Geo34 63

Industrial Training at B.S.N.L. Training Report’12

CAP divides the signals of the telephone line into three bands: voice, upstream channel
and downstream channel. Voice conversations are carried in the 0 to 4
KHz band as they are in all POTS circuits. The upstream channel that carries data
from the user to the server is between 25 and 160 KHz. The downstream channel begins
at 240KHz with a maximum of 1.5 MHz which depends on a number of conditions
such as distance, line noise and number of users. CAP by keeping the three channels
widely separated, minimises the possibility of interference both between channels on one
line and signals on different lines.

DMT also operates by dividing signals into separate channels without using two quite
broad channels for upstream and downstream. The modulation technique that has
become standard for ADSL is called the Discrete Multitone Technique, which combines
QAM and FDM. In ADSL, the available bandwidth of 1.104 MHz is divided into 256
channels. Each channel uses a bandwidth of 4.312 KHz. Each channel is 4KHz wide with
a guard band of .312KHz.. Hence the name Discrete Multitone.

Each sub carrier can support maximum15 number of bits. Depending on signal to noise
ratio for that sub carrier, a decision is taken as to how many bits that particular sub
carrier can support. Every channel is monitored and if the quality is low, the signal is
shifted to another channel. DMT constantly shifts signals between different channels,
looking for the best channels for transmission and reception. Moreover, some of the
lower frequency channels, are used as bi- directional channels for upstream and
downstream. Keeping up with the quality of all channels, monitoring and sorting the
information on the bi-directional

Applied Electronics 34 Model Polytechnic College, Poonjar


Wireless Access
T echnologies
Wi-Fi stands for Wireless Fidelity. As the words indicates, the system seeks to do away
with-wires. This system is known as wireless LAN, as with Wi-Fi we can connect different
computers in a LAN using radio waves. This is as shown below in Fig-1.

Figure 1

This standard is known as IEEE 802 .11. There are different versions of this standard
available. They are 802.11b, 802.11a & 802.11g. A comparison of the different standards
is given below. WiFi generally refers to any type of IEEE
802.11 standard.

Wi-Max
WiMAX is an acronym that stands for Worldwide Interoperability for Microwave Access,
a certification mark for products that pass conformity and interoperability tests for the
IEEE 8802.16 standards.(IEEE 802.16 is working group number 16 of IEEE 802
specializing in point-to-multipoint Broadband wireless access). WiMAX covers wider,
metropolitan or rural areas. It can provide data rates up to 75 megabits per second
(Mbps) per base station with typical cell sizes of 2 to 10 kilometers. This is enough



bandwidth to simultaneously support (through a single base station) more than 60
businesses with T1/E1-type connectivity and hundreds of homes with DSL-type
connectivity. It will provide fixed, portable, and eventually mobile wireless broadband
connectivity and also provides POTS services.

WiMAX actually provide two forms of wireless service :

1. Non-line-of-sight, WiFi sort of service, where a small antenna on your
computer connects to the tower. In this mode, WiMAX uses a lower
frequency range – 2 GHz to 11 GHz

2. Line-of-sight service, where a fixed dish antenna points straight at the WiMAX
tower from a rooftop or pole. Line-of-sight transmissions use higher frequencies,
with ranges reaching a possible 66 GHz.

IEEE 802.16 Specifications

Range - 30-mile (50-km) radius from base station

Speed - 70 megabits per second



Line-of-sight not needed between user and base station

Frequency bands - 2 to 11 GHz and 10 to 66 GHz

Two key features of Wi Max is the use of OFDM (Orthogonal Frequency Division
Multiplexing) and Adaptive modulation techniques for achieving greater bit rates and
stable connection. In OFDM, the data will be sent over narrow band carriers
transmitted in parallel at different frequencies. These carrier frequencies are
closely spaced and they are orthogonal. In adaptive modulation, depending on the signal
to Noise ratio (SNR) value of the radio link, the modulations will the
automatically changed and bit rates will be adjusted accordingly. This also ensures a
stable connectivity between the subscriber station and the Wi Max base station.



Overview of the GSM RF Interfaces
Interfaces For the connection of the different nodes in the GSM network, different
interfaces are defined in the GSM specifications. The GSM interfaces
discussed in this lesson are:

Air interface or U m –interface
The Air Interface is the interface between the BTS (Base Transceiver
Station) and the MS (Mobile Station). The air interface is required for
supporting:

— Universal use of any compatible mobile station in a GSM
network

— A maximum spectral efficiency

A bis -interface
The A bis -interface is the interface between the BSC (Base Station
Controller) and the BTS. The interface comprises traffic and control
channels. Functions implemented at the A bis -interface are:
— Voice-data traffic exchange

— Signaling exchange between the BSC and the BTS

— Transporting synchronization information from the BSC to the
BTS

A-interface
The A-interface is the interface between the BSC and the MSC.



