................................................15
A.C. INPUT 350V, 370V, 390V, 410V, 430V+- 10%.............................................15
D.C. INPUT FOR 24 CELLS CHARGING...........................................................15
COMPONENTS 3-PHASE STEP-DOWN MAIN TRANSFORMER..........................16
FLOAT SYSTEM: IN THIS SYSTEM THE BATTERY SUPPLIES ONLY THE
EMERGENCY RESERVE REQUIREMENTS OF THE EQUIPMENTS DURING MAINS
FAILURE. THE BATTERY IS ALWAYS KEPT CONNECTOR ACROSS THE LOAD IN
THE FLOAT SYSTEM IN A FULLY CHARGE STATE BUT THE OPERATING
CONDITIONS ARE SO MAINTAINED THAT THE FLOATED BATTERY DOES NOT
DISCHARGE IN TO THE LOAD AS LONG AS THE MAINS POWER SUPPLY IS
AVAILABLE. AS SOON AS THE MAINS FAIL THE FLOATED BATTERY TAKES
THE LOAD OF THE EACH. THUS THERE IS ABSOLUTELY NO BREAK IN D.C.
POWER SUPPLY WHEN THE MAINS POWER SUPPLY FAILS................................17
Optical fibers have following two parts...........................18
ADVANTAGE OF OPTICAL FIBRE..............................................................................18
FUNCTIONS...................................................................................................................19
Address Bus-..................................................................19
Control Bus-...................................................................19
Facilities To Subscriber-..................................................20
Facilities To Administration-...........................................21
Versatility-.....................................................................21
Facilities To Maintenance personal-................................21
PROCESSORS WHEN........................................................................23
XGE WHEN.......................................................................................................24
FUNCTION FUNCTION.......................24
5

6. Function of a electronic exchange...................................26
PART OF BASE MODULE............................................................................................34
TIME SWITCH UNIT.......................................................................................................34
TERMINAL INTERFACE CONTROLLER .............................37
R.A.X CONTROLLED PROCESSOR (R.C.P.)........................37
……………………………………………………………………38
6

7. INTRODUCTION:
From the ancient days it has been deepest human desire to stay closed with
those they need. Many different ways have been evolved and implemented
to fulfill this; Telephony is also such a mechanism that uses Telephone,
electronic equipment that converts sound into electrical signals that can be
transmitted over distances and then converts received signals back into
sounds.
Electronic exchanges play major role in this system. It’s a workplace
that serves telecommunication facilities where lines from telephones can be
connected together to permit communication. The switching center that
houses the terminating and switching equipment is called TELEPHONE
EXCHANGE. Following are the types of exchanges where upon we have done
training and prepared this report:
1) E10B- This was the first ever fully electronic exchange of our country
established in 1990. This name stands for Electronic-10,000
subscribers’ capacity- B (series code). It had given first ever speeded
dialing feature in telephony.
2) OCB- This name stands for Optical-Cable-B (series code). It was
established in India in 1997. And it had first ever introduced concepts
of mobile exchange, call tracing, caller ID, ISDN etc. in our country
along with significantly increasing the number of connected
subscribers per exchange.
3) 5ESS- This name stands for 50,000 subscribers’ limit- Electronic-Sub-
Subscribers. It had implemented concept of sub-subscribers to
increase the subscribers’ limits.
7

8. TELEPHONE EXCHANGE
The switching center that houses the terminating and switching
equipment is called TELEPHONE EXCHANGE.
TELEPHONE EXCHANGE
Manual exchange Automatic exchange
Cordless Cord Type Strowger Crossbar Electronic
(PBX Board) magneto switch
CB switchboard
Analog
Digital
Fig 1: Types of Telephone exchanges
• Manual Exchange- Manually operated by a attendant
called telephone operator.
• Automatic Exchange-Operated by automatic switching
equipment.
• Local Battery System-Battery bar locally provided with
the subscribess for energising the transmitter of their
telephone for speech service.
• Magneto Switch-Ringing was done by a hand generator
producing 17 cps at 75v Magneto type subscriber telephone
instrument is used with this system which has a hand
-Generator set within it.
• Central Battery-Battery was provided centrally at
exchange and it provided energy to all the telephone
connected to the exchange.
8

