basics of pstn

7/28/2019 Basics of PSTN

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MaGee Academy 1

Basics of PSTN


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MaGee Academy 2

Our Agenda

PSTN Architecture & Its Components

Understanding Local Loop

Digital Transmission & Switching

Switching & Functions Of Exchange Trunk Types And Trunk Signalling

Teletraffic modeling and analysis

Number Plan International, National ISDN Principles, Interfaces and Services

Role of SS7 and its applications


3/98

MaGee Academy 3

The Evolution of Telecommunication

Telecommunication is more than 100 years old

Graham Bell invented the telephone that enabled people

separated by distance to talk to each other

Telecommunication network has evolved from simple local

network into a global network

Has passed through transitions manual, Strowger, Crossbar,

SPC, Digital, Mobile Communication, VoIP ..

Initially catered to only voice and later on data networks evolved

We are seeing the merging of voice and data networks so calledconvergence

Though voice is still the dominant application, data services are

gaining prominence


4/98

MaGee Academy 4

What happens when we speak?

Speech is a very very important means ofcommunication

When we speak we create acoustic pressure variations

and ears perceive the speech through this

Human speech contains most energy in 0-4 kHz band

Voice communication involves converting this variation

into electrical form and transmitting over distance and

converting back to sound waves at the other end

Speech has been extensively analysed & researched

Speech coding, speech & speaker recognisation,

speech synthesis and so on


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MaGee Academy 5

How Voice Communication Takes Place?

When you establish a call with another person

A duplex path is set up one carrying voice fromyou and another bringing voice to you

Establishing this path is the job of one or more

switches or exchanges

During very early days this was done manuallythrough operators

Now, most of these jobs are performed by

automatic switches

This path remains till you disconnect The same elements may be used for establishing

another call later

This is broadly what we call circuit switching


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MaGee Academy 6

State-level

trunk exchange

Area-level trunk

exchange

Local

Exchange

Subscribers

Hierarchical Telephone Network (SingleOperator)

A wide area, circuit-switched, mesh (partially-connected)network of star-connected sub networks

vulnerable to failure

high-traffic routes can be directly connected

requires sophisticated routingstrategy


7/98MaGee Academy 7

Typical Network of Single Operator

Mesh connected Trunk exchanges at the state/regional level

Mesh connected local exchanges at bottom in metropolitanareas

Large number of direct routes between area-level trunkexchanges

hierarchical route selected only if direct route is notavailable

Metro area LE

State TE State TE

Area TE

Town area LE


8/98MaGee Academy 8

PSTN Building Blocks (Indian View)

Subscriber terminal: telephone instrument

Simple, robust, low-cost, powered by exchange Local loop: a pair of wires from telephone to local

exchange

Carries power to telephone, voice and signals both ways

Local exchange (LE): a switching node that switches callsfrom one subscriber to another, as well as to/from trunkscomplex equipment

Call processing

ssubscriber administration

Configuration / health monitoring

Battery

Local Loop terminations (Main Distribution Frame)

Trunk terminations (Digital Distribution Frame)


9/98MaGee Academy 9

PSTN Building Blocks (Indian View) (Contd.)

Trunk: interconnects exchange to one another. Medium could

be optical fiber, microwave radio, coaxial cable, carrying

multiplexed trunks

Trunk Exchange (TE): Switching node which only switches

calls between trunks

TAX (Trunk Automatic Exchange): switches long-distance

calls (dial 0 : call is routed to TAX)

Tandem Exchange: switches calls between LEs in a metro;

handles spillover traffic from direct routes

L

E

L

E

L

E

L

E

Tandem


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MaGee Academy 10

PSTN Network Topology: A Summary

Fully mesh-connected TAXs at highest level

International gateways also connected to thissubnetwork

Second-level TAXs for states/regions/metros :connected to level-1 TAXs, but also partially amongstthemselves

Third-level TAXs (often, TAX-cum-local) also deployedat times

LEs (sometimes, LE-cum-TE) at lowest level connect tosubscribers

Size of trunk group between any two TEs, depends onamount of traffic

Trunks often segregated as outgoing andincoming, but can also be bothways


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MaGee Academy 11

Classification of Exchanges Indian View

The exchanges are classified into categories based upon

the functions performed by them Local exchange (or also called end office exchange)