The U m –interface

Introduction One of the most important interfaces is the U m or Air interface. This
interface is thoroughly specified to achieve a full compatibility
between mobile stations of various manufacturers and networks of
different operators.

FDMA and To achieve a high spectral efficiency in the cellular network a
TDMA combination of FDMA (Frequency Division Multiple Access) and
methods TDMA (Time Division Multiple Access) is used. The FDMA part
involves the division by frequency of the 25 MHz bandwidth into
124 carrier frequencies spaced 200 KHz for GSM-900. For GSM-
1800 the frequency spectrum of the 75 MHz bandwidth is divided
into 374 carrier frequencies spaced 200 KHz. One or more
frequencies are assigned to each BTS. Each of these carrier
frequencies is then divided in time, using a TDMA scheme to
increase the number of channels per carrier frequency.

Each carrier frequency channel carries eight time-division
multiplexed physical channels. A physical channel is determined by
the carrier frequency (or a number of carrier frequencies and a
defined hopping sequence) and the timeslot number. A mobile station
can transmit speech data only during its assigned timeslot.

Uplink and In the frequency range specified for the GSM-900 mobile radio
downlink networks, 124 frequency channels with a bandwidth of 200 KHz are
available for both the uplink and downlink direction. The uplink
(mobile station to BTS) uses the frequencies between 890 MHz and
915 MHz and the downlink (BTS to mobile station) uses
the frequencies between 935 MHz and 960 MHz. The duplex spacing,
the spacing between the uplink and downlink channel, is 45 MHz.

GSM-1800 uses a similar scheme. The difference is that for GSM-
1800 the uplink uses the frequencies between 1710 MHz and 1785
MHz and the downlink the frequencies between 1805 MHz and 1880
MHz. The duplex spacing is 95 MHz.



BASIC TYPES OF POWER PLANTS AND SPECIFICATIONS

Classification of power plants (3 piece)
-Power plant comprises 3
parts
-Float Rectifier
-Battery Charger
-Switching Cubicle.
Power plants are classified based on their capacity.

Feature Small exchange Medium exchange Large exchange
power plants power plants power plants
Capacity: 5/12A 25/50A > 50A
Input Single Phase Single Phase Three Phase
Paralleling Not possible Only manual Auto paralleling is
of rectifier paralleling is possible
possible

Another classification of power plant is

Single unit type Two unit Type Three unit type
F.R,B.C and SC -One unit is FC/BC/SWC -FR, BC, and SC are
will be in Single (Float rectifier cum in individual units.
container battery charger, cum
Switching cubicle )
Ex: 5/12A PP - another unit is FR

Note: Nowadays mostly 2 units p/p are used with maintenance
free batteries and all transmission power plant are 2-unit type only. The latest
being P/P of SMPS with VRLA batteries.



SWITCHING SYSTEMS

Introduction

The telephone is a telecommunication device that is used to transmit and receive
electronically or digitally encoded speech between two or more people conversing. It is one of
the most common household appliances in the developed world today. Most telephones operate
through transmission of electric signals over a complex telephone network which allows almost
any phone user to communicate with almost any other user.

Telecommunication networks carry information signals among entities, which are
geographically far apart. An entity may be a computer or human being, a facsimile machine, a
teleprinter, a data terminal and so on. The entities are involved in the process of information
transfer that may be in the form of a telephone conversation (telephony) or a file transfer
between two computers or message transfer between two terminals etc.

With the rapidly growing traffic and untargeted growth of cyberspace,
telecommunication becomes a fabric of our life. The future challenges are enormous as we
anticipate rapid growth items of new services and number of users. What comes with the
challenge is a genuine need for more advanced methodology supporting analysis and design of
telecommunication architectures. Telecommunication has evaluated and growth at an explosive
rate in recent years and will undoubtedly continue to do so.

The communication switching system enables the universal connectivity. The universal
connectivity is realized when any entity in one part of the world can communicate with any
other entity in another part of the world. In many ways telecommunication will acts as a
substitute for the increasingly expensive physical transportation.

The telecommunication links and switching were mainly designed for voice
communication. With the appropriate attachments/equipments, they can be used to transmit
data. A modern society, therefore needs new facilities including very high bandwidth switched
data networks, and large communication satellites with small, cheap earth antennas.