9. TELEPHONE INSTRUMENT
Telephone instruments are mainly of three types –
1 Magneto Telephone Instrument
2 C.B. Telephone Instrument
3 Automatic Telephone Instrument
AUTOMATIC TELEPHONE INSTRUMENT
Rotary Dial Type Electronic Push button
Type
IPT677 Type is allowed for
Electronic Exchange
D.T.M.F. Decadic
Fig 2: Telephone Instruments
Rotary Dial Telephone - It comprises three type of circuit
(Fig)
1. Dialing Circuit – Impulse spring produces make break
pulses. Dial of normal (DON springs are also provided).
2. Ringing Circuit – In ringing circuit 1.8 Micro Farad
condensor allows only A.C. ringing current and does not
allow D.C. to flow throw it and thus save energy.
3. Speech Circuit – Transmitters converts acoustical energy
into electrical energy and receiver converts electrical energy
into acoustical energy.
Push Button Type – In decadic dialing make break pulses
are generated and in dual tone multi-frequency (DTMF) is
generated by mixing of two frequencies from two sets of
frequency one set of lower and other set of higher frequency
(fig-2).
9

10. Difference between DTMF & Decadic Dialing
Decadic Dialing DTMF Dialing
• Make break pulse dialing
• Store dialing.
• Dialing speed slow 0.8
digits per second.
• Different digits take
different times in dialing.
• Frequency dialing.
• Instantaneous dialing.
• Dialing is faster, 7digits
per second.
• Same time is consumed in
dialing any digit.
TELEPHONING SYSTEM
Telephonic System can be divided into two parts (fig-3)
1 Internal – Exchange side equipment fall in this category such
as –
• Main Distribution Frame (MDF)
• Switching Equipment
• Power Plant
• Carrier Station and Microwave station.
2 External – it comprises equipments from MDF to rosselt of
telephone such as –
(a) U/G Cable Section: MDF to Distribution point box.
(b) O/H Line Section: Distribution point to Subscribes window.
(c) Internal Wiring: Subscribes window to rosselt of telephone.
10

11. Primary Cables - From main Distribution Frame to cabinet the
underground cables are called Primary Cables. It has 600, 800,
1000, 1400 pairs.
D.P.
O/H
CABINET PILLER SUBS
WINDOW IIIII
U/G
ROSSETT
Fig 3: TELEPHONING SYSTEM
SWITCHING EQUIPMENT
ANALOG SWITCHING- the term analog switching is used as a
synonym for space division switching. A path in an analog
switching system consists of number of path elements called
links, which are interconnected by cross points.
CROSS POINTS- It is a miniaturized switch, which is used to
connect various links of speech path. It has two stable states-
• Closed connecting (0.1 to 200 ohms)
• Open blocking (10 to 25 mega ohms)
11
SW
R
O
O
m
M
D
F

12. ABBREVIATIONS USED IN TELEPHONE EXCHANGE
PCM Pulse code modulation
SSTD Subscriber’s trunk dialing
OTD Operators trunk dialing
ISD International subscriber’s dialing
FAX Facsimile auto Xerox
TAX Trunk auto exchange
PCO Public call office
IDN Integrated digital network
ISDN Integrated service digital network
CBNM Central battery non multiple
CBM Central battery multiple
PABX Private auto branch exchange
PBX Private branch exchange
PMBX Private Manual Branch Exchange
EPABX Electronic Private Auto Branch Exchange
ALU Arithmetic Logic Unit
LDCA Long Distance Charging Area
SDCA Short Distance Charging Area
CDOT Center for Development of Telematics
LAN Local Area Network
WAN Wide Area Network
E-MAIL Electronic Mail
I-NET Computer Network within Land
INTERNET International Computer Network
TELEX Telegraphic Exchange
CTO Center Telegraph Office
DTO District Telegraph Office
SPC Stored Programme Control
DTMF Dual Tone Multi Frequency
RAX Rural Auto Exchange
12

13. Fig 4: Various types of cross points
Various types of cross points have been evolved, but sealed
contact type reed relays are commonly used in analog switching
networks. Memories are preferred.
A basic switching stage consists of reed relay arranged in a
row and columns in the form of a matrix. Several such stages
connected in tandem constitute a switching network. There are
two types of switching network viz. Unidirectional and bi-
directional. Transit exchanges are unidirectional network.
Whereas the subscriber local exchange are bi-directional
networks.
DIGITAL SWITCHING: Digital switching has become a
synonym for time division multiplex. In digital switching system,
13
CROSS
POINT
ELECTRO MECHANICAL ELECTRONIC
CROSSBAR
DERIVATIVE
SWITCHES
SEALED
SWITCHES BIPOLAR
FIELD
EFFECT
REED
RELAY
OTHERS
THYR
ISTER
TRANSISTOR FET MOS CMOS
ELECTRICALLY
LATCHEDMAGNETIC