This serves only subscribers connected to it

Local cum trunk exchange This serves subscribers

and also switches trunk circuits Trunk exchange This switches calls between trunks

Tandem exchange A trunk exchange thatinterconnects local exchanges, usually in metro areas

Trunk automatic exchange Services long distanttrunk circuits, will have to generate metering pulses

International Gateway Exchange Performs routingfunction for international calls, transcoding and otherrelated functions


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MaGee Academy 12

Classification Of Exchanges (NorthAmerican Terminology)

Class 5: the end office where the calls originate and

terminate (19000) Class 4: a tandem toll center which interconnect Class

5 offices typically serve to connect ILECs to RBOCs;first point of entry to toll network (1500)

Class 3: a primary toll center connecting Class 4 officesprimarily intrastate toll calling (200)

Class 2: a sectional toll center connects Class 3 officesfor interstate calling within a geographic region (67)

Class 1: a regional toll center connects Class 2 offices

to support interregional toll calling (10)


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MaGee Academy 13

Elements in a Typical PSTN Call

Subs 1

Subs 2

Local

Exchange 1

Trunk

Exchange

Local

Exchange 2

Subs 3

Subs 4

Calls usually involve more than one exchange

Exchanges are interconnected by trunks

Exchanges and trunks can be of many different types

Proper compatibility is important for calls to be successful

Trunks

Subscriber

Nodes

Exch Nodes Subs Lines

Subscriber

Nodes


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MaGee Academy 14

Steps in a Typical Call Set-up / Release

Subscriber goes off hook and dials the required digits

Local exchange analyses the digits and routes the call

If another exchange is involved, the digits have to be

conveyed to that exchange

This process is carried till the terminal exchange is

reached

Terminal exchange routes the call by ringing the

subscriber

When subscriber wants to disconnect, he goes on hook

and similar but simpler steps are repeated

Information about billing / metering also get exchanged

during call

Voice is carried on the trunk path on end-to-end basis

(circuit switched connection)


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MaGee Academy 15

Call Setup Successful

SubscriberActions

Local Exchange /Network Actions

Loop is closedLifts Receiver

Hears dial tone

Dials first digit

Dials next digit.

.

Dials last digit

Hears RBTConversation

begins

Current flow is sensed

Dial tone fed

Analyses digit

Removes Dial tone

Analysesdigits

and

routes call

Called subscriber rungCalled subscriber answers& Conversation begins

Ring back tone

.

.

LocalExchange

Local Loop

Dial tone

Voice signals


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MaGee Academy 16

Call Setup Unsuccessful

SubscriberActions

Local Exchange /Network Actions

Loop is closedLifts Receiver

Hears dial tone

Dials first digit

Dials next digit.

.

Dials last digit

Hears busy toneDisconnects

Current flow is sensed

Dial tone fed

Analyses digit

Removes Dial tone

Analysesdigits

and

routes call

Called party busyCurrent flow stops

Busy tone

.

.

LocalExchange

Loop is open

Local Loop

Dial tone


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MaGee Academy 17

Recap Of PSTN Architecture

List different types of exchanges

What is Class5 switch?

What is Class4 switch? Is there another name for this?

Class1 switch will be _______ connected

Link interconnecting exchanges is called _____

TAX has subscribers connected to it True or False?

Explain circuit switching


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MaGee Academy 18

What Is Local Loop?


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MaGee Academy 19

Concept of Local Loop

The pair of wires that interconnects the local exchange with

subscriber instrument (telephone) is called Local Loop

This pair carries voice signals in both directions and we say

that the voice is carried on 2 wires

The voice signals (incoming and outgoing) are separated at

instrument & exchange by using a device called Hybridtransformer or equivalent device

Not done at the exchange sied if the 2 wire switching is

done (as in Strowger exchanges)

The exchange supplies 48V to the instrument and monitors

the current flow continuously

When the telephone is not in use, the handset (or receiver) is

kept in its place


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MaGee Academy 20

Concept of Local Loop (Contd.)