Voice Signal Characteristics

Telecommunication is mainly concerned with the transmission of messages between
two distant points. The signal that contains the messages is usually converted into electrical
waves before transmission. Our voice is an analog signal, which has amplitude and frequency
characteristics.



Voice frequencies: - The range of frequencies used by a communication device
determines the communication channel, communicating devices, and bandwidth or information
carrying capacity. The most commonly used parameter that characterizes an electrical signal is
its bandwidth of analog signal or bit rate if it is a digital signal. In telephone system, the
frequencies it passes are restricted to between 300 to 3400 Hz.

In the field of telecommunications, a Telephone exchange or a Telephone switch is a
system of electronic components that connects telephone calls. A central office is the physical
building used to house inside plant equipment including telephone switches, which make
telephone calls "work" in the sense of making connections and relaying the speech information.

Switching system fundamentals

Telecommunications switching systems generally perform three basic functions: they
transmit signals over the connection or over separate channels to convey the identity of the
called (and sometimes the calling) address (for example, the telephone number), and alert (ring)
the called station; they establish connections through a switching network for conversational
use during the entire call; and they process the signal information to control and supervise the
establishment and disconnection of the switching network connection.

In some data or message switching when real-time communication is not needed, the
switching network is replaced by a temporary memory for the storage of messages. This type of
switching is known as store-and-forward switching.

Signaling and control

The control of circuit switching systems is accomplished remotely by a specific form of
data communication known as signaling. Switching systems are connected with one another by
telecommunication channels known as trunks. They are connected with the served stations or
terminals by lines.

In some switching systems the signals for a call directly control the switching devices
over the same path for which transmission is established. For most modern switching systems
the signals for identifying or addressing the called station are received by a central control that
processes calls on a time-shared basis. Central controls receive and interpret signals, select and
establish communication paths, and prepare signals for transmission. These signals include
addresses for use at succeeding nodes or for alerting (ringing) the called station.

Most electronic controls are designed to process calls not only by complex logic but
also by logic tables or a program of instructions stored in bulk electronic memory. The tabular



technique is known as translator. The electronic memory is now the most accepted technique
and is known as stored program control (SPC). Either type of control may be distributed among
the switching devices rather than residing centrally. Microprocessors on integrated circuit chips
are a popular form of distributed stored program control.

Switching fabrics

Space and time division are the two basic techniques used in establishing connections.
When an individual conductor path is established through a switch for the duration of a call, the
system is known as space division. When the transmitted speech signals are sampled and the
samples multiplexed in time so that high-speed electronic devices may be used simultaneously
by several calls, the switch is known as time division.

In the early stages of development in telecommunication, manual switching methods
were deployed. But later on to overcome the limitations of manual switching; automatic
exchanges, having Electro-mechanical components, were developed. Strowger exchange, the
first automatic exchange having direct control feature, appeared in 1892 in La Porte (Indiana).
Though it improved upon the performance of a manual exchange it still had a number of
disadvantages, viz., a large number of mechanical parts, limited availability, inflexibility, bulky
in size etc. As a result of further research and development, Crossbar exchanges,having an
indirect control system, appeared in 1926 in Sweden.

The Crossbar exchange improved upon many short- comings of the Strowger system.
However, much more improvement was expected and the revolutionary change in field of
electronics provided it. A large number of moving parts in Register, marker, Translator, etc.,
were replaced en-block by a single computer. This made the exchange smaller in size, volume
and weight, faster and reliable, highly flexible, noise-free, easily manageable with no
preventive maintenance etc.

Influence of Electronics in Exchange Design.

When electronic devices were introduced in the switching systems, a new concept of
switching evolved as a consequence of their extremely high operating speed compared to their
former counter-parts, i.e., the Electro-mechanical systems, where relays, the logic elements in
the electromechanical systems, have to operate and release several times which is roughly equal
to the duration of telephone signals to maintain required accuracy.

Research on electronic switching started soon after the Second World War, but
commercial fully electronic exchange began to emerge only about 30 years later. However,



electronic techniques proved economic for common control systems much earlier. In
electromechanical exchanges, common control systems mainly used switches and relays, which
were originally designed for use in switching networks. In common controls, they are operated
frequently and so wear out earlier. In contrast, the life of an electronic device is almost
independent of its frequency of operation. This gave a motivation for developing electronic
common controls and resulted in electronic replacements for registers, markers, translators etc.
having much greater reliability than their electromechanical predecessors.