14. digital signals are switched in two nodes, time switching and
space switching. Time switching basically involves rearrangement
of channel sequence in the same PCM from whereas space
switching involves interconnection of same channels of a
different PCM high way. In a practical digital switching both time
and space switching stage 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.
Synchronization of various exchange
clocks in a digital network is essential to provide required
services. Causing a slip due to mismatch between the clocks will
result in multination of information and description of services.
Hence, slip has to be controlled using plesiochronous and
synchronous approaches.
MODULATION: Often the message signal is not suitable for
transmission over the medium in its original forms. Therefore,
the message signal is located in a carrier wave, which is more
suited for transmission over the medium. This process is called
modulation of the carrier wave. At the receiving end station the
message signal received back from the incoming modulated
carrier wave by a process is called demodulation which is inverse
of modulation.
MULTIPLEXING: Modulation enables multiple message signals
to be simultaneously transmitted over the same medium without
interfering each other. Transmission of multiple message signals
simultaneously over a medium is called multiplexing.
Modulation is essentially a process of relocation of a
message signal in a carrier wave, which is more suitable for
transmission for transmission over the medium than the
message signal itself.
Three types of modulation can occur-
1. Amplitude modulation
2. Frequency modulation
3. Pulse modulation
14

15. POWER PLANT
One of the primary requirements of any computer system is that
service will be available to the subscriber at any time. In the vast
majority of exchanges the electrical energy required for
signaling, switching, speed transmission mains. The exchange
power system is designed to provide continuous and
uninterrupted telephone service even when the public electric
fails. We can divide power plant in three major blocks such as
• FLOAT RECTIFIER
• BATTERY CHARGER
• SWITCHING CUBICAL
FLOAT RECTIFIERS: - It constitutes one item of telephone
exchange power plant. The function of float rectifier is to drive
power supply A.C. mains and deliver to the telephone exchange
equipments continuous and filtered D.C. supply at nominal
voltage of 50 volt (actual terminal voltage is 51.5 volt). The
voltage of supply is to be maintain with closed limits of +-0.5
volt unvarying impact voltage and output load currents, on set or
more batteries are normally floated across the float rectifier in
parts are as follows
 3-phase step down transfer
 Half controlled 3phase bridge rectifier
 Filter circuit and controller
BATTERY CHARGER- It is a part of power plant in an exchange
and provides initial, refresher and boost charging for the
batteries. The 24 lead acid cell battery can be charged from
1.8V per cell to 2.7 volt per cell.
A.C. INPUT 350V, 370V, 390V, 410V, 430V+- 10%
50Cps+-2Cps
3-phase 4wire system
D.C. INPUT For 24 cells charging
44volt to 54volt at 150 Ampere
45 volt to 65 volt at 75 Ampere
15

16. COMPONENTS 3-phase step-down main transformer
Ballast chokes
3phase full wave rectifier
SWITCHING CUBICAL: -
It provides facility for
 Floating any one or more batteries
 Charging any one battery at a time
 Supervisory alarms for all associated units
 Incorporates auto-paralleling panel, which contains the
circuiting for sequential switching i.e., for switching ‘ON’ and
‘OFF’ automatically to suit exchange load one or more float
rectifiers whose output are in parallel.
In case where exchange load is met directly from the mains,
provision is also made for alternate source of supply in the event
of mains failure .The emergency energy is derived from
 Battery of secondary cells.
 A combination of batteries of secondary cells and prime
mover generator sets.
Secondary cell or accumulators used in department are
mostly of bad acid type and hard rubber container
16

