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SILVER CERTIFICATION COURSE DIGITAL SWITCHING SYSTEM

OVERVIEW OF TELECOM

NETWORK AND PCM PRINCIPLES

Version 2 June 2014

Digital Switching Systems (DSS) Overview of Telecom Network

EETP/ BSNL Silver Certification Course /Ver.02/June 2014 Page 1 of 41

For Restricted Circulation

OVERVIEW OF TELECOM NETWORK AND PCM

PRINCIPLES

INDEX

1.1 Objective .............................................................................................2

1.2 Introduction ........................................................................................3

1.3 Telecommunication Network ............................................................3

1.4 Evolution Of Electronic Exchange ................................................ 16

1.5 Components Of PSTN Network ..................................................... 21

1.6 Main Distribution Frame (MDF) ................................................... 24

1.7 Installation ....................................................................................... 28

1.8 Routine Maintenance ...................................................................... 30

1.9 PCM Principles ................................................................................ 32

1.10 Summary .......................................................................................... 39

1.11 Self Assessment Questions .............................................................. 39

1.12 References And Suggested Further Readings .............................. 40




1 OVERVIEW OF TELECOM NETWORK AND PCM

PRINCIPLES

STRUCTURE

1.1 OBJECTIVE

1.2 INTRODUCTION

1.3 TELECOMMUNICATION NETWORK

1.4 EVOLUTION OF ELECTRONIC EXCHANGE

1.5 COMPONENTS OF PSTN NETWORK

1.6 MAIN DISTRIBUTION FRAME (MDF)

1.7 PCM PRINCIPLES

1.8 SUMMARY

1.9 SELF ASSESSMENT QUESTIONS

1.10 REFERENCES AND SUGGESTED FURTHER READINGS

1.1 OBJECTIVE

After reading this chapter you will be able to

List the different blocks of telecom network, their types.

Explain about the national networks and numbering

Enumerate the present trends like Next generation networks

Differentiate between FDM and TDM.

Describe Pulse Code Modulation.

Describe Frame & Multi-frame

Describe Synchronization

Describe the development of electronic exchanges

Describe advantages of electronic exchanges over electromechanical exchanges

Enumerate Facilities provided by Electronic Exchanges.

Describe the facilities supported by the electronic exchanges




1.2 INTRODUCTION

The telecom services have been recognized the world-over as an important tool

for socio-economic development of a nation. Telecommunications is one of the few

sectors in India, which has witnessed the most fundamental structural and institutional

reforms since 1991.

Traditionally, telephone networks have been provided to carry voice traffic.

Because of their versatility, they have also often been used to carry data services. Early

analogue networks have been replaced by digital networks.

Basically there are two ways in which information of any type can be electrically

transmitted over telecommunication media analog or digital. Analog means that the amplitude of the transmitted signal varies over a continuous range. Digital transmission

means that a stream of on/off (high/low) pulses is sent on the transmission media. The

pulses are referred to as bits. Examples of analog signals are human voice, hifi music, temperature reading, etc. While that of digital are data, telegraphy signals. With the

invention of Pulse Code Modulation by Reeves in 1938, the basic principles for digitizing

analog speech signals were established.

Cellular radio and intelligent control processes have led to huge progress in

mobile data services. More recently, the availability of the Internet and the Voice over

Internet Protocol has provided an entirely new paradigm for data and multimedia

services. Mobile networks themselves are developing into ubiquitous networks, able to

offer a wide range of data and video, as well as voice services. The Internet, which began

as a data network, is now able to support voice and other real-time services.

The electronic exchanges which have replaced the electro mechanical exchanges

are rich in delivering lot of facilities. This is one of the main advantages of electronic

exchanges. This developmental step opened a new era of innumerable additional facilities

to the subscribers, administration and maintenance personnel

These three networking approaches (circuit-switched, Internet, and cellular

mobile) therefore provide the basis for a telecom network infrastructure. Finally, all these

architectures are migrating towards a common IP network infrastructure called Next

Generation Network (NGN).

1.3 TELECOMMUNICATION NETWORK

A telecommunication network is required to transmit messages between any of its

users (who are usually called customers or subscribers), and the messages may be

conveyed by signals that are either digital or analogue. Channels used to transmit signals

also may be either digital or analogue, but it does not follow that a digital signal always

requires a digital channel and an analogue signal always requires an analogue channel. A

digital signal may be transmitted over an analogue channel by modulating a carrier wave.

An example is the use of a modem (modulator/demodulator) to transmit digital data over

a telephone line.




Fig 1. A telephone call scenario

An analogue signal can be transmitted over a digital channel by using analogue-

to-digital conversion. An example is the use of pulse-code modulation (PCM) to send

speech as a digital signal. It was necessary for the lines of a calling and a called customer

to be connected together in a switching center for the duration of their call. This was

called circuit switching and has formed the basis of all telephone networks.

In addition to its transmission and switching functions, a telecommunication

network must perform signaling functions. Signals must be sent between customers and

their exchanges and between exchanges in order to instruct the switches when to set up

and release connections and how to route the calls. A telecommunication network may

therefore be considered as a system consisting of three interacting subsystems as follows:

Transmission

Switching

Signaling

Fig 2. Elements of Telecom Network




1.3.1 Transmission

Transmission is the process of transporting information between end points of a

system or network. The end-to-end communication distance is often very long and there

are many electrical systems on the line.

Elements of a Transmission System are shown below

Fig 3. Elements of Transmission System

1.3.2 Elements of a Transmission System

Public Switched Telecom Networks(PSTN) use a range of digital transmission

systems, operating over different transmission media like copper, microwave radio, and

optical fiber. The basic building block of these systems is the primary multiplex. This

uses PCM to modulate the signal of each telephone channel onto a 64 Kbit/s pulse train

and the signals are combined by time-division multiplexing to form an assembly of 30

channels occupying approximately 2 Mbit/s. These assemblies can form tributaries that

are combined to form higher-order multiplexes.

The synchronous digital hierarchy (SDH) is sent over optical fibers. It assembles signals from modules called synchronous transport modules at

level 1 (STM-1). This module has a bit rate of approximately 155Mbit/s

and can accommodate 63, 2Mbit/s bit streams (i.e., 1,890 telephone

channels).

The signals of four STM-1 modules can be multiplexed together to provide a synchronous transport module, STM-4, at 622Mbit/s

The four of these can be combined to form a synchronous transport module, STM-16, at 2.48Gbit/s. Thus, STM-16 can accommodate up to

30,240 telephone channels.

Because the number of telephone channels that can now be transported over a

single optical fiber is so enormous, the cost per kilometer of providing each channel is

extremely small. The transport networks are deployed using different technologies in the

different parts of the network.

DSL is a popular technology deployed in the access network today.

SDH/SONET technologies are widely being used for the deployment of the Metro network and

DWDM is used for the core network.