Under this condition the loop is having high impedance

and no current flows

When we lift the handset, a contact closes at the

instrument and a low impedance path is created

Consequently there is a current flow and this is

sensed by exchange to know that the subscriber

wants service

When the subscriber gets an incoming call, ringing

voltage is fed on the local loop and the bell (or buzzer) atthe subscriber end alerts subscriber

Local loop is a dedicated path between exchange and

each subscriber and this is a major cost component


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MaGee Academy 21

The Local Loop

A pair of copper wires connecting subscriber terminal to

the exchange is called Local Loop this is a dedicatedpair for each subscriber

The pair is twisted (~ 3-4 twists every foot or so) to

reduce induced coupling to other pairs in same bundle

Called twisted pair, or tp Characteristic impedance is 600 ohms

tp comes in various gauges (diameter)

26 gauge 0.4 mm

9 gauge 0.9 mm Typically 0.5 mm dia used to get of exchange

(primary cable), dropping to 0.4 mm dia for the last

segment to the subscriber


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MaGee Academy 22

Balance Wire Transmission

....

....

Single wire transmission with ground return

Balanced 2-wire transmission


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MaGee Academy 23

Balance Wire Transmission - Advantages

2-Wire transmission has been adapted for local loop for

the following reasons

It can carry both transmit and receive signals

Transmission over long distance is affected by

interference from

Powerline signals 50 Hz or 60 Hz and their

harmonics

Lightning and other impulsive signals

Voice band of 0-4 kHz gets affected by these

Balanced 2 wire transmission can reject most of the

common mode interference

Hence 2-wire transmission is being used always


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MaGee Academy 24

Subscriber Terminal Or Telephone Instrument

Normally it is of passive type requires nopower for its operation

It is being powered by the local exchange

Usually 48 V

It has the following components

Mechanism for converting ringing voltage toaudible ringing

Mechanism for dialling digits

Converting into decadic or DTMF form

Receiver having earphone and mothpiece

Hook-switch that operates when the receiveris placed in idle position


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MaGee Academy 25

Concept of 2 Wire / 4 Wire

Exchange

Both signals from/to subscribercarried on 2wires

Local

Loop

mic

ear

2W

4W

2 Wire 4 Wire

Hybrid transformer at telephoneinstrument separates the 2w

signal into 4w signals (receive /send signals)

Terminology:2 Wire: Composite signal

4 Wire: Separate receive andsend signal


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MaGee Academy 26

2 Wire / 4 Wire Conversion

Separating receive signal in the presence of transmit

signal is performed by 2W/4W conversion This is usually done by an inexpensive transformer

called Hybrid Transformer

This depends upon some form of impedance

matching Perfect matching will result in perfect separation of

signals

Otherwise send signal has small portion of receivedsignal

We say part of received signal is reflected

Whenever there is a 2W/4W conversion, some mixingof signals will result


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MaGee Academy 27

2 Wire / 4 Wire Conversion (Contd.)

2W/4W

Converter

ReflectedSignal

Long transmissionpath

2W/4WConverter

ReflectedSignalReflected signal

with long delay

(Echo)


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MaGee Academy 28

Generation Of Echo

In a normal telephone instrument part of microphone

output is fed to the earphone

Called sidetone, is meant to give feedback to

earphone to regulate the speakers loudness

If the reflected signal is not delayed less than 50-75msec, this will be similar to sidetone and user will not

perceive this

If the reflected signal delay is more than this, then

user will start noticing this and will be annoying This delay will be experienced on international

calls, satellite circuits and cellular networks due to

coding, processing and other delays


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MaGee Academy 29

Handling Echo

Echo is the effect of reflected signal arriving with a

large delay Either reflection or long delay alone will not be a

problem

Echo can be handled by two methods

Echo Suppressor this is a very simplemechanism that makes the communication semiduplex one direction at a time

Whenever you talk, your receive signal iscutoff this happens at both ends

Users should learn to talk one at a time

This is rather obsolete now


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MaGee Academy 30

Handling Echo (Contd.)

