Geo34 63

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

Applied Electronics 34 Model Polytechnic College, Poonjar

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



Wireless AccessT echnologies

Wi-Fi stands for Wireless Fidelity. As the words indicates, the system seeks to do away

with-wires. This system is known as wireless LAN, as with Wi-Fi we can connect different

computers in a LAN using radio waves. This is as shown below in Fig-1.

Figure 1

This standard is known as IEEE 802 .11. There are different versions of this standard

available. They are 802.11b, 802.11a & 802.11g. A comparison of the different standards

is given below. WiFi generally refers to any type of IEEE

802.11 standard.

Wi-MaxWiMAX 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

FDMA andTDMAmethods

Uplink anddownlink

One of the most important interfaces is the U m or Air interface. Thisinterface is thoroughly specified to achieve a full compatibilitybetween mobile stations of various manufacturers and networks ofdifferent operators.

To achieve a high spectral efficiency in the cellular network acombination of FDMA (Frequency Division Multiple Access) andTDMA (Time Division Multiple Access) is used. The FDMA partinvolves the division by frequency of the 25 MHz bandwidth into124 carrier frequencies spaced 200 KHz for GSM-900. For GSM-1800 the frequency spectrum of the 75 MHz bandwidth is dividedinto 374 carrier frequencies spaced 200 KHz. One or morefrequencies are assigned to each BTS. Each of these carrierfrequencies is then divided in time, using a TDMA scheme toincrease the number of channels per carrier frequency.

Each carrier frequency channel carries eight time-divisionmultiplexed physical channels. A physical channel is determined bythe carrier frequency (or a number of carrier frequencies and adefined hopping sequence) and the timeslot number. A mobile stationcan transmit speech data only during its assigned timeslot.

In the frequency range specified for the GSM-900 mobile radionetworks, 124 frequency channels with a bandwidth of 200 KHz areavailable for both the uplink and downlink direction. The uplink(mobile station to BTS) uses the frequencies between 890 MHz and915 MHz and the downlink (BTS to mobile station) usesthe 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 1785MHz and the downlink the frequencies between 1805 MHz and 1880MHz. The duplex spacing is 95 MHz.



BASIC TYPES OF POWER PLANTS AND SPECIFICATIONS

Classification of power plants (3 piece)

-Power plant comprises 3parts

-Float Rectifier

-Battery Charger

-Switching Cubicle.

Power plants are classified based on their capacity.

Feature Small exchangepower plants

Medium exchangepower plants

Large exchangepower plants

Capacity: 5/12A 25/50A > 50AInput Single Phase Single Phase Three Phase

Parallelingof rectifier

Not possible Only manualparalleling is

possible

Auto paralleling ispossible

Another classification of power plant is

Single unit typeF.R,B.C and SCwill be in Singlecontainer

Two unit Type-One unit is FC/BC/SWC

(Float rectifier cumbattery charger, cum

Three unit type-FR, BC, and SC are

in individual units.

Ex: 5/12A PPSwitching cubicle )- another unit is FR

Note: Nowadays mostly 2 units p/p are used with maintenancefree batteries and all transmission power plant are 2-unit type only. The latestbeing 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 callsbeing made and can prevent incoming calls when wishing to be left in peace.

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

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

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

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

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

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

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

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



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

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)

Subs interface

Otherexchanges

CONTROLPROCESSOR

Operation &Maintenance

Trunks interface

Other auxiliary inter faces

Such as,

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

Digital Switch



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

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.

year




Figure 3
















Telecommunication.

year




Figure 3
















Telecommunication.

year



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

8 Channel system

12 Channel system

24 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.





B. 12 MHz System



Micro Wave Systems











B. 12 MHz System



Micro Wave Systems









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.

1 (

2 (

3 (

n (

(

(

(

(

1

2

3

n

CH1

CH2

CH3

CHn

CH1

CH2

CH3

CHn

)

)

)

)

)

)

)

)

~~Medium

Gate

Gate

Trans

Receive

Time Division MultiplexingMultiplexing



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

SDH Optical Fiber Systems

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

M

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X

O

L

T

E

R

E

G

N

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R

A

T

O

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M

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Telephonecallsfrom/toExchange

Telephone callsfrom/toExchange



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.

ADM

“B”

ADM

“A”

ADM

“D”

ADM

“C”

EXCHANGE

EXCHANGE

EXCHANGE

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



Satellite Systems









systems is assumed.




Satellite Systems









systems is assumed.


Geo34 63

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Transcript of Geo34 63