17. FLOAT SYSTEM: In this system the battery supplies only the
emergency reserve requirements of the equipments during
mains failure. The battery is always kept connector across the
load in the float system in a fully charge state but the
operating conditions are so maintained that the floated
battery does not discharge in to the load as long as the mains
power supply is available. As soon as the mains fail the
floated battery takes the load of the each. Thus there is
absolutely no break in D.C. power supply when the mains
power supply fails.
Various forms of standby supply arrangements are available, out
of which the engine generator sets are most common. This
consists of two separate machine an engine and the other
generator or alternator. A generator is an electromagnetic
machine, which converts natural form of energy to mechanical
energy.
A diesel engine is an internal combustion engine, which operates
on liquid fuel.
OPTICAL COMMUNICATION
Optical fiber is a latest means by which we transmit information
(speech, data, video), in the form of light through optical fibers
(fig. 5). This communication requires five stages.
 Information is converted in to electrical signals by encoder.
 Electrical signals are converted in to optical signal by means
of a light source.
 Optical signals are transmitted to the distant station
through optical fiber media.
 At the distant station optical signal are again converted in to
electrical signal by means of detector.
 The electrical signal is further converted in to information
(speech data and video) by means of decoder at the distant
station.
17

18. Optical fibers have following two parts
 Core (Internal part)
 Cladding (Outer periphery)
Out surface of cladding creates a difference in refractive index
due to which total internal refraction phenomenon happens in
central core. Core and cladding made of transparent substance
like silica or Borosilicate, but the refractive index of cladding is
less than core.
ADVANTAGE OF OPTICAL FIBRE
 Thousands of circuit can be provided through optical fiber.
 The cable is less in size and diameter and so easily
transportable.
 Noiseless circuit can be provided through this media
 There is no risk of fire, electric shock and short-circuiting.
 There is no effect of excessive temperature on it
 Repeaters are provided at long intervals and repeater less
chain is also possible.
Fig 5: Organization of typical microcomputer
18
MICROPROCESSOR
I/P DEVICE
O/P DEVICE
SEMICOND
UCTOR
MEMORY
FLOPPY
DISKS
C.D.
CASSETTS

19. Microprocessor
A microprocessor is a complex electronic processor on a single
silicon chip (Large-scale integrated circuit). It is capable of
performing arithmetic and logical functions under the control of
program.
Functional units of microprocessor-
1. Arithmetic and logic unit
2. Registers (Temporary memory)
3. Timing and control unit
Functions
1. FETCH
2. DECODE
3. EXECUTE
Bus System
Various switching peripherals are connected to central processor
by means of a common bus system. A bus is a group of wires on
which data and command pulses are transmitted between the
various sub units of a switching processor and switching
peripherals. The device to be activated is addressed by sending
its address on address bus. The common bus system avoids
costly interconnection among various devices (Fig-6).
Address Bus-
The address bus normally consists of 16 lines. To access a
particular device or memory location, the microprocessor
transmits its address on these lines.
Data Bus-
The data bus usually has parallel conductors. It is a path for
transferring data in and out of the processor.
Address bus is unidirectional and data bus is bi-directional.
Control Bus-
Control bus is used for transmitting and receiving control signals
between microprocessor and various I/O devices.
19

20. Electronic Exchange Facilities
Electronic exchange facilities can be divided in three major
groups-
1. Facilities to subscribers
2. Facilities to Administration
3. Facilities to maintenance personal
Facilities To Subscriber-
• MFC Push Button Dialing ( DTMF dialing)
• Priority subscriber lines
• Toll (outgoing call) detraction
• Service interception
• Abbreviated dialing
• Call forwarding
• Do not disturb
• Conference call
• Camp on busy
• Call waiting
• Call repetition
• Third party inquiry
• Priority of calls to emergency position
• Subscriber charge indicator
• Call charge print out or immediate billing
• Malicious call identification
• Interception or announcement
• Connection without dialing
• Automatic wake-up alarm service
• Hot line or private wire
• Denied incoming call
• Instrument locking
• Free of charge call
• Collect call
20

21. Facilities To Administration-
• Reduced switch room accommodation
• Faster installation and easy extension
• Versatility
Versatility-
1. Adopting various signaling and call charging pattern
2. Increase facilities in rating and numbering by simple
commands
3. MANAGEMENT FACILITIES: Automatic billing, centralized
supervision and maintenance, automatic load control and
traffic data recording and analysis
4. Wide range of subscriber services
5. Wide range of dial speed( 5 to 22 digits/ second)
6. Easily handling of group line (PBX)
7. Unlimited possibilities for line translation, digits storage and
route analysis
8. Compatibility with new techniques in signaling an data
switching
Facilities To Maintenance personal-
• Automatic fault processing (Hardware checking)
• Automatic changeover to standby unit in case of fault in one
unit
• Diagnosis
• Statistical programme
• Blocking facilities
• Overloading security
I.D.N (Integrated Digital Network)-
I.D.N is a network in which both the transmission media and
switches employ digital techniques. This network combines the
advantages of both switching and transmission. In this system,
speech signals are converted to digital signals at transmitting
and receiving stations. This system has easy maintenance and
supervision, which is carried through computers. This enables
more flexibility in terms of remote controller and subscriber units
and reserved in saving of circuit cost.
21