Echo Canceller this is a sophisticated method

by which the reflected signal is totally eliminatedby using DSP techniques

Most widely used nowadays

Earlier this was fairly expensive and was

selectively used

Due to dropping costs of DSP, this usage has

increased

Users will not be aware of the presence of echo

canceller

Allows full duplex communication


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MaGee Academy 31

Twisted Pair Frequency Response

Frequency response of twisted pair is bandbass

Increase in attenuation with frequency

Restricts the response to higher frequencies

On very long loops ( 5 kms) loading coils are

placed periodically to improve the frequencyresponse

Results in flat frequency response upto 4 kHz, but

has poor response for higher freq

These effects do not have significance for POTS

Have serious implications for DSL (Digital Subscriber

Line) applications


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MaGee Academy 32

Role of Signalling PSTN calls are circuit switched calls

Dedicated voice path is established between the end

users

This may involve one or more exchanges and may also

involve trunks

This path is set up using information supplied by

subscriber (called party number)

Once the path is set up, this allows end to end voice

communication

The process of setting up this path and releasing it

subsequently is called signalling

Broadly divided into two classes

Between subscriber node & exchange node

Between exchange nodes

S ll S b b d h


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MaGee Academy 33

Signalling Between Subscriber and Exchange

ExchangeSubSubscriber to Exchange

Loop (Flow of - To indicate desire for service (either originatingcurrent) or terminating)

No loop - To indicate end of request for service

Pulse Dialing - To convey digits of the called subscriber

(make/brake) by loop / no loop repetition

Hook Flash - To invoke features / special services

DTMF - More reliable method to convey digits

Si lli B S b ib d E h


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MaGee Academy 34

Signalling Between Subscriber and Exchange

ExchangeSub

Exchange to Subscriber

Ringing - To indicate incoming call with

distinctive ringing

Tones / announcements - To indicate the status of call

Special signals - To convey calling line identification

Battery Reversal - To indicate the start of metering

on PCO lines (called party answer)

16 kHz tone - Metering indication to Home Meter


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MaGee Academy 35

Summary Of Outband Signalling

Purpose of

Signalling

Achieved by Results in

Changing conditionof service

On Hook / Off Hook/ Flash

Flow /interruption ofcurrent

Indication ofincoming call

Applying ringingvoltage throughrelay (at exchange)

Bell ringing

Indication of answerto coin operated

telephones(charging)

Operation of relayat exchange

Battery reversal

Conveying digits bydecadic method

Operation of relayat telephone

Flow /interruption ofcurrent


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MaGee Academy 36

Outband Signalling Decadic Dialling

Pulse, or decadic, dialling

Break Make Ratio = 2 : 1

Make + Break duration is 100 ms

10 pulses per sec

Inter-Digital Pause > 1 sec

~10V

48V IDP

B BM MB

2 3


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MaGee Academy 37

Dual-Tone Multi-Frequency (DTMF) Signalling

Pulse dialling was too slow

Each digit takes average of 0.5 sec (max 1 sec)

Timing gets changed when transmitted

Prone to wrong interpretation and wrong number

being switched

DTMF signalling evolved to overcome these defects This is an inband signalling using voice path

Two tones (1 tone from low frequency group and 1

tone from high frequency group sent)

Signalling is about 5 digits per sec DTMF signalling can be used after establishing calls

DID (Direct Inward Dialing), IVRS (Interactive Voice

Response Service)


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MaGee Academy 38

DTMF Frequency Combinations

fH (Hz)

fL (Hz)1209 1336 1477

6971 2 3

7704 5 6

8527 8 9

941* 0 #


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MaGee Academy 39

Recap Of Local Loop

What is local loop?

Explain 2 wire concept

How echo is generated?

Balanced transmission explain the importance ofthis

Differentiate between echo suppressor and echo

canceller

Explain decadic dialling and its disadvantages

What is DTMF signalling?


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MaGee Academy 40

Why Digital Networks?

N t f S h Si l


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MaGee Academy 41

Nature of Speech Signals

Human speech signals have most of the energy contained

in 0 to 4 kHz This is called voice band

Musical Instruments produce signals in the range of 0-20

kHz and music requires higher bandwidth

There are many advantages of representing speech

signals in digital form

A speech signal in analog form is continuous in both

amplitude and time

It can take any continuous value in both these

dimensions

Di iti i S h Si l


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MaGee Academy 42

Digitising Speech Signal

It is preferable to represent speech in digital forms

Done by sampling the speech waveforms at periodicalintervals and represent the signal in digital form

These are called Sampling and Quantisation

How often do we need to sample the waveform? This is governed by Nyquist criterion

Any waveform having maximum frequency of f Hz has to

be sampled at a minimum rate of 2f samples per second

Otherwise a phenomenon call Aliasiing occurs that

makes a higher frequency component appearing as a

lower frequency component

Di iti i S h Si l ( td )


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MaGee Academy 43

Digitising Speech Signal (contd.)