In electromechanical switching, the various functions of the exchange are achieved by
the operation and release of relays and switch (rotary or crossbar) contacts, under the direction
of a Control Sub-System. These contracts are hard - wired in a predetermined way. The
exchange dependent data, such as subscriber’s class of service, translation and routing,
combination signaling characteristics are achieved by hard-ware and logic, by a of relay sets,
grouping of same type of lines, strapping on Main or Intermediate Distribution Frame or
translation fields, etc. When the data is to be modified, for introduction of a new service, or
change in services already available to a subscriber, the hardware change ranging from
inconvenient to near impossible, are involved.

In an SPC exchange, a processor similar to a general-purpose computer is used to
control the functions of the exchange. All the control functions, represented by a series of
various instructions, are stored in the memory. Therefore the processor memories hold all
exchange dependent data. such as subscriber date, translation tables, routing and charging
information and call records. For each call processing step. e.g. for taking a decision according
to class of service, the stored data is referred to, Hence, this concept of switching. The
memories are modifiable and the control program can always be rewritten if the behavior or the
use of system is to be modified. This imparts and enormous flexibility in overall working of the
exchange.

Digital computers have the capability of handling many tens of thousands of
instructions every second, Hence, in addition to controlling the switching functions the same
processor can handle other functions also. The immediate effect of holding both the control
programme and the exchange data, in easily alterable memories, is that the administration can
become much more responsive to subscriber requirements. both in terms of introducing new
services and modifying general services, or in responding to the demands of individual
subscriber. For example, to restore service on payment of an overdue bill or to permit change
from a dial instrument to a multi frequency sender, simply the appropriate entries in the
subscriber data-file are to be amended. This can be done by typing- in simple instructions from
a teletypewriter or visual display unit. The ability of the administration to respond rapidly and
effectively to subscriber requirements is likely to become increasingly important in the future.



The modifications and changes in services which were previously impossible be
achieved very simply in SPC exchange, by modifying the stored data suitably. In some cases,
the subscribers can also be given the facility to modify their own data entries for supplementary
services, such as on-demand call transfer, short code (abbreviated) dialing, etc.

The use of a central processor also makes possible the connection of local and remote
terminals to carry out man-machine dialogue with each exchange. Thus, the maintenance and
administrative operations of all the SPC exchanges in a network can be performed from a single
centralized place. The processor sends the information on the performance of the network, such
as, traffic flow, billing information, faults, to the centre, which carries out remedial measures
with the help of commands. Similarly, other modifications in services can also be carried out
from the remote centre. This allows a better control on the overall performance of the network.

As the processor is capable of performing operations at a very high speed, it has got
sufficient time to run routine test programmes to detect faults, automatically. Hence, there is no
need to carry out time consuming manual routine tests.

In an SPC exchange, all control equipment can be replaced by a single processor. The
processor must therefore be quite powerful, typically it must process hundreds of calls per
second, in addition to performing other administrative and maintenance tasks. However, totally
centralized control has drawbacks. The software for such a central processor will be
voluminous, complex, and difficult to develop reliably. Moreover, it is not a good arrangement
from the point of view of system security, as the entire system will collapse with the failure of
the processor. These difficulties can be overcome by decentralizing the control. Some routine
functions such as scanning, signal distributing, marking, which are independent of call
processing, can be delegated to auxiliary or peripheral processors.

Stored program control (SPC) has become the principal type of control for all types of
new switching systems throughout the world, including private branch exchanges, data and
Telex systems. Two types of data are stored in the memories of electronic switching systems.
One type is the data associated with the progress of the call, such as the dialed address of the
called line.

Another type, known as the translation data, contains infrequently changing
information, such as the type of service subscribed to by the calling line and the information
required for routing calls to called numbers. These translation data, like the program, are stored
in a memory, which is easily read but protected to avoid accidental erasure. This information
may be readily changed, however, to meet service needs. The flexibility of a stored program



also aids in the administration and maintenance of the service so that system faults may be
located quickly.

SPC exchanges can offer a wider range of facilities than earlier systems. In addition,
the facilities provided to an individual customer can be readily altered by changing the
customer’s class-of-service data stored in memory. Moreover, since the processor’s stored data
can be altered electronically,some of these facilities can be controlled by customers. Examples
include:-

1. Call barring (outgoing or incoming): The customer can prevent unauthorized calls
being made and can prevent incoming calls when wishing to be left in peace.

2. Call waiting: The ‘Call waiting’ service notifies the already busy subscriber of a third
party calling him.

3. Alarm calls: The exchange can be instructed to call the customer at a pre-arranged time
(e.g. morning alarm).

4. Call Forwarding: The subscriber having such a feature can enable the incoming calls
coming to his telephone to be transferred to another number during his absence.