22. I.S.D.N (Integrated service digital network)-
Presently, there is a multiplicity of services such as telephoning,
telex, data cable television etc. that are provided by separate
dedicated network for each of them. These non-integrated
services require separate switching system. Special subscribers
access plants, individual trunk networks and separate operations
and maintenance staff. These services are costly to the
subscriber as well as to department. In I.S.D.N, these services
are provided by one integrated network, which can be accessed
by different customer premises equipment.
In I.S.D.N, subscriber loop from the local exchange will be
terminated at the network terminal (N.T). From N.T the signal
will be distributed to the information sockets. These information
sockets will be distributed in the premises just like power sockets
are distributed for providing on the information sockets, different
terminals for voice and nonvoice services can be connected.
22

23. Control System
A system can perform any work by a single processor or
multiprocessor.
Fig 6: Functional Division & Segmenting
Functional Division- each processor does only a certain
function exclusively allotted to it for the entire exchange.
Segmenting –each processor carrying out all the functions for a
certain segment of the exchange (Fig 7).
Redundancy- there is different standby methods available in
control structure of electronic telephone exchange.
1. Hot standby redundancy
2. N+1 redundancy
3. Synchronous replica duplication
4. Load sharing (fig 11)
Hot standby redundancy- there is a standby available for all
functional units separately. Even anyone or more units fail. The
working of exchange will not be affected (fig 8).
Processors when
Segmenting is used
23
Control System
Single Processor System (Monarchic)
Multiprocessor System (Distributed Control)(Oligrachic)
One level System Hierarchical System
Functional Division Segmenting 2 Levels 3 Levels

24. P1 P2 P3 PN
F1 P1’
F2 P2’
F3 P3’
Processes
XGE when
Function function
division
is used
Fn Pn’
Ultimate capacity of XGE
Fig 7: Functional Division & Segmenting
To switching network
Fig 8: Hot Standby
N+1 Redundancy- one unit is extra added in parallel with N
units. In the failure of any one unit, this extra unit takes the load
of the failed unit, but if two units will fail, the exchange cannot
work (fig 9).
24
A1 A2 B1 B2 N1 N2

25. Synchronous replica duplication- the standby unit is always
connected and compared by a clock pulse. If there is a difference
in two units, it means one unit is faulty (fig 10).
Electronic Exchange
These are entirely computerized and electronic devices, which
work on store programme control. Electronic logic component
can do ten thousand times operation during signal in the
comparison of electromechanical exchange.
In an electronic exchange a single control device
simultaneously process a number of calls on time-sharing basis.
Such centralization is more economical, but it is disadvantageous
in making switching system more vulnerable to system failure.
This, however, can be overcome a standby control devices.
These electronic exchanges have powerful processing unit.
Part of control equipment capacity utilize for function other than
for processing. There are two types of electronic exchange.
Analog Exchange- this exchange uses one physical path for one
call. Hence, full availability could not be achieved due to internal
blocking.
To switching network
25

26. CC
Fig 9: N+1 Redundancy
Clock
Copartor
Fig 10: Synchronous Replica Duplication
Digital exchange- digital switching enables sharing a single
path by several calls. Hence, it provides full availability and there
is no change of internal blocking.
Function of a electronic exchange
The primary purpose of an exchange is to provide a
temporary path for simultaneously bi-directional transmission of
speech between
26
MA MB
A BC
~
TELEPHONE ENVIRONMENT