The evolution of transport technology with the increase in bandwidth demand is shown in Figure.

The analog voice was digitized and the Plesiochronous Digital (PDH) techniques were discovered for the transportation of information.

Though, these techniques were popular in the old days, the increasing demand for bandwidth proved that these techniques have many drawbacks.

The highest data rate available in PDH is 140 Mbps and the hardware required for multiplexing and de multiplexing of the signal is much more

than that of in SDH/SONET.

One approach to handle the continuously increasing data traffic can be to adopt the existing SDH/SONET based infrastructure for data.

By replacing SONET/SDH add/drop multiplexers (ADMs) with multi-service provisioning platforms (MSPP). MSPP support Ethernet and other

packet-based protocols, as well as TDM and multiple optical wavelengths.

This approach today is known as the Next-Generation SDH/SONET. It came into existence around 2002.

Fig 4. bbbb

1.3.3 Switching

Graham Bell demonstrated point to point telephone connection. In such a point to

point network appropriate link is to be chosen by the calling subscriber. These networks

are called as fully connected networks. Practical use of telephones on large scale paved

the way for switching system or switching office or the exchange. With this the

subscribers are not connected directly with each other instead they are connected to

switching system

Switches establish, maintain, and change connectivity between circuits. When you

or your computer places a call (Telephone), the switching equipment within the telephone

system search for a physical copper path all the way from your telephone to receivers telephone, this technique is called as circuit switching.

Once the call has been setup, a dedicated path between both ends exists & will

continue to exist until the call is finished.

The earliest telephone exchanges used switchboards worked by operators. The

manual exchange was unique among switching systems, since a single switch (i.e., the

operator) could make a connection to any of several thousand lines by inserting a plug in




a jack within arms reach. When electromechanical systems were introduced, they used switches of much smaller capacity

Fig 5. Point to point connection

The introduction of electromechanical switches controlled by telephone relays

made exchanges much less flexible in the services they could offer, since they had lost the

intelligence provided by the operator. A modern electronic exchange performs basic

actions very rapidly, so it can be controlled by a single central digital processor. The

processor is controlled by stored programs, so the actions it performs can be changed by

modifications to the software. The use of stored-program control (SPC) has enabled some

of the versatility of the manual exchange to be regained and new services can be offered

to the subscribers.

Month Year Landmark

October 1851 The Line completed and opened for East India Companys traffic.

February 1855 Electric telegraph opened to public traffic.

August 1875 The first Private Telephone line supplied by the Telegraph

department.

January 1882 Opening of telephone exchange at Bombay.

August 1907 Central Battery working of telephones was first introduced in

Kanpur.

1913-

14

First automatic exchange at Simla with a capacity of 700 lines

with 400 actual connections.

1953 First Automatic Exchange in Calcutta. Telex Service in

Bombay.

Mechanization of Telephone Revenue Accounting.

November 1960 First subscriber trunk dialing route commissioned between

SWITCHING SYSTEM




Kanpur and Lucknow

December 1967 First crossbar TAX put into service at Madras

March 1975 First PCM system between city and Andheri telephone

exchanges commissioned in Mumbai.

1976 Installation of SPC gateway telex exchange and introduction

of International Subscriber dialed telex service.

1982 First SPC electronic digital telex exchange commissioned at

Bombay.

1983 First SPC analogue electronic trunk automatic exchange

commissioned at Bombay.

1991 I-Net exchange commissioned

2005 BSNL launched Broadband services

1.3.4 Signaling

In telephony, signaling is the exchange of information between involved points in

the network that sets up, controls, and terminates each telephone call. The purpose of

signaling is to exchange control information between systems. Signaling in the period

1900 to 1980 had three characteristics. Its application was limited to setup and release.

Signals were carried by the same circuit that carried speech. It took place only between

customer and his local exchange and between exchanges

a) Customer Line Signaling

A customers line is normally fed with current by a central battery in the exchange. If the telephone handset is in the on-hook position, the line loop is open-circuited and no current flows. If the handset is lifted, the switch hook completes the

circuit; current flows in the line and is detected by the exchange. The following

conditions are thus applied to the line by the customer:

1. Idle: no current

2. Busy: line current

3. Calling or answering: change from idle to busy

4. Clearing (i.e., disconnecting at the end of a call): change from busy to idle

In the reverse direction, when the telephone is on-hook, the exchange calls the

customer by sending low-frequency alternating current to ring the bell. When the

telephone is off hook, the exchange sends back audio-tone signals (e.g. busy tone) or

recorded announcements.

With the introduction of automatic exchanges, the customer was no longer able to

tell an operator the number of the customer to be called. Telephones were therefore fitted

with dials and the customer signaled each digit of a called number by using the dial to

break the line loop, with the corresponding number of disconnections at about 10 pulses

per second. The introduction of subscriber trunk dialing and then international dialing

resulted in long strings of digits and the dial was considered to be too slow. Push-button

telephones were therefore introduced with multi frequency signaling.




b) Channel-associated Signaling

If a customer whose line is on one exchange wishes to make a call to a customer

on another exchange, signals must be sent between them in order to establish the

connection. The traditional way of exchanging these signals is to send them over the

circuits being taken into use for the connection. This is called channel-associated

signaling. Signals sent in the direction away from the caller and toward the called

customer are said to be in the forward direction and those in the reverse direction are

called backward signals. The basic signals required between exchanges for a simple

telephone call are as follows:

Call request or seize (forward)

Address signal (forward)

Answer (backward)

Clear (forward and backward)

c) Common-channel Signaling

In a network consisting of SPC exchanges a high-speed data link is provided

between the central processors of the two exchanges. It can provide a channel for all the

signals needed between the two exchanges. This is known as common-channel signaling

(CCS). Common-channel signaling has the following advantages:

1. Signals can be exchanged much more quickly than with channel-associated signaling.

2. A much wider variety of signals can therefore be used. As a result, customers can be offered more services.

3. Signals may be sent while a call is in progress. This enables customers to alter connections that have already been set up (e.g., to transfer a call elsewhere or

to enable an additional party to join in).

4. Signals can be exchanged between processors for purposes other than call processing (e.g., for maintenance or network management functions).

CCS systems use message-based signaling. Processors assemble messages into

message units containing sequence numbers and check bits for error control. Since

successive messages between two processors usually relate to different calls, each

message must contain a circuit-identity code that indicates the speech circuit to which it

refers.

1.3.5 TELECOMMUNICATION NETWORK

The telecommunication network which allows us to speak or send faxes or other

data is an aggregation of interconnected networks of different types. Networks can be

classified as shown below




Fig 6. Network classification

The telecommunication network consists of national networks and the

international networks. In turn the national network consists of public networks and

private networks. Public networks are for public use. Private networks can be used by

the employees of the organization who owns the network. A public network consists of

fixed networks and cellular mobile networks. The fixed networks are known as PSTN and

the mobile networks are as PLMN.A cellular network has one or more MSCs. Each MSC

is connected by TG to a nearby tandem exchange of a fixed network

Private branch exchanges are owned by private or government agencies and

located in buildings. A PBX enables the employees to call each other and to make or

receive calls served by PSTN. A PBX is connected to a nearby PSTN exchange by access

line groups. PBX is also known as PABX and EPABX (Private Automatic branch

Exchange and Electronic Private Automatic branch Exchange.