Quantisation is the process of representing a

continuously varying signal to a value closest to one

from a finite set of discrete values

This will result in Quantisation Error

There are many quantisation techniques

Uniform quantisation

The step sizes are all uniform & equal

This is wasteful of bandwidth

Not widely used in communication sytems



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MaGee Academy 44


Nonuniform quantisation is widely used in communication

systems

Probability of smaller amplitude signals is more than

larger amplitude signals

Step size is varied depending upon the signal amplitude

Lower the signal amplitude, smaller the step size andvice versa

This results in smaller quantisation error even while

maintaining smaller number bits to represent signal

This type of quantisation is called companding (since this isequivalent to compressing the signal values before A/D

conversion and expanding the signal after D/A conversion)



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MaGee Academy 45


PCM (Pulse Coded Modulation) Coding

Speech signal is band pass filtered (band 0.3-3.4kHz) & sampled at 8000 times per second

Sampled value is quantised using 8 bits non-uniformquantisation scheme

Each speech signal is represented by 8000 x 8 =64000 bits per second or 64 kb/sec

Two international standards

American networks follow -Law

European networks follow A-Law

They appear similar, but incompatible

Optimises the bandwidth 8 bit equivalent to 10 ormore bits

64 kbps PCM coding is de facto PSTN standard

Major Functions of a Network


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MaGee Academy 46

Major Functions of a Network

There are broadly 2 functions performed by atelecommunication network

Switching This is a function related toestablishing the calls based upon info provided bysubscriber and releasing the call after usage

This function is carried out by Switches or

Exchanges Transmission This is related to carrying the

voice (in analog or digital form) from one exchangeto another

This involves carrying the trunks aftermultiplexing many of them

Earlier this was performed by Frequency DivisionMultiplexing (FDM) Or Time Division Multiplexing(TDM) systems

L Di t T i i


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MaGee Academy 47

Long Distance Transmission

Circuits carry large number of individual channels

100s or even more at a time Need reliable, high capacity, economical eqpts

Earlier methods were analog

4 kHz channels multiplexed FDM systems

12 channel, 60 channel & so on

Transmitted through coaxial cables or microwave

Performance was just ok

Some of the problems of FDM system Cross talk

Noise build up

Expensive

Long Distance Transmission (contd )


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MaGee Academy 48

Long Distance Transmission (contd.)

Late 70s saw increasing use of digital transmission

PCM was the adopted standard

Americans adopted 24 channel (called DS1 at 1.544

Mbps) 96 channel (called DS2 at 6.312 Mbps), ..

rates

Europeans adopted 30 channel (called E1 at 2.048

Mbps), 120 channel (called E2 at 8.192 Mbps), ..

rates

Performance improvement was fantastic Cost of bandwidth began to drop with fibre optic

medium

Ad t f Di it l N t k


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MaGee Academy 49

Advantages of Digital Network

Ease of Multiplexing

Better Signalling

Benefit from Semiconductor Technology

Integration of Transmission and Switching

Preserving the Signal Integrity

Performance Monitoring

Supporting Other Services

Operation Under Low SNR conditions

Ability for Encryption

E f M lti l i


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MaGee Academy 50

Ease of Multiplexing

FDM Systems with adequate performance are difficult

to design Cost of higher order multiplexer prohibitively

expensive

Every stage has to be carefully built for

Linearity Cross talk and other impairments

TDM systems can be built economically due toadvances in semi conductor technology

Once digitisation is done, subsequent stages arelike a normal digital design

Higher order multiplexers only marginallyexpensive

This effect is accelerated by advent of fibre

B tt Si lli


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MaGee Academy 51

Better Signalling

Signalling is inherently digital in nature

On-hook / off-hook, ringing on / off, dialled digits,

answer, etc.