27. 1. Subscriber lines connected to the same exchange (local
switching)
2. Subscriber lines and trunks to other exchange (outgoing
trunk calls)
3. Subscriber lines and trunks from other exchange (incoming
trunk call)
4. pairs of trunks towards different exchange (transit
switching)
These are called the switching function of an exchange and
are implemented through the equipment called the switching
network.
An exchange which can set up the just first three types of
connection is called a subscribers of local exchange it is called a
transit or random exchange.
Other distinguish function of an exchange
1. Exchange of information with the external environment
(subscriber’s line or other exchange) that is signaling.
2. Processing and signaling information and controlling the
operation of switching network that is control.
3. Changing and billing.
All these function can be provided more efficiently using
computer controlled electronic exchange and functional division
of a store programme. Control electronic exchange can be given
as (fig 11)
Line & Trunk Terminal Equipment
27
Switching
Network

28. BUS
Switching
processor
Fig 11: Functional Subdivision
Of an SPC Exchange
28
Distributor
Scanner Marker
Common
Channel
Signalling
terminal
Channel
Associated
Signalling
terminal
Central
Control
----------------
Memories
Man-Machine Dialogue
peripheral

29. 1. Terminal equipment- it provides an individual basis for
each subscriber’s line and for inter exchange trunk.
2. Switching network- the main function of switching is
setting up the temporary connection between two or more
exchange termination and transmission of speech and
signals between terminations with reliable accuracy.
Types of switching system
There are following two types of switching system in India
(a) Space division switching system- in this system a
continuous physical path is being set up between input
and output terminations. This path is separated for each
connection and is held for the entire duration of the call.
(b) Time division switching system- in this system a
number of connections shares the same path on time
division sharing basis. The path is not separate for each
connection, rather, is shared sequentially for a fraction of
time by different path. This process is repeated
periodically at a suitable high rate. The repetition rate is
8 KHz in ones every 125 microseconds for transmitting
speech on telephone network.
Functional subdivision of an SPC exchange
1. Switching processors consisting mainly of processors and
memories.
2. Switching peripherals (scanners distributors and marker)
are interface circuit between control system terminal
equipment and switching network.
3. Signalling interface depending on type of signaling used.
4. Data processing peripherals (tele-type writers, printers
e.t.c.) for man machine dialogue for operator and
maintenance of exchange.
29

30. Central Control
It is a high-speed data processing unit that controls the
operation of the switching network (fig 12)
Programme Store- in this, set of instructions called programme
are stored. The programmes are interpreted and executed by the
central control.
Data Store- it provides for the temporary store of transient
data, required in processing calls such as digits dialed by
subscribers, busy/idle status of lines and trunks e.t.c.
Data Store is temporary memory whereas translation and
programmes stores are semi permanent type. The information in
semi transparent memory does not change during the processing
of call but the information in data store changes the continuously
origination and termination of each call.
Switching peripherals
The time intervals in which the processor operates is in the
order of micro second, while the components in telephone
switching system operate in millisecond. The equipment known
as switching peripherals is interface between two equipment
working at different speeds. The interface equipments act as
speed, buffer as well as enables conversion of digital logic signals
from the processor to the appropriate electrical signal to operate
relay cross points etc scanner, distributor and marker fall under
this category of devices.
Scanner-its purpose is to detect and inform the central control
of all significant events on subscriber lines and trunks connected
to the exchange.
Marker-it performs physical set up and release of paths through
the switching network under the control of central control.
Distributor- it is a buffer between high speed, low power central
control and relatively slow speed high power signaling terminal
circuit.
30

31. Line Interface
To enable an electronic exchange to function to the existing
outdoor telephone network certain interfaces are required
1. Analog subscriber line interface
2. Transmission interface
3. Signaling interface
Analog subscriber line interface-
The function of a subscribers line interface for each two-wire line
is often known by the acronym “BORSHCT” which is
B- Battery Feed
O- Overload Protection
R- Ringing
S- Supervision of loop status
H- Hybrid
C- CODEC
T- Connection to test equipment
All these functions cannot be performed directly by
electronic circuiting so, suitable interfaces are required.
Transmission Interface- transmission interfaces between
analog trunks and digital trunks such as A/d and D/A converters
known as CODEC. These may be provided per line and per trunk
basis ( on the basis of one per 30 speech channels).
Signalling Interface- A typical telephone network may have
various exchange systems (manual, stronger, crossbar and
electronic e.t.c.) each having different signaling schemes. In
such an environment an exchange must be in accordance with
several different signaling codes.
31