5. Conference calls: Subscriber can set up connections to more than one subscriber and
conduct telephone conferences under the provision of this facility.

6. Dynamic Barring Facility: Subscriber having STD/ISD facilities can dynamically lock
such features in their telephone to avoid misuse. Registering and dialing a secret code will
extend such such a facility.

7. Abbreviated Dialing: Most subscribers very often call only limited group of telephone
numbers. By dialing only prefix digit followed by two selection digits, subscribers can call up
to 100 predetermined subscribers connected to any automatic exchange. This shortens the
process of dialing all the digits.

8. Malicious call Identification: Malicious call identification is done immediately and the
information is obtained in the print out form either automatically or by dialing an identification
code.

9. Do Not Disturb: This facility enables the subscriber to free himself from attending his
incoming calls. Using this facility the calls coming to the subscriber can be routed to an
operator position or to an answering machine. The operator position or the machine can inform
the calling subscriber that the called subscriber is temporarily inaccessible. Today SPC is a
standard feature in all the electronic exchanges.



Implementation of Switching Network.

In an electronic exchange, the switching network is one of the largest sub-system in
terms of size of the equipment. Its main functions are Switching (setting up temporary
connection between two or more exchange terminations), Transmission of speech and signals
between these terminations, with reliable accuracy.

There are two types of electronic switching system. viz. Space division and Time
Division.

Space Division switching System

In a space Division Switching system, a continuous physical path is set up between
input and output terminations. This path is separate for each connection and is held for the
entire duration of the call. Path for different connections is independent of each other. Once a
continuous path has been established., Signals are interchanged between the two terminations.
Such a switching network can employ either metallic or electronic cross points. Previously,
usage of metallic cross-points using reed relays and all were favored. They have the advantage
of compatibility with the existing line and trunk signaling conditions in the network.

Time Division Switching System

In Time Division Switching, a number of calls share the same path on time division
sharing basis. The path is not separate for each connection, rather, is shared sequentially for a
fraction of a time by different calls. This process is repeated periodically at a suitable high rate.
The repetition rate is 8 KHz, i.e. once every 125 microseconds for transmitting speech on
telephone network, without any appreciable distortion. These samples are time multiplexed
with staggered samples of other speech channels, to enable sharing of one path by many calls.
The Time Division Switching was initially accomplished by Pulse Amplitude

Modulation (PAM) Switching. However, it still could not overcome the performance
limitations of signal distortion noise, cross-talk etc. With the advent of Pulse Code Modulation
(PCM), the PAM signals were converted into a digital format overcoming the limitations of
analog and PAM signals. PCM signals are suitable for both transmission and switching. The
PCM switching is popularly called Digital Switching.



Digital Switching Systems

A Digital switching system, in general, is one in which signals are switched in digital
form. These signals may represent speech or data. The digital signals of several speech samples
are time multiplexed on a common media before being switched through the system.

To connect any two subscribers, it is necessary to interconnect the time-slots of the two
speech samples, which may be on same or different PCM highways. The digitalized speech
samples are switched in two modes, viz., Time Switching and Space Switching. This Time
Division Multiplex Digital Switching System is popularly known as Digital Switching System.

The general architecture of a Digital Switching System is depicted in Fig2.



General architecture of Digital Switching System

Subs interface

Digital Switch
Trunks interface
Other
exchanges

CONTROL
PROCESSOR
Other auxiliary inter faces
Such as,

(a) Tone generator
(b) Frequency receives
(c) Conference call facility
(d) CCS# 7 Protocol Manager Operation &
(e) V 5.2 access manager Maintenance

Figure-2

The ESS No.1 system was the first fully electronic switching system but not digital.
But later came ESS No.4 system which was digital for trunk portion only. When designed, the
cost of A/D conversion (CODEC) on each subscriber line was seen as prohibitive. So the ESS
No.4 system was acting as a Trunk/Tandem exchange but not as a local exchange. So the main
difficulty for implementing a digital local exchange was the implementation of the subscriber
line interface. This was solved by the introduction of Integrated Circuits, which made the
digital local exchange economically feasible. This implementation handles the following
functions:

B-Battery feed

O-Over-voltage protection (from lightning and accidental power line contact)



R-Ringing

S-Supervisory Signaling

C-Coding (A/D inter conversion & low pass filtering)

H-Hybrid (2W to 4W conversion)

T-Testing the connectivity of Subscriber

Examples of digital exchanges (switching systems) include CDOT, OCB, AXE, EWSD, 5ESS
etc.