Telecommunication

networks

National networks Inter National

networks

Public Network Private Networks

Fixed Networks

Cellular mobile

networks

LE l

lyy

ylll

LLl

llL

VL

1

Lvl 1 Lvl 2 GSM CDMA

PABX

LE

PABX PABX

LE




Fig 7. PBX

1.3.6 PSTN (ANALOG) and ISDN subscriber services

Digital Switching Systems offer a wide range of telephony features and

supplementary services. In addition to the services available for PSTN/Analog as well as

ISDN subscribers, a number of supplementary services are offered only to ISDN-

subscribers.

Number Identification Service

Call Offering Supplementary Services

Call Completion Services

Multi-Party Services

Miscellaneous Services like reminder call/alarm, hot line, Subscriber Controlled Call Restriction Services etc are available to both analog and

ISDN subscribers

These services are invoked by the customers by dialing service codes.

1.3.7 Intelligent Network

Over the last thirty years, one of the major changes in the implementation of

Public Switched Telephone Networks (PSTNs) has been the migration from analogue to

digital switches. Coupled with this change has been the growth of intelligence in the

switching nodes. From a customer's and network provider's point of view this has meant

that new features could be offered and used.

Since the feature handling functionality was resident in the switches, the way in

which new features were introduced into the network was by introducing changes in all

the switches. This was time consuming and fraught with risk of malfunction because of

proprietary feature handling in the individual switches. To overcome these constraints the

Intelligent Network architecture was evolved both as a network and service architecture.

1.3.8 IN Services

Account Card Calling (ACC).

Free Phone (FPH).

Universal Access Number (UAN).

Virtual Private Number (VPN).

Televoting (VOT).

Premium Rate (PRM).

Virtual Card Calling (VCC).

Universal Personal Number (UPN).

Number Portability (NP).

Different service codes are used to invoke these services which have been dealt in

detail later.




1.3.9 The Internet and Data Networks

At first, the Internet was only used by the academic and military communities, but

much has taken place in the last 20 years, both through exploiting new technologies and

through broadening its user base.

The two principal devices in data networks are routers and hosts. A router is a

specially optimized computer that is part of the network routing mechanism, which

ensures that packets are forwarded in the correct way to reach the destination, whereas a

host is a computer running a user application program.

Fig 8. Generic data network

Processes A and B are not aware of the details of network operation, but are aware

that they are running on separate computers.

There is variety of technologies deployed in the present-day Internet; for example,

wireless networks, local area networks such as Ethernet, and connections via the

telephone system via both 56 K modems and Asymmetric Digital Subscriber Line

(ADSL). TCP/IP unifies these diverse technologies, making them all appear to be one

network to the applications programmer.

Each host and router has a unique Internet-wide IP address that identifies the

device and the network that it is on, which could be, for example, an Ethernet. This is 32

bits long in IPv4 or 128 bits long in IPv6. No two devices on the Internet have the same

IP address. To be exactly correct, the IP address refers to a network interface on a device,

and not the device itself. For example, routers have more than one network interface, each

on a different network.

1.3.10 Cellular Telephone Networks

The public land mobile networks (PLMN) use a landline component that employs

many of the basic principles of landline telephony, either based on circuit or packet

techniques. However, they require additional functionality to enable radio connections to

be made to mobile phones and to keep track of where the network users are located. Just

as major change is reshaping fixed, or landline, telephone networks, so, too, is massive

change being seen in mobile networks with third-generation networks now being widely

introduced.

1.3.11 Generations of Cellular Systems

a. First-generation Systems

The first-generation (1G) wireless networks primarily targeted voice and carried a

very low speed data signal.




b. Second-generation Systems

The second-generation (2G) cellular systems are digitally transmitted over the air

and are based on a circuit-switching core network. The main technologies of this

generation are GSM (Global System for Mobile communication) and CDMA One Code

Division Multiple Access system One.

c. Second-and-a-half generation Systems

The 2.5G systems use packet technology in their core network and in their air

interface to improve the network ability to support a wider range of data. These systems

are based on the same infrastructure and the same frequencies as the 2G systems and

usually coexist using the same core network. In these common systems, the 2G usually

carries the voice services and the 2.5G carries the data as an overlay system. The main

technologies of this generation are:

GPRS for GSM, where GPRS stands for General Packet Radio Service

EDGE for GSM, where EDGE stands for Enhanced Data GSM Environment

d. Third-generation Systems

The new features and capabilities of the 3G provide operators with the opportunity

to enhance the relationship they already enjoy with their customers and drive new

revenue opportunities by encouraging additional traffic, stimulating new usage patterns,

and strengthening customer loyalty. It defines a system capable of supporting broadband

data and multimedia services that are not possible with 2G systems. The main 3G

technologies are the Universal Mobile Telecommunications System (UMTS) and CDMA

2000. Standard technologies used today for 3G improvements are High Speed Downlink

Packet Access (HSDPA) for UMTS and Evolution Data Only/Evolution Data Optimized

(EVDO) for CDMA2000 (3GPP2).

e. Beyond 3G

The expectations and the requirements of Mobile users are growing day by day.

To keep pace with the growing demands of high data rate for high end Mobile broadband

services and on the same time keeping a check on effective spectrum utilization is the

biggest challenge for the mobile operators and the equipment vendors. This data thrust

gave rise to various Beyond 3G (B3G) technologies like WCDMA, HSPA, HSPA+, Wi-MAX, LTE and LTE-A etc. All of these technologies are one of the ways to provide

Mobile broadband.

f. LTE (Long-Term Evolution,)

Also called as 4G LTE, is a standard for wireless communication of high-speed

data for mobile phones and data terminals. It is based on the GSM/EDGE and

UMTS/HSPA network technologies, increasing the capacity and speed using a different

radio interface together with core network improvements.LTE is a standard for wireless

data communications technology and an evolution of the GSM/UMTS standards. The

goal of LTE was to increase the capacity and speed of wireless data networks using new

DSP (digital signal processing) techniques and modulations that were developed around

.The LTE specification provides downlink peak rates of 300 Mbit/s, uplink peak rates of

75 Mbit/s and QoS provisions permitting a transfer latency of less than 5 ms in the radio

access network.




The LTE standard only supports packet switching with its all-IP network. Voice

calls in GSM, UMTS and CDMA2000 are circuit switched, so with the adoption of LTE,

carriers will have to re-engineer their voice call network.