32. Signaling
There are four types of signaling possible: -
1. Ring Down Signalling
2. DC signaling
3. E & M signaling
4. Digital signaling
Digital Signalling can be performed in two ways: -
Channel Associated Signalling- Initially all signaling between
automatic exchanges was decadic i.e. telephone numbers were
transmitted as trains of 1 to 10 pulses, each train representing
one digit. To increase the speed at which the calls could be set
up and to improve the reliability of signaling, compelled
sequence multi frequency signaling system was then introduced.
In this system each signal transmitted as a combination of two
out of a group of say 5 or 6 frequencies. In both decadic and
multi frequency systems the signal for each call are sent over a
channel directly associated with the inter exchange speech
transmission circuit is used for that call. This is termed as
channel associated signaling.
Common Channel Signalling- recently a different technique
has been developed known as common channel signaling. In this
technique all the signaling information for a number of calls is
sent over a signaling link independent of the inter-exchange
speech circuit. Higher transmission rates can be utilized to enable
exchange of much larger amount of information. This results in
faster called set up. Introduction of new services i.e. abbreviated
dialing and more retrials ultimately accomplish higher call
completion rate. Moreover it can provide network management
and traffic engineering.
32

33. To switching Network
Fig 12: Central Control
Configuration and is based on 32 channels PCM structure. It uses
a basic rate of 64 kilo bit per second and 2 mega bit per second .
Primary multiplexing control is distributed over the system by
using 16 bit microprocessors. All the critical control circuitry as
built in redundancy.
System hardware utilized advanced concepts
in microelectronics for a compact and optimum design. Basic
memory unit has been implemented as 2 mega bit dynamic RAM
based with 256 kilo bit as the basic dynamic RAM chip.
Single chip digital signal processors are used for implementing
DTMF and MF receiver.
A high performance, high density
VLSI chip detects multiple tones simultaneously performs signal
filtering on four channels. This approach reduces costs, power
dissipation and saves space on PCM. Analog to digital conversion
on the line circuit has been achieved by using a per channel
coder decoder (CODEC) chip.
33
Central Control Processor
Programme
Store
Translation
Store
Data
Store

34. BASIC MODULES
1. Base module (Fig 13)
2. Central module
3. Administrative module
4. Input output module
PART OF BASE MODULE
1. Terminal units for interfacing analog lines and trunks and
providing special circuit.
2. Digital terminal unit – for interfacing digital trunks.
3. Time switching unit - for message switching and provision
of service circuit.
4. Base processor unit - for control message, communication
and call processing function.
CARDS IN TERMINAL UNIT
1. Line circuit card.
2. Analog trunk card.
3. Signalling processor card - scan/drive function origination
detection, answer detection, digital detection analysis
transmission and several detection.
4. Terminal interface card.
TIME SWITCH UNIT
It has a time switch for time switching within the base
module, a message switch for controlled message communication
within the base module and between base module and a service
unit for providing call processing support. Like MF/DTMF circuits
Answering circuits and tones e.t.c. 128 channel multiplexing
circuits and 16 bit microprocessors.
BASE PROCESSOR UNIT
34

35. 1. C.P.U card.
2. Base interface controller card (B.I.C).
3. Bus interface device (B.I.D).
4. Memory unit (M.U).
CENTRAL MODULE
It is responsible for space switching of inter module calls and
communication between base module and administrative module.
LINE MODULE BASE MODULE LINE MODULE
Fig 13: Concentration of base module
35
T U
T U
T U
T U
T U
T U
T U
T U
T U
T U
B P U
T S U
T U
T U
T U
T U
T U
T U

36. CENTRE FOR DEVELOPMENT OF TELEMATICS
 SYSTEM FEATURE OF C.DOT
Single frame terminal unit capable of 128 terminations
Stored program control
Non blocking
Maximum 24 trunks
Modular growth
Man machine communication
Low power consumption
No air conditioning required
No signal fault effect more than 8 terminations
Easy installation
Flexible system dimensioning
Individual subscriber meter up to 1, 00,000 units
 SYSTEM ARCHITECTURE
Terminal groups=4 nos
Terminal interface=4 per terminal group
One terminal interface has 8 terminations of pairs (fig 15)
Analog information over the termination are first change
to digital format at the bit rate of 64 bit per second. Such 32
channels PCM from the4 terminal interfaces are time division
multiplexed to generate 32 PCM link with the data rate of 2.048
Megabyte per second thus from 16 terminal interfaces We get the
4PCM 32 link which are connected to the switching network.
 SWITCHING NETWORK
The switching network is completely non-blocking
network and allows for higher traffic handling capacity. The 4PCM
32 links from terminal groups undergo time division multiplexing
TDM to generate PCM 128 channel link at bit rate of 8 megabit per
second. After switching the channel 128 PCM channel link is
demultiplexed to regenerate 4PCM 32 links at 2 megabit per
second, which carry the switched information back to the terminal
interfaces.
36