The next evolutionary step was to move the PCM codec from the exchange end
of the customer’s line to the customer’s end. This provides digital transmission over the
customer’s line, which can have a number of advantages. Consider data transmission. If there
is an analog customer’s line, a modem must be added and data can only be transmitted at
relatively slow speeds. If the line is digital, data can be transmitted by removing the codec
(instead of adding a modem). Moreover, data can be transmitted at 64 kbit/s instead of at, say,
2.4 kbit/s. Indeed, any form of digital signal can be transmitted whose rate does not exceed 64
kbit/s. This can include high-speed fax, in addition to speech and data.

This concept had led to the evolution of Integrated services digital network
(ISDN), in which the customer’s terminal equipment and the local digital exchange can be used
to provide many different services, all using 64 kbit/s digital streams. In simple terms, we can
say ISDN provides end-to-end digital connectivity.

Access to an ISDN is provided in two forms:

1. Basic-Rate Access (BRA)

The customer’s line carries two 64 kbit/s “B” channels plus a 16 kbit/s “D”
channel (a common signaling channel) in each direction.

2. Primary Rate Access (PRA)

The line carries a complete PCM frame at 2 Mbit/s in each direction. This
gives the customer 30 circuits at 64 kbit/s plus a common signaling channel, also at 64 kbit/s.

Control of switching systems



Switching systems have evolved from being manually controlled to being controlled by
relays and then electronically. The change from the manual system to the Strowger step-by-
step system brought about a change from centralized to distributed control. However, as
systems developed and offered more services to customers, it became economic to perform
particular functions in specialized equipments that were associated with connections only when
required, thus, common control was introduced.

Later, the development of digital computer technology enabled different functions to be
performed by the same hardware by using different programs; thus switching system entered
the era of stored-program control (SPC).

There are basically two approaches to organizing stored program control: centralized
and distributed. Early electronic switching systems (ESS) developed during the period 1970-75
almost invariably used centralized control. Although many present day exchange designs
continue to use centralized SPC, with the advent of low cost powerful microprocessors and very
large scale integration (VLSI) chips such as programmable logic arrays (PLA) and
programmable logic controllers (PLC), distributed SPC is gaining popularity.



Development of exchanges

year

Figure 3

The figure above shows the evolution of electronic switching systems from the manual
switching systems. The figure also depicts the changing scenario from digital switching to
Broadband where the focus will be for high bit rate data transmissions.

Signaling In Telecommunication

A telecommunication network establishes and releases temporary connections, in
accordance with the instructions and information received from subscriber lines and inter
exchange trunks, in form of various signals. Therefore, it is necessary to interchange
information between an exchange and it external environment i.e. between subscriber lines and
exchange, and between different exchanges. Though these signals may differ widely in their
implementation they are collectively known as telephone signals.

A signaling system uses a language, which enables two switching equipments to
converse for the purpose of setting up calls. Like any other language. it possesses a vocabulary
of varying size and varying precision, ie. a list of signals which may also vary in size and a
syntax in the form of a complex set of rules governing the assembly of these signals. This
handout discusses the growth of signaling and various type of signaling codes used in Indian
Telecommunication.



Types of Signaling

1. Subscriber Line Signaling

2. Inter exchange Signaling

The signaling information exchanged between a subscriber and an exchange for the
establishment and release of a call is termed as Subscriber line signaling. Example for
Subscriber line signaling –Calling subscriber going off-hook, feeding of dial tone to the
subscriber by the exchange, feeding of Ringing current, digits dialed by the subscriber
(Address information) in the form of pulses or tones etc.

The signaling information exchanged between different exchanges via inter exchange
trunks for the routing of calls is termed as Inter exchange Signaling. Earlier in band /out of
band frequencies were used for transmitting signaling information. Later on, with the
emergence of PCM systems, it was possible to segregate the signaling from the speech channel.



TRANSMISSION SYSTEMS
Telephonic conversation is possible on a single wire connected between two telephones
but distance is limited, approximately 5 to 10 Kms. If two wires are used which run parallel
between two telephones, long distance communication can be provided. However, this is a
costly affair therefore, many different systems of transmission are designed.

What is a Transmission System?

A transmission system consists of two terminals Transmitter and Receiver, with media
for transmission between the two. There may be few repeaters (for amplification) at
intermediate stations if required. There are different types of Transmission systems.