1.3.12 Next Generation Networks And The IP Multimedia Subsystem

Next Generation Networks

Although the term NGN can sound vague, through standardization it has become a

clearly defined architecture. The main aim is stated in Y.2001 and can be summarized as

follows:

It is a packet-based network that can use multiple transport network technologies. The transport network has QoS capabilities. Service-related functions are separated from the transport technologies. The access and core networks are clearly separated so that users can have a choice

about who delivers the services.

Generalized mobility is supported so that users can have ubiquitous access to services.

NGN Architecture

NGN is a layered architecture consisting of transport, access, and control

and application layer. It is important to note that all the layers are independent from each

other. Change in one layer should not affect other layers.

The current generation is that these networks have been developed during

different time zones. Thats why they are separate network infrastructure. There is no sharing of infrastructure among them. However some gateways are available for inter

network communication .Because all the three networks are having separate access

transport and switching network service provider has to invest in all the three networks

separately




4

SEPARATE NETWORKS

Service / Application Layer

Control Layer

Transport Layer

Access Layer

Each vertical on the left has to be split into Network Elements that map onto the

horizontal layers on the right.

Fig 9. NGN Vision:

Next Generation Network is the framework where operator will have a common

transport network based on Internet Protocol for providing all kinds of telecommunication

services. Hence operators will have to install and maintain only a single network

8

Service A Service B

Separated control

NB Wireless

BB Wireless

BB Wireline

IP/MPLS Transport Core

Management

Service Layer

Control Layer

Access Layer

Open interfaces

SIP

H.248 QoS Mechanism

FMC

Usage Measurement

Transport Layer

Fig 10. NGN Layers




The IP Multimedia Subsystem

The IP Multimedia Subsystem (IMS) was originally proposed for mobile systems

and is specified by 3GPP in TS 23.228. However, the IMS has been incorporated into the

model of an NGN for handling multimedia sessions. IMS uses mainly IETF-based

standards; for example, SIP is the main session layer protocol. Investigation of the IMS

infrastructure shows that it provides a scalable architecture. The IMS mainly uses two call

control protocols: SIP and H.248/Megaco. SIP is used by end terminals and internally by

the IMS for session layer control; the media gateway control function terminates the SIP

session for sessions requiring end points in external networks and uses H.248/Megaco.

1.4 EVOLUTION OF ELECTRONIC EXCHANGE

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 1832. 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 1326. The Crossbar exchange improved upon many

short- comings of the Strowger system. 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.

The first electronic exchange employing Space-Division switching (Analog

switching) was commissioned in 1365 at Succasunna, New Jersey. This exchange used

one physical path for one call and, hence, full availability could still not be achieved.

Further research resulted in development of Time-Division switching (Digital Switching)

which enabled sharing a single path by several calls, thus providing full availability. The

first digital exchange was commissioned in 1370 in Brittany, France.

1.4.1 ADVANTAGES OF ELECTRONIC EXCHANGES

Electromechanical Exchanges Electronic Exchanges

Category, Analysis, Routing, translation, etc

done by relays.

Translation, speech path Subs Facilities, etc., managed by MAP and other DATA.

Any changes in facilities require addition of

hardware and/or large amount of wiring

change. Flexibility limited

Changes can be carried out by simple

commands. A few changes can be made by

Subs himself. Hence, highly flexible.

Testing is done manually externally and is

time consuming. No logic analysis carried

out.

Testing carried out periodically automatically

and analysis printed out

Partial full-availability, hence blocking. Full availability, hence no blocking. A




limited facilites to the subscribers large number of different types of services

possible very easily

Slow in speed. Dialing speed is max. 11 Ips

and switching speed is in l milliseconds

Very fast. Dialing speed up to 11 digits /sec

possible. Switching is achieved in a few

microseconds.

Switch room occupies large volume. Much lesser volume required floor space of

switch room reduced to about one-sixth

Lot of switching noise. Almost noiseless.

Long installation and testing time. Short installation and testing period.

Large maintenance effort and preventive

maintenance necessary.

Remedial maintenance is very easy due to

plug-in type circuit boards. Preventive

maintenance not required.

1.4.2 ISSUES IN EXCHANGE DEVELOPMENT.

Though there are a number of definite advantages of Electronic exchanges, over

the electromechanical exchanges, there are certain constraints, which should be

considered, at the planning stage for deciding between the two systems.

Traffic Handling Capacity

Apparently, the traffic handling capacity of an exchange is limited by the number

of subscriber lines and trunks connected to the switching network, and the number of

simultaneous paths available through the switching network. However, in electronic

exchanges, the prime limitation is the number of simultaneous calls, which can be

handled by the control equipment, as it has to execute a number of instructions depending

on the type of the call. Therefore the extent of loading of the exchange will be guided

solely by the amount of processor loading. Moreover, the facilities to the subscribers will

also have to be limited accordingly.

Power Supply

The power supply should be highly stable for trouble free operation as the

components are sensitive to variations beyond +10%. It is almost essential to have a

stand-by power supply arrangement.

Total Protection from Dust

All possible precautions should be observed for ensuring dust-free environment.

Temperature and Humidity Control

Due to the presence of quiescent current in the components and because of their

compactness, heat generated per unit volume is highest in electronic exchanges.

Moreover, as the component characteristics drift substantially with the temperature and

humidity, the air-conditioning load is higher. Obviously, the air-conditioning system




should be highly reliable and preferably there should be a stand-by arrangement. The

installation is also carried out in air-conditioned environment.

Static Electricity and Electromagnetic interference.

Due to the presence of static electricity on the body of persons handling the

equipment, the stored data may get vitiated. Handling of PCBs therefore, should be done with utmost care and should be minimized care should also be taken to protect the cards

from exposure to stray electromagnetic fields.

PCB Repair

The repair of PCBs is extremely complicated and sophisticated equipments are required for diagnosing the faults. This results in having costly inventory and a costly

repair centre. With the frequent improvement and changes in the cards, proper

documentation of cards becomes essential.

Faster Obsolescence

The changes in the field of electronics are almost revolutionary with the very fast

improvements. Hence, the current technology becomes obsolete at a very fast rate. The

equipment becomes obsolete before it can possibly complete one third of its life and it

might be impossible to get spare parts for the entire currency of the life of the system.

1.4.3 IMPORTANT FACILITIES OFFERED BY ELECTRONIC EXCHANGES

Facilities offered by electronic exchanges to subscribers are as under. They can be

categorized in three parts.

Facilities to the Subscribers.

Facilities to the Administration.

Facilities to the Maintenance Personnel.

Here some of the Facilities to the Subscribers only are dealt with. Rest two will be

explained later.

MFC Push-button Dialing.

All subscribers in an electronic exchange can use push-button telephones, which

use Dual Tone Multi- frequency, for sending the dialed digits. Sending of eleven digits

per second is possible, thus increasing the dialing speed.