Ssignalling depended on the type of trunk

E&M trunk, 2 wire, FDM system in band, out of

band, etc

PCM based transmission improved signalling

performance and ultimately resulted in CCS based on

messages Faster & reliable call set up

New services

B fit F S i d t T h l


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MaGee Academy 52

Benefit From Semiconductor Technology

Advances in semiconductor technology make it easier tointegrate digital circuits

This results in much reduced cost of higher order

multiplexers

Scope for integrating more analog circuits not much

Many multiplexing and switching blocks have been

standardised

Results in lower cost & speedier implementation

Performance improvement in space occupied, power

consumed, etc equally important

I t ti f S it hi & T i i


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MaGee Academy 53

Integration of Switching & Transmission

AnalogSwitch

AnalogSwitch

ChannelBank

ChannelBank

TDMLink

DigitalSwitch

DigitalSwitch

TDMLink

If switching & transmission are done digitally, then thereare many benefits like, less A/D & D/A conversions, lower

cost, less complexity, etc

P i Si l I t it


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MaGee Academy 54

Preserving Signal Integrity

In the case of analog signals once noise is injected, it

remains always

There is continuous deterioration (cumulative effect)

Digital signals are based upon thresholds and if the

thresholds are not exceeded, signal can be restored

Even if one bit is in error, error coding can preserve the

integrity of digits

Totally error free transmission end to end possible

Noise injected atfirst stage

Noise injected atlater stage



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MaGee Academy 55


In the case of analog links, pilot signals areintroduced to monitor the link condition

This is effective only under limited condtions

CRC check of bits transmitted is used in digitallinks

This provides a very good measure of linkcondition



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MaGee Academy 56


Analog network was used for voice communication The same network when used for data

communication, data has to be converted to look

like voice waveforms using modem

Digital network can carry Digitally coded Voice

Any form of data

Digitally coded images, video, etc.

Same network can cater to variety of services

Network can remain same while many services can

be adapted



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MaGee Academy 57


SNR

Quality

Analog Transmission



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MaGee Academy 58


SNR

Quality

Digital Transmission

Ease of Encryption


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MaGee Academy 59

Ease of Encryption

Signals have to be encrypted to prevent others

tapping the signal

Analog signal is not amenable to encryption

Digital signals can be easily encrypted thus

ensuring privacy

Encrypted signal is secure and is intelligible only

to the intended person

Digital methods provide variety of encryption From the simplest to the most complex

Recap Of Digital Network


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MaGee Academy 60

Recap Of Digital Network

Explain the difference between sampling and quantisation

What is PCM coding?

List the advantages of PCM coding

Name the advantages and disadvantages of digitalmultiplexing compared to analog multiplexing

List 3 advantages of digital networks

List 3 advantages of digital networks

What is encryption?


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MaGee Academy 61

Switching Concepts & Functions OfExcahnge

Functions of a Local Exchange (Digital)


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MaGee Academy 62

g ( g )

Local exchange (LE) has subscribers connected to it

Connected to tandem, TAX,.. through trunks

Subscriber instrument connected to local exchange

through local loop

Major functions of LE

Performing subscriber functions called BORSCHT

B Feeding battery supply to power subscriber

instrument

O Providing overload protection to subscriber line

RFeeding ring voltage to announce incoming call

S - Supervision of subscriber line

Functions of a Local Exchange (contd.)


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MaGee Academy 63

g ( )

C Performing audio coding to convert to digital form

H - Hybrid function for converting to 4 wire signal

T Line testing function

Switching tones & announcements as per call status

Analysing & switching the call based upon the digits dialledby subscriber

Performing signalling functions for trunks

Monitoring incoming trunk status and switching calls

Managing traffic & generating

Functions of a Local Exchange (contd.)


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MaGee Academy 64

g ( )

Performing billing related functions

Periodically doing maintenance functions & raising

alarm

Allow local and remote monitoring of exchange status

Provide man machine commands for

Accessing and monitoring exchange and subscriberdata

Carrying out maintenance / replacement of cards

Replacement under power-up condition should

not bring down system

With multilevel privilege protection


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Concept of Circuit Switched Connection


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MaGee Academy 66

Concept of Circuit Switched Connection

....

Switch1

Switch3

Switch2

..

..

..

..

User1

User2

User3

Conncetion1 User2

Connection2 User3

Trunk1

Trunk2

Different Types of Switching


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MaGee Academy 67

yp g

Electromechanical switching

Control by electrical, but switching is by physicalcontact of moving parts

2 wire switching

Oldest Strowger exchanges belong to this

Other electromechanical forms

4 wire switching done by electromagnetic relays /

reed relays

Electronic switching Analog, this belongs to Storedprogram Control (SPC) using cross point swithces

2 wire

4 wire

Different Types of Switching (Contd.)