Carrier Systems

This system is installed between two cities and both systems are
connected by two wires called line. It provides three telephonic subscribers conversation
simultaneously. The distance between two cities is 50Km. to 100 Km. These systems work on
230 V AC.

Different Carrier systems are,

 3 Channel system




Co-axial cable systems

In the initial stages of Tele –Communication two wires were used as
line (as explained above). The disadvantage is that, it cannot handle more traffic. With the
increasing demand of more simultaneous telephone- calls different measures were adopted. Co-
axial cable is one of them. The systems working on Co-Axial Cables are called Coaxial Cable
systems. After multiplexing telephone calls are transmitted by different coaxial cable systems.
The description is as follows.

A. 4 MHz System



960 telephone calls are multiplexed by multiplexing equipment and it comes in
this system. This system amplifies the power and transmits on coaxial cable to distant station.

B. 12 MHz System

2700 multiplexed signals are fed in this system for distant end. This system
amplifies the power and makes it suitable for transmission to other stations.

Micro Wave Systems

Microwave working is resorted to provide reliable communication especially in difficult
terrains where communication by coaxial cable and other means cannot be provided. It has the
following advantages.

 It can provide a very large bulk of speech circuits.

 It can be provided over rough and in-accessible terrain where other types of
communication cannot be economically arranged.



 The fault liability in M/W system is very low. Provision of remote fault
localization and standby microwave channel almost provides uninterrupted
communication.

 The annual maintenance cost is very low as compared with other transmission
system.

Pulse Code Modulation System (PCM System)

PCM systems use Time Division Multiplexing technique to provide a number
of circuits on the same transmission medium viz-open wire or underground cable pair of a
channel provided by carrier, coaxial, microwave or satellite system

Time Division Multiplexing

Basically, time division multiplexing involves nothing more than sharing a
transmission medium by a number of circuits in time domain by establishing a sequence of time
slots during which individual channels (circuits) can be transmitted.

Each channel is sampled at a specified rate and transmitted for a fixed
duration. All channels are sampled one by one and transmitted one by one, the cycle is
repeated again and again. The channels are connected to individual gates, which are opened
one by one in a fixed sequence. At the receiving end also similar gates are opened in unison
with the gates at the transmitting end.

The signal received at the receiving end will be in the form of discrete
samples and these are combined to reproduce the original signal. Thus at a given instant of
time, only one channel is transmitted through the medium, and by sequential sampling a
number of channels can be staggered in time as opposed to transmitting all the channel at the
same time as in FDM systems. This staggering of channels in time sequence for transmission
over a common medium is called Time Division Multiplexing (TDM).

To develop a PCM signal from several analogue signals, the following
processing steps are required:

 Filtering

 Sampling

 Quantization

 Encoding



 Line Coding

Optical Fiber Systems

Optical fibre is a medium, in which information (voice, data or video) is
transmitted through a glass or plastic fibre, in the form of light, following the transmission
sequence given below:

 Information is encoded into electrical signals.
 Electrical signals are converted into light signals.
 Light travels down the fibre.
 A detector changes it into electrical signals at receiver.

 Electrical signals are decoded into information.

Recei
Tra
Gat ve Ga
ns
CH 1 ( )
e ) (
te 1 CH
1 1
CH 2 ( ) ) ( 2 CH
2 2
~~
Mediu
CH 3 ( ) m ) ( 3 CH
3 3

CH n ( ) ) ( n CH
n n

Time Division Multiplexing
Multiplexing


Advantages Of Optical Fibre

 Optical Fibre is non conductive (Dielectric)

 Electromagnetic immunity

 Large Bandwidth

 Low Loss

 Small, light weight cables

 Available in Long lengths

 Security

 Safety

 Universal medium

PDH Fiber Optic Transmission Systems: -

Present telecom technology provides both PDH (Plesiochronous Digital
Hierarchy) and SDH (Synchronous Digital Hierarchy) optical fiber equipments. The PDH
systems are basically used for the Point-to-Point transmission (carrying a signal between two
end points) e.g. for connecting two cities. The different PDH Fiber Optic Transmission Systems
which are used in the network of BSNL, capacity wise, are given below: -

 8 Mbps system 120 channels capacity

 34 mbps system 480 channels capacity



The above PDH Fiber Optic Transmission Systems available either in
separate OLTE (Optical Line Terminating Equipment) and MUX (Multiplexer) or in integrated
OLTE + MUX (OPTIMUX) version housed in a single rack. The application of these systems
depends upon the traffic of that particular route. These PDH Fiber Optic Transmission Systems
are supplied by different manufacturers to BSNL