Priority Subscriber Lines

Priority Subscribers lines may be provided in electronic exchanges. These

subscribers are attended to, according to their priority level, by the central processor, even

during heavy congestion or emergency.

Toll (Outgoing Call) Restriction

The facility of toll restriction or blocking of subscriber line for specific types of

outgoing traffic, viz., long distance STD calls, can be availed of by all subscribers. This

can be easily achieved by keying-in certain service codes.




Service Interception

Incoming calls to a subscriber can be automatically forwarded during his absence,

to a customer service position or a recorded announcement. The customer service position

answers the calls and forwards any message meant for the subscriber.

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.

Call Forwarding

The subscriber having the call forwarding facility can keep his telephone in the

transfer condition in case he wishes his incoming calls to be transferred to another

telephone number during his absence.

Do Not Disturb

This service enables the subscriber to free himself from attending to his incoming

calls. In such a case, the incoming calls are routed to an operator position or a talking

machine. This position or machine informs the caller that called subscriber is temporarily

inaccessible.

Conference Calls

Subscribers can set up connections to more than one subscriber and conduct

telephone conferences under the provision of this facility.

Camp On Busy

Incoming call to a busy subscriber can be Camped on until the called subscriber gets free. This avoids wastage of time in redialing a busy telephone number.

Call Waiting

The Call Waiting service notifies the already busy subscriber of a third party calling him. He is fed with a special tone during his conversation. It is purely his choice

either to ignore the third party or to interrupt the existing connection and have a

conversation with the third party while holding the first party on the line.

Call Repetition

Instead of camp on busy a call can automatically be repeated. The calling party

can replace his hand set after receiving the busy tone. A Periodic check is carried out on

the called partys status. When idle status is ascertained, the connection is set up and ringing current fed to both the parties.

Third party Inquiry

This system permits consultation and the transfer of call to other subscribers.

Consultation can be initiated by means of a special signal from the subscriber telephone

and by dialing the directory number of the desired subscriber without disconnecting the

previous connection.

Priority of calls to Emergency Positions




Emergency calls such as ambulance, fire, etc., are processed in priority to other

calls.

Subscriber charge Indicator

By placing a charge indicator at the subscribers premises the charges of each call made can be ascertained by him.

Call Charge printout or immediate Billing

The subscriber can request automatic post call charge notification in the printout

form for individual calls or for all calls. The information containing called number, date

and time, and the charges can be had on a Tele-type-write.

Malicious Call Identification

Malicious Call Identification is done immediately and the information is obtained

in the printout form either automatically or by dialing an identification code.

Interception or Announcement.

In the following conditions, an announcement is automatically conveyed to calling

subscribers.

Change of a particular number of transferred subscriber.

Dialing of an unallocated cods.

Dialing of an unobtainable number.

Route congested or out of order.

Subscribers line temporarily out of order.

Suspension of service due to non-payment.

Connection without Dialing.

This allows the subscribers to have a specific connection set up, after lifting the

handset, without dialing. If the subscriber wishes to dial another number, then he has to

start dialing within a specified time period, say 10 seconds, after lifting the handset.

Automatic Wake Up.

Automatic wake up service or morning alarm is possible, without any human

intervention.

Hot Line or Private Wire.

Hot line service enables the subscriber to talk to a specific subscriber by only

lifting the handset. This service cannot be used. Along with normal dialing facility. The

switching starts as soon as the receiver is lifted.

Denied Incoming Call

A Subscriber may desire that no incoming call should come on a particular line.

He can ask for such a facility so that he can use the line for making only outgoing calls.

Instrument Locking

Subscribers may like to have their telephone sets locked up against any misuse.

Dialing of a secret code will extend such a facility to them.

Free of charge Calls




Calls free of charge are possible on certain special services such as booking of

complaints, booking of telegrams, etc.

Collect call

If so desired, the incoming subscriber is billed for all the calls made to him,

instead of the calling subscriber.

1.5 COMPONENTS OF PSTN NETWORK

1.5.1 Customer Premises Equipment

Customer-premises equipment or customer-provided equipment (CPE) is

any terminal and associated equipment located at a subscriber's premises and connected

with a carrier's telecommunication channel at the demarcation point ("demarc"). The

demarc is a point established in a building or complex to separate customer equipment

from the equipment located in either the distribution infrastructure or central office of

the Communications Service Provider.

CPE generally refers to devices such as telephones, routers, switches, residential

gateways (RG), set-top boxes, fixed mobile convergence products, home

networking adaptors and internet access gateways that enable consumers to access

Communications Service Providers' services and distribute them around their house via

a LAN (Local Area Network).




1.5.2 DISTRIBUTION POINT :

Distribution point is the last point in local Cable network from where

subscriber line is connected.

Fig 11. Distribution Point

Fig 12. Cabinet

1.5.3 EXTERNAL PLANT STRUCTURE IN A LOCAL TELEPHONE EXCHANGE SYSTEM

The factors driving towards replacement of present analog networks by digital

networks are mainly:

1. Technological evolution, with Optical fibers, VLSI

2. Reduced costs due to OFC

3. Need for new services with a long term objective of ISDN lines.




The services which can be provided with existing subscriber network based on

copper cable pairs are Telephony, Telefax, Teletext with phased digitalization the

services that can be made available on copper wire are Telephony (7KHz), Audio

conferencing (64Kb/s), Videotext, Image transmission, Computer communications

and ISDN

Nomenclature used

LOCAL EXCHANGE AREA:

A Local exchange together with equipment, cables, overhead lines and equipment at

customer premises

LOCAL NETWORK:

A local network include more than one exchange ( each having its own area and

network ) and include those sections of Cables both local and Junction

MULTI EXCHANGE AREA:

A group of local exchanges for providing services to customers of one area served by

one or more tandem exchanges.

Subscriber line:

The circuit connecting customer premises equipment to the local exchange.

DIRECT JUNCTION CIRCUIT:

Circuit between two local exchanges.

MAIN DISTRIBUTION FRAME (MDF) :

The connection frame in a local exchange on which local cable pairs and exchange

equipment are terminated for inter connecting.

Distribution point:

The last point in local Cable network from where subscriber line is connected.

PRIMARY CABLE :

Cable usually large in size starting from exchange and connecting many smaller size

cables.

DISTRIBUTION CABLE :

Cable serving a distribution point or interconnecting two cross points

DUCT :

Any arrangement made for laying cables with out excavating or digging at a later

stage

JOINTING CHAMBER / MANHOLE :

Work place where jointing of cables is carried out comfortably in a ducted system.

Conventional external plant

In the conventional network , external plant comprises of




Primary cables from exchange MDF to cabinet (Underground cable).

Secondary cables from cabinet to pillar (Underground cable).

Distribution cable from pillar to Distribution Point (D.P).

Overhead alignment with drop wire.

LJU (Line Jack Unit) with telephone instrument in the subscriber premises.