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MaGee Academy 68

g

Digital switching done on digitised PCM streams

Speech digitsed and multiplexed to PCM format

Switching is done on PCM streams

Major types of digital switching

Time Switching

Time & Space Switching or a combination of these

This is the prevalent form of switching

Has superior performance in terms of cost,

performance and reliability

Challenges In Implementation of Switch


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MaGee Academy 69

Switches need to have large capacities

Switches with 100,000 lines are common Such switches need to support large number of

simultaneous calls

Switches have to be efficient, less costly, consume less

power, occupy less space They also need good performance

Minimum cross talk

Less blocking during peak hours

There is need for switching systems with small,

medium, large and very large capacities

They need to be modular enabling scalability

Challenges In Implementation of Switch


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MaGee Academy 70

Switches are designed with peak hour considerations

Switch does not allow every body to talk

simultaneously

It is assumed that only 10% or 15% of subscribers

will talk at any point of time Switch with 100,000 lines can support only 10,000

peak hour calls (10% active users)

If more users try to call, their calls will be

blocked

This makes the systems economical and viable

There are concepts called concentration & blocking

Concept Of Blocking


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MaGee Academy 71

Concept Of Blocking

There are certain resources that are critical

One could be the switch, that is the essence of

exchanges or switching systems

Another resource could be ringing voltage

This will dictate only x numbers of lines can befed ringing voltage

Another critical resource could be availability of

trunk towards a particular exchange

Availability of codec could be another reason

Non availability of this resource could lead to blocking

subscriber may be fed busy tone

An Example of Blocking


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MaGee Academy 72

An Example of Blocking

Digital switching is accomplished by

Converting the analog voice into PCM form

Multiplexing the individual PCM samples and

forming PCM streams

Doing the switching by Time / Space switching

to the required destination

This could be PCM trunk or another

subscriber

Economics may dictate that not all subscribersPCM output can be multiplexed

We talk that there is concentration

PCM Multiplexer With 4:1 Ratio


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MaGee Academy 73

PCM Multiplexer With 4:1 Ratio

Codec

Codec

Codec

Codec

Codec

.

.

.

Line1

Line2

Line3

Line 128

Multiplexer32 ChannelPCM Output Stream at2.048 Mbps

Clock & othersignals

..

Analog Space Switching


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MaGee Academy 74


N x N space switch needs N(N-1) N2 cross points

Cross point array has been implemented in many

technologies

Strowger, crossbar, electronic

Space switching can be either 2 wire switching or4 wire switching

Major disadvantage of space switching is that it is not

scalable

Larger arrays with good performance are difficultto build

Cross talk goes up sharply



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MaGee Academy 75


.

.

.

. . .

Inlet1

Inlet2

Inlet3

Inlet N-1Inlet N

Outlet 1 2 3 4 5 m-1 m

.

.

. .

.

.

PCM Format


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MaGee Academy 76

0 1 2 . . . 15 16 17 . . . 29 30 31

1 2 3 4 5 6 7 8

Time Slot =

8 bits

Time Slots

PCM Format

PCM Frame

= 125 sec =256 bit time

Gross data rate is 2.048 Mbps

Time Switching


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g

64 kbps PCM voice coding has been standardised for

transmission & switching Two standard transmission rates called E1 (2.048

Mbps) and T1 (1.544 Mbps) are used

A multiplexed signal at 2.048 Mbps can carry 32 voice

channels of 64 kbps each Let us assume that we want user1 (his voice is in time

slot # 5) to be connected to user2 (his voice is time

slot 20) and the these two users to be communicating

This can be done by interchanging their signalsappearing in the respective slot

Voice in TS#20 should be switched to user1 and

Voice in TS#5 should be switched to user2

Time Switching (Contd.)


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Time Switching (also called Time Slot Interchanger -TSI)

interchanges these time slots at the output

Bit streams received in TS#20 interchanged with bit

streams received in TS#5 and vice versa

This function is easily done by converting to 8 bit form

(equivalent to time slot), writing into Switch Memory

(SM) and reading out memory with proper sequence

This is achieved by having two banks of memories, called

SM0 and SM1

One for writing into and one for reading out from

Writing into is done sequentially and reading out done as

per switching requirement

Time Switching (Contd.)