Main parts of PDH Fiber Optic Transmission Systems: -

 Multiplexing Equipments
 Line Equipments



 OLTE (Optical Line Terminating Equipment)

 Repeater (Regenerator)

 Transmission Media (Optical Fiber)

Block diagram of FOTS

O R O
M
E M
Telephon Telephone
e calls U L calls
G L
from/to from/to
Exchange X
N U Exchange
T
E T
E R X
A
E
T

SDH Optical Fiber Systems O

R
It is an international standard networking principle and a multiplexing
method. The name of hierarchy has been taken from the multiplexing method, which is
synchronous, by nature. The evolution of this will assist in improving the economy of
operability and reliability of a digital network

SDH was very quickly perceived to be a better way of deploying optical
networks. SDH played a crucial role in the fast and efficient deployment of high speed
backbone connecting routers. SDH starts its hierarchy at 155.52 mbps and available in different
capacity systems like: -

Name of system Speed Capacity

 STM-1 155.52 Mbps 1890 channels

 STM –4 622.08 Mbps 7560 channels

 STM –16 2.50 Gbps 30,240 channels

 STM - 64 10.0 Gbps 1,20,960 channels



The first attempt to formulate standards for optical transmission started in
USA as SONET (Synchronous Optical Network) The aim of these standards was to simplify
interconnection between network operator by allowing inter connection of equipment from
different venders to the extent that compatibility could be achieved.

When in year 1986 USA reported about SONET, which was developed by
AT&T USA .UK and Japan took the interest in the technique and it was discussed in ITU (T)
[International Telecommunication Union (telecommunication standardization Sector)] which is
a world standard in telecommunication field , and SONET was renamed as SDH (Synchronous
Digital Hierarchy ), after making some modifications in SONET .Thereafter SDH become the
global of transmission system.

Merits of SDH

1. Simplified Multiplexing and De-multiplexing process
2. Direct access to lower speed channel without need to demultiplex the entire high-
speed signal.
3. Enhanced OAM & P: - Due to enhanced Operation, Administration, Maintenance
and Provisioning capabilities user can control the whole network from a central
location i.e. remote supervision and control is very easy.
4. Easy Growth: - Easy growth to high capacity systems.
5. Capable of transporting existing PDH signals.
6. Capable of transporting future broadband signals like, interactive multimedia &
video conferencing.

7. Capable of operating in a multivender and multi-operator environment: - Before
SDH optical solutions for the long distance transmission were intensely vendor
specific but SDH were firm standards for vendor inter operability.

8. Synchronous networking: - SDH supports multi-point, Hub and Ring
Configuration whereas PDH networking only supports Point-to-Point
Configuration.

SDH Network Elements: -

 Terminal Multiplexer (TM) – TM an end point device of SDH network, it is used
at terminals of point to point SDH network.



 Add/Drop Multiplexer (ADM) – ADM is network element, which allows
configurable add/drop of a subset of traffic channels from higher rate data stream.

 Regenerator (REG): -The most basic element is regenerator. They terminate and
regenerate the optical signal. These are not simple regenerator but have alarm and
performance monitoring capability.

 Synchronous Digital Cross Connect (SDXC): -

This device will form the cornerstone of new SDH. They can function as semi
permanent switch for transmission channels and can switch at any level from 64 kbps to STM-
1 under software control .The previous systems like analogue transmission systems and PDH
based digital transmission systems are only point to point or in bus configuration. In bus
configuration repeaters are having drop and insert facility of channels and in point to point
configuration repeaters do not have drop and insert facility. But in both of these configuration if
media breaks or repeater fails, the full system goes out of order.

EXCHANGE

ADM

“A”
E
E X
X C
ADM ADM
C H
H A
A N
N “B” “D”
G
G E
E

ADM

“C”

EXCHANGE



In SDH this problem is overcome by using other type of configurations
specially RING TYPE as shown in fig.

In the fig. Four nodes (A, B, C, D) are shown, when media between say node
A and B breaks, the traffic interrupted is automatically rerouted between nodes A&B via nodes
D&C.



Satellite Systems

Long distance communications, particularly to remote locations, using
conventional terrestrial media is both uneconomic and unreliable. A geo-stationary
communication satellite which acts as a repeater hung in the sky can cover a very large
area and provide a reliable and cost effective alternative.

Although satellite communication would seem to be a straightforward extension
of Conventional radio system, the use of satellites for communications brings in new
operational features not found in terrestrial systems. In this hand out, some of the
features of satellite communication are discussed. Basic knowledge of terrestrial radio
systems is assumed.

Structure of a Satellite Link


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