Fig 13. External plant structure

1.6 MAIN DISTRIBUTION FRAME (MDF)

In telephony, a main distribution frame ( or main frame) is a signal distribution

frame for connecting equipment (inside plant) to cables and subscriber carrier equipment

(outside plant). The MDF is a termination point within the local telephone

exchange where exchange equipment and terminations of local loops are connected by

jumper wires at the MDF. All cable copper pairs supplying services

through user telephone lines are terminated at the MDF and distributed through the MDF

to equipment within the local exchange e.g. repeaters and DSLAM. Cables

to intermediate distribution frames terminate at the MDF.

Exchange

SECONDARY CABLE

DISTRIBUTION CABLE SUBS EUIPMENT

Cabinet Pillar

Over Head Line

D.P.

Primary

Cable




Fig 14. MDF

Fig 15. DDF




1.6.1 DIGITAL DISTRIBUTION FRAME

A Digital Distribution Frame (DDF) is the interface when coaxial cable has to be

terminated, organized or cross-connected in long-distant transport networks, or in access

networks close to subscribers.

In fixed networks, a DDF is installed between the exchange and transmission

equipment, to mention one example. In mobile networks, DDFs can also serve as the

interface between an MSC (Mobile Services Switching Centre) or BSC (Base Station

Controller) and the transmission equipment.

75 ohm Digital Distribution Frames are used to terminate, cross-connect and inter-

connect 75 ohm coaxial cables, and to supervise digital transmission equipment. In the

DDF, signals can be extracted from the desired level to measure incoming and outgoing

signals, allowing the rearrangement or disconnection of traffic.

1.6.2 CABLES

Fig 16. PCM CABLE




Fig 17. POWER CABLE

1.6.3 CONNECTORS

Connector Description

RJ 11

4 wire Telephone Plug for Flat

Stranded Phone Cable

RJ 45

8 wire, for flat stranded phone

cable

DB 9-pin

DB 9-pin Socket Type

Connector, Used in Home

Appliances

D-

50 PIN

D50_male_pcb_mounting_d-sub_connector

D -SUB connectors

D-Sub Connector




1.7 INSTALLATION

As a first step towards the iron work installation the floor is properly marked as per the exchange layout. The installation activities can be broadly divided into two main

phases:

o Pre-Power ON: Physical installation o Power ON and After.

1.7.1 STANDARD TERMS

Floor

Floor means the normal floor covered with suitable antistatic material.

Flooring

Floor is generally covered with 2mm thick antistatic vinyl strips to give antistatic

property to the floor which is necessary to avoid damage to the sensitive circuitry

which incorporates many CMOS devices.

False Ceiling

The false ceiling comprises of panels made of Supersil (aluminium), some of which have holes for fire detectors and lighting fixtures. It is not mandatory and it is needed

for aesthetics improvement.

Datum Line

A datum line is a straight line drawn through the maximum no. of points located at a

distance X from the wall of the Switch Room.

Reference Point

It is a point of the Switch Room floor area to which the datum lines and subsequent

measurements are related. It is defined as the intersection point of the two datum

lines drawn at right angles to each other.

1.7.2 PHYSICAL INSTALLATION

Floor Marking:

Marking of First Datum Line

Mark some points along the length of the room at a distance of 600mm from the wall.

Draw a line such that it passes through a maximum number of the points. To draw the

line, soak a silk string of required length in ultra marine indigo or blue ink. Place it over

the points by holding the string tightly at both the ends. Pull the string from the middle

and release it so that it leaves an impression on the floor. This line is called the first

datum line. Refer Fig.




Locating Reference Point

Mark a point on the first datum line at a distance of 600mm from the adjacent wall. This

point is called the Reference Point. Fix a brass screw at the Reference point to mark it

permanently. This point is denoted as `X' in Fig.

Marking the Second Datum Line

The second datum line should be perpendicular to the first datum line. To ensure this, the

3-4-5-triangle rule is adopted and the second datum line is drawn passing through the

reference point.




With a steel tape measure a length of 900mm or a multiple of 300mm (say Nx300mmm)

from the reference point and mark a point on the first datum line. Let us call this point as

'Y'. (Fig. 1.2). From the reference point 'X' draw an arc at a distance of 1200mm or a

multiple of 400mm (say Nx400 mm) in the direction approximately perpendicular to the

first datum line. To draw an arc use a pen tied to one end of a string. Draw another arc at

a distance of 1500mm or a multiple of 500mm (say Nx500mm) from the point 'Y' on the

first datum line such that it intersects the earlier arc. Let us call the intersection point as

'Z'. Obtain several points in a similar way. Draw a line from the reference point such that

it passes through maximum number of these points. To draw the line use the string dipped

in ultra marine indigo or blue ink.

Marking of lines to mount the equipments Iron work installation

Power DC distribution panel installation

Mounting of Racks

Cable preparation and laying

Powering ON

Software installation

Equipping different modules

Testing

1.8 ROUTINE MAINTENANCE

Initiatives to be taken by maintenance personnel in the best interest of the system's health

ROLE OF MAINTENANCE PERSONNEL

Keeping a watch on the system's health, trouble fixing and programming periodic routing

strategy in advance form the major functions to be performed by the maintenance

personnel. In addition to above, following functions also require human attention:




1. Co-ordination with remote exchanges for trunk testing.

2. Providing the necessary feedback to the support centre.

3. Day to day logging of important observations and maintenance actions.

Watch on System's Health This involves ensuring periodic dump of desired informations, scanning reports

generated by the system and verifying systems integrity with a view to uncover any

abnormalities in system's behavior, and being vigilant towards the audio-visual alarms

raised by the system

Ensuring Periodic Dump of Desired Information

System, on its own initiative, keeps generating various reports regarding system's health

as and when significant events take place. Maintenance personnel too can programme the

system in advance, for generating various periodic reports including the following. Such

reports are to be scanned daily to enable them to track system's health on a day-to-day

basis.

Scanning Spontaneously Generated Reports

Verifying System's Integrity

Trouble-Fixing

Periodic Routing

Deciding a Schedule

Conveying the Schedule to System

Scanning Routing Reports

LOGS TO BE MAINTAINED

All observations and maintenance actions are to be logged in a sequential manner. From

this log, a daily report can then be prepared which provides useful information in a

structured manner. Daily reports help in compiling information regarding system's

performance which is of interest to external agencies such as the support centre.

Deliverable Register

This contains all the information regarding the software related to the system ie. Master

cartridges , Retrofit cartridges ,etc.

System Log Book

In this log book, faults description of every type is written. Any activity related to

exchange is to be noted down in this log book.

MDF Log Book

This important book contains the details of MDF locations for all subscriber lines, line

equipment number, DSLAM port details, etc.




Complaint Book

Complaints from subscribers are noted in this register under the following headings :

Docket number, telephone number, date and time of booking, fault reported, action

taken, remarks, sign, date and time of clearance.