37.  SIGNAL PROCESSOR
Signaling information as the dialed digits,
ring trip, trunk seizure etc is separated at the terminal interfaces
and carried to SP on a time-multiplexed link. The SP passes on
this information to the T.I.C similarly, T.I.C uses S.P for effective
any device in the terminations.
E.g. ringing current.
 TERMINAL INTERFACE CONTROLLER
It drives the identities of the calling and the called terminals
and stabilizes a path through the switching network between
these terminals. The T.I.C communicates with the R.C.P on the
high data link controller (H.D.L.C) link for all call related
information.
 R.A.X CONTROLLED PROCESSOR (R.C.P.)
It is a microprocessor-based processor, which handles the call
processing function. R.C.P stands the terminal connection related
information to the T.L.C over the H.D.L.C link.
 MANTENANCE PANNEL
It is connected to R.C.P viz RS232C link for system for
operation and maintenance function
A DC-to-DC converter generates the various voltages
required for the system operation and also provides ringing for
the subscriber loop.
CDOT RAX uses integrated circuit based on metal oxide
silicon, MOS, CMNOS low power schottkey transistor. In,
Transistor logic bipolar 10 types of cards are used.
37

38. DATA PROCESSING PERIPHERALS
1. Man machine dialog terminal
a. Tele type writer
b. Visual display unit
c. Printer
2. Special purpose peripherals employed for carried out repeated
function such as subscriber line testing which requires high
speed.
3. High-speed large capacity data storage
a. Magnetic tape drive
b. Magnetic disk unit
These are used for loading software in processor memory.
4. Maintenance peripheral-Such as alarm initiator special console
are used primarily to indicate that automatic maintenance
procedures are failed and manual attention is necessary.
CONCLUSION-
The electronic exchange work on the principle of stored
program control (SPC). At the call processing are performed on
the basis of a pre designed program which is stored in the
memory of a central processor. Though the initially designed
electronic exchange had single centralized processor, the control
is being decentralized providing dedicated microprocessor
controlled subsystem for improved efficiency and security of the
system. This modular architecture also adds future expansion.
38

39. Requirements
A.TERMINAL INTERFACE
1.Terminator card
2. Subscriber line card (4 wire trunk)
3. Loop trunk card (two wire trunk)
B.SERVICE CIRCUIT
1. Conference card
2. Tone generating card (dial tone, busy tone, number
unattainable tone, ring back tone conference idle tone, RAX
test tone, idle tone not used ton
C.TIME SWITCH AND COMMON CONTROL
1. T.I.C / switching card
2. S.P card
3. RCP card (call processing, administrative and maintenance).
POWER SUPPLY CARD (-48 V)
+5 v_8A regulated
-9V_0.5A unregulated
-5V_0.1 A unregulated
+12 V_1.0 A regulated
Sine wave PWM inverter is used and 25 hz sine wave is out put
Maintenance panel card
Line circuit card(L.C.C)
C DOT BSS MAX
IT is a modular and flexible digital switching
system, which provides economical means of metropolitan, urban and
rural environment. This system employs an open-ended architecture, bar
flexibility of configuration and growth. The syste4m employ a T.S.T
(Time-Space-Time) switching.
39

40. INTRODUCTION:
From the ancient days it has been deepest human desire to stay closed with
those they need with. Many different ways have been evolved and
implemented to fulfill this; Telephony is also such a mechanism that uses
Telephone, electronic equipment that converts sound into electrical signals
that can be transmitted over distances and then converts received signals
back into sounds.
Electronic exchanges play major role in this system. It’s a workplace
that serves telecommunication facilities where lines from telephones can be
connected together to permit communication. Thus the switching center
that houses the terminating and switching equipment is called TELEPHONE
EXCHANGE. Following are the types of exchanges where upon we have done
training and prepared this report:
1) E10B-
2) OCB-
3) 5ESS-
40