Backup Register

A register to record the billing traffic and data backup is maintained. This contains the

date, expiry date, signature of person taking the backup,

Generator Record Registers

A register is kept to record the time for which the generator has been run in case of AC

power failure.

Power Plant & Environment Register

Note down the voltage and current reading of the power plant after every hour. Similarly

log the temperature of the switch room every hour. Input 3 phase voltage should also be

logged.

Spare Card inventory Register

In this register all the entries related to the spare cards is made like the number of cards

available etc.

Faulty Card Register

In this register all the entries related to the faulty cards is made like the name , serial

number of cards, when sent , where sent , when received, etc.

1.9 PCM PRINCIPLES

1.9.1 Analog And Digital Signals

It is electrical, electronic or optical representation of data, which can be sent over

a communication medium. Stated in mathematical terms, a signal is merely a function of

the data. For example, a microphone converts voice data into voice signal, which can be

sent over a pair of wire. Analog signals are continuous-valued; digital signals are discrete-

valued. The independent variable of the signal could be time (speech, for example), space

(images), or the integers (denoting the sequencing of letters and numbers in the football

score). Figure shows an analog signal.




Fig 18. Analog signal

Digital signal can have only a limited number of defined values, usually two

values 0 and 1, as shown in Fig. 12.

Fig 19. Digital Signal

Digital Signals- A digital signal is a discrete signal. It is depicted as discontinuous

(Discretely variable (on/off) as opposed to an analog signal which is continuously

variable (Sine wave) A digital signal has the following characteristics:

Holds a fixed value for a specific length of time

Has sharp, abrupt changes

A preset number of values allowed

Each pulse (on/off) is known as a bit. Bit is a contraction of the words binary and

digit. A binary (two-level) signal (1 or 0) is the most common digital signal in the

telecommunication industry. The number of bits transmitted per second is the bit rate of

the signal. To convert analog signals to digital signals, a coding system called Pulse Code

Modulation or PCM is used. This process is also called Analog-to-Digital, or A/D,

conversion. When changing a digital signal to an analog signal, the process is called

Digital-to-Analog, or D/A, conversion.

1.9.2 MULTIPLEXING

Due to fast industrial development and increased telephone awareness, demand

for trunk and local traffic went on increasing at a rapid rate.

To cater to the increased demand of traffic between two stations or between

two subscribers at the same station we resorted to the use of an increased number of pairs

on either the open wire. Similarly increasing the number of open wire pairs that can be

installed on one alignment due to headway consideration and maintenance problems.

Similarly increasing the number of pairs to the underground cable is uneconomical

and leads to maintenance problems.

It, therefore, became imperative to think of new technical innovations hitch could

exploit the available bandwidth of transmission media such as open wire lines or

underground cables to provide more number of circuits on one pair. The technique used

to provide a number of circuits using a single transmission link are called Multiplexing.




This could solve the problem for some time only as there is a limit to the number of

open wire pairs that can be installed on one alignment due to headway consideration

and maintenance problems.

There are basically two types of multiplexing techniques

Frequency Division Multiplexing (FDM)

Time Division Multiplexing (TDM)

1.9.3 Frequency Division Multiplexing Techniques

The FDM techniques are the process of translating individual speech circuits (300-

3400 Hz) into pre-assigned frequency slots within the bandwidth of the transmission

medium. The frequency translation is done by amplitude modulation of the audio

frequency with an appropriate carrier frequency. At the output of the modulator a filter

network is connected to select either a lower or an upper side band. Since the intelligence

is carried in either side band, single side band suppressed carrier mode of AM is used.

This results in substantial saving of bandwidth mid also permits the use of low power

amplifiers.

FDM techniques usually find their application in analogue transmission systems. An

analogue transmission system is one which is used for transmitting continuously varying

signals.

Fig 20. Frequency Division Multiplexing

1.9.4 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. Thus

the entire bandwidth is periodically available to each channel. Normally all time slots1 are

equal in length. Each channel is assigned a time slot with a specific common repetition

period called a frame interval.

Each channel is sampled at a specified rate and transmitted for a fixed

duration. All channels are sampled one by; 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 EDM systems. This staggering of channels in time sequence for

transmission over a common medium is called Time Division Multiplexing (TDM).

Fig 21. Time Division Multiplexing

1.9.5 PULSE CODE MODULATION

It was only in 1938; Mr. A.M. Reaves (USA) developed a Pulse Code

Modulation (PCM) system to transmit the spoken word in digital form. Since then

digital speech transmission has become an alternative to the analogue systems.

PCM systems use TDM technique to provide a number of circuits on the same

transmission medium viz open wire or underground cable pair or a channel provided by

carrier, coaxial, microwave or satellite system.

1.9.6 Basic requirements for PCM system

To develop a PCM signal from several analogue signals, the following

processing steps are required

Filtering

Sampling

Quantization

Encoding

Line Coding

a. FILTERING

Filters are used to limit the speech signal to the frequency band 300-3400 Hz.

b. SAMPLING

It is the most basic requirement for TDM. Suppose we have an analogue

signal then minimum number of samples is to be sent for any band limited signal to get a




good approximation of the original analogue signal and the same is defined by the

sampling Theorem.

A complex signal such as human speech has a wide range of frequency

components with the amplitude of the signal being different at different frequencies. Let

us say that these frequency components occupy a certain bandwidth B. If a signal does not

have any value beyond this bandwidth B, then it is said to be band limited. It States

"If a band limited signal is sampled at regular intervals of time and at a rate equal to

or more than twice the highest signal frequency in the band, then the sample contains

all the information of the original signal." Example:

Let us say our voice signals are band limited to 4 KHz and let sampling frequency be

8 KHz.

Time period of sampling TS = 1 sec 8000

Or TS = 125 micro seconds

If we have just one channel, then this can be sampled every 125 microseconds and

the resultant samples will represent the original signal. But, if we are to sample N

channels one by one at the rate specified by the sampling theorem, then the time available

for sampling each channel would be equal to TS/N microseconds.

c. Quantization

In FDM systems we convey the speech signals in their analogue electrical

form. But in PCM, we convey the speech in discrete form. The sampler selects a number of

points on the analogue speech signal (by sampling process) and measures their instant

values. The output of the sampler is a PAM signal. The transmission of PAM signal will

require linear amplifiers at Trans and receive ends to recover distortion less signals. This

type of transmission is susceptible to all the disadvantages of AM signal transmission.

Therefore, in PCM systems, PAM signals are converted into digital form by using

Quantization Principles. The discrete level of each sampled signal is quantified with

reference to a certain specified level on an amplitude scale.

The process of measuring the numerical values of the samples and giving them

a table value in a suitable scale is called "Quantizing". Of course, the scales and the

number of points should be so chosen that the signal could be effectively reconstructed

after demodulation.

Quantizing, in other words, can be defined as a process of breaking down a

continuous amplitude range into a finite number of amplitude values or steps. The finite

number of amplitude intervals is called the "quantizing interval". Thus, quantizing means

to divide the analogue signal's total amplitude range into a number of quantizing

intervals and assigning a level to each interval.

d. Encoding

Conversion of quantized analogue levels to binary signal is called encoding. To

represent 256 steps, 8 level codes are required. The eight bit code is also called an eight bit

"word".

The 8 bit word appears in the form: PABCWXYZ




Polarity bit 1Segment Code Linear encoding

For + ve and '0' for - ve. in the segment

The first bit gives the sign of the voltage to be coded. Next 3 bits gives the segment

number. There are 8 segments for the positive voltages and 8 for negative voltages. Last

4 bits give the position in the segment. Each segment contains 16 positions.

The quantization and encoding are done by a circuit called coder. The coder

converts PAM signals (i.e. after sampling) into an 8 bit binary signal. The coding shows

a relationship between voltage V to be coded and equivalent binary number N.

For the purpose of transmission, these levels are given a binary code. This is called

encoding. In practical systems-quantizing and encoding is a combined process. For the

sake of understanding, these are treated separately.

e. Line Coding

The digital output of PCM equipment contains "1s and '0's. For transmission of

the digital signals between two points, the 1s and 0s contained by the signal are transmitted in the form of pulses as shown in Fig. 5.

Fig 22. Pulse representation of digital signals

The transmission medium normally used for transmitting PCM signals is the VF

cable pair. If the stream of pulses shown in above fig. is transmitted as it is, the signal

undergoes high frequency attenuation distortion and also suffers from other kinds of

distortion such as cross talk etc. This is because of the electrical characteristics of the VF

pair. Moreover the signal passed through the cable pair has strong DC content. This is

because of the characteristics of the signal and those of the medium do not match.

For distortion free transmission, the PCM output should be converted into a

suitable code which will match the characteristics of the medium. This code is called the

"line code" and the signal converted to the line code is called a line signal.

Since the invention of PCM by A.M. Reeves in 1938, a number of line' codes

has been designed. Following are some line codes:

NRZ Binary Code

RZ Binary

Bipolar Coding (Alternate Mark Inversion: AMI Code)

HDB-3 (High-Density Bipolar Code )

CMI Code (Coded Mark Inversion)

1.9.7 SYNCHRONIZATION

The duration, which is the width of the sampling pulse, is called the "time slot"

for a given channel. One full set of samples for all channels taken within the duration Ts

is called a "frame". Thus the set of all first samples of all channels is one frame; the set of all

second samples is another frame and so on.




For a 30 channel PCM system, we have 32 time slots.

Thus the time available per channel would be 3.9 micro seconds.

Thus for a 30 channel PCM system,

Frame = 125 microseconds

Time slot per channel = 3.9 micro seconds.

A frame of 125 micro seconds duration has 32 time slots. These slots are

numbered TS 0 to TS 31.Information for providing synchronization between trans and

receive ends is passed through a separate time slot. Usually the slot TS 0 carries the

synchronization signals. This slot is also called Frame alignment word (FAW).

The signaling information is transmitted through time slot TS 16.TS 1 to TS 15

are utilized for voltage signal of channels 1 to 15 respectively. TS 17 to TS 31 are

utilized for voltage signal of channels 16 to 30 respectively.

The output of a PCM terminal will be a continuous stream of bits. At the

receiving end, the receiver has to receive the incoming stream of bits and

discriminate between frames and separate channels from these. That is, the receiver

has to recognize the start of each frame correctly. This operation is called frame

alignment or Synchronization and is achieved by inserting a fixed digital pattern

called a "Frame Alignment Word (FAW)" into the transmitted bit stream at regular

intervals. The receiver looks for FAW and once it is detected, it knows that in next

time slot, information for channel one will be there and so on. The FAW is transmitted

in the TS 0 of every alternate frame.

1.9.8 MULTI FRAME

The signaling information can be transmitted in the form of DC pulses (as in

electromechanical step by step exchange) or multi-frequency pulses (as in

electromechanical X-Bar systems) etc.

The signaling pulses retain their amplitude for a much longer period than the

pulses carrying speech information. In a 30 Chl PCM system, time slot TS 16 in

each frame is allocated for carrying signaling information.

The time slot 16 of each frame carries the signaling data

corresponding to two VF channels only. Therefore, to cater for 30 channels, we

must transmit 15 frames, each having 125 micro-seconds duration.

For carrying synchronization data for all frames, one additional frame

is used. Thus a group of 16 frames (each of 125 micro seconds) is formed to

make a "multi-frame". The duration of a multi-frame is 2 milliseconds. The multi-

frame has 16 major time slots of 125 microseconds duration. Each of these (slots)

frames has 32 time slots carrying, the encoded samples of all channels plus the

signaling and synchronization data. Each sample has eight bits of duration and each bit

is of 0.488 microseconds (3.9/8 = 0.488)

We have 32 time slots in a frame; each slot carries an 8 bit word.

The total number of bits per frame = 32 x 8 = 256

The total number of frames per seconds is 8000




The total number of bits per second is 256 x 8000 = 2048 K/bits.

Thus, a 30 channel PCM system has 2048 K bits/sec.

Fig 23. Frame Structure

1.10 SUMMARY

This unit gives an understanding about the different elements of the

Telecommunication Network. The three sub sets in a Telecommunication Network are

switching, transmission and signaling. In this unit an insight into the various signaling is

also given. A brief of the basic principles of Pulse Code Modulation is also given in the

chapter. In the end phased development of electronic exchanges is given with a brief

about the various facilities provided by the Electronic Exchanges.

An insight is also given in the different types of cables, connectors, DDF, MDF,

cabinet, etc.

1.11 SELF ASSESSMENT QUESTIONS

1. A telecommunication network may therefore be considered as a system consisting of interacting subsystems. (2,3,4)

2. The fixed networks are known as .. and the mobile networks are as

3. PSTN stands for.and PLMN stands for 4. STM1 can accomate_____________32 Mbps streams (64 /63 /16 ) 5. PABX is -------------network (access/distribution /core) 6. . is a packet-based network that can use multiple transport

network technologies (PSTN/NGN)

One time slot (8 bits)

3.9s

Tb=0.488s

One frame

(32 time slots)

125s

One Multi frame (16 Frames)

2 milli seconds




7. To develop a PCM signal from several analogue signals, the following processing steps are required: Filtering , Sampling, . Encoding, Line Coding.( digitization , Quantization)

8. Define the processes involved in developing a PCM signal. 9. The duration of one frame is .. and of one time slot is . 10. Any changes in facilities require addition of hardware and/or large amount

of wiring change in Electronic Exchange.(T/F)

1.12 REFERENCES AND SUGGESTED FURTHER READINGS

1. https://en.wikipedia.org

Documents

W1 Telecom Switching Network