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g ( )

PCMCoding

&Multiplexing

User1

User2

.

.

.

User32

PCMDecoding

&Demultiplexing

TimeSlot

Interchanger(Time Switch)

User1

User2

.

.

.

User32

User1 inTime Slot5

User2 inTime Slot20

User2 in

Time Slot5

User2 in

Time Slot5

PCM frame= 125 sec

Time Switching Implementation


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g p

32 x 8SwitchMemory

Bank0(SM0)

32x 6Switch

MemoryBank1(SM1)

SwitchAddressMemory(SAM)

Input PCMStream

SwitchedOutputPCMStream

clock, control andstatus signals

serial toparallelconverter

parallel toserialconverter

Time Switching Implementation (Contd )


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Time Switching Implementation (Contd.)

Initially Writing takes place into SM0, then into SM1,

then into SM0 and so on Reading from SM takes place as per the switching to be

done

This is controlled by Switch Address Memory that

holds the mapping details Read out takes place from the bank that has been just

written when SM0 is being used for writing, readout

will be from SM1, similarly when SM1 is used for

writing, SM0 will be used for writing With reference to the need for switching User1s voice

output recd on TS#5 to User2 on TS#20, the following

points can be observed

Time Switching Implementation (Contd )


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MaGee Academy 82

Time Switching Implementation (Contd.)

User2s PCM samples received in TS#20 will be

written into memory location 20 These bits need to be read out during TS#5

SAM will have this information that specifies the

locations to be read out during TS#5

Similarly User1s PCM samples recd on TS#5 need tobe switched to User2 on TS#20

User2s PCM samples received in TS#20 will be

written into memory location 20

These bits need to be read out during TS#5

SAM will have this information that specifies the

locations to be read out during TS#20

Time Switch With More Capacity


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MaGee Academy 83

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Building large capacity time switch is possible

It will have multiple PCM input stream andmultiple PCM output stream

Memory size will be larger Operating speeds will increase

Time switch with 4 input / output streams willachieve 128 x 128 switch

Time

SlotInterchanger

(32 x 32switch)

Input PCMStream

SwitchedOutput PCMStream (128 x 128

Switch)

Output PCM

Streams

InputPCMStreams

Building Large Switch


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Building large capacity switch (say 512 x 512) is

possible

Beyond this we look for multistage switch

Switching is done in stages

32

x 32

32x 32

32x 32

32x 32

PCM1

PCM4

PCM5

PCM8

PCM9

PCM12

PCM13

PCM16

32

x 32

32x 32

32x 32

32x 32

PCMOut1

PCMOut4

PCMOut5

PCMOut8

PCMOut9

PCMOut12

PCMOut13

PCMOut16

Building Large Switch (Contd.)


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MaGee Academy 85

Advantages of 2 stage 4 stream Time Slot Interchanger

(TSI0 TSI Chipsets are available cost is affordable

System cost is low

Disadvantages

Time switch introduces frame delays

Every stage will involve 2 or 3 frame delays

There are allowable upper bounds on delays in

switching systems

There is blocking in a multistage switch

You will not be able switch any input to any

output

Space Switch


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MaGee Academy 86

Building a very large capacity time space will be

expensive Clock rates will get increased

Memory capacity gets increased

Design becomes complex

PCB size will get increased

No of input output pins will get increased

Instead space switch can be considered

Space switch

Has no memory but has multiplexer

Cannot change time position but instead

switches to another output

Simple Space Switch


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MaGee Academy 87

SpaceSwitch

128 x 128

TS#2

TS#2

TS#8

TS#8

TS#18

TS#18

TS#25

TS#25

PCM1

PCM2

PCM3

PCM4

PCMOut1

PCMOut2

PCMOut3

PCMOut4

Space Switch


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MaGee Academy 88

PCM1

PCM4

PCM5

PCM8

PCM9

PCM12

PCM13

PCM16

SpaceSwitch

PCM1

PCM4

PCM5

PCM8

PCM9

PCM12

PCM13

PCM16

PCMOut1

PCMOut4

PCMOut5

PCMOut8

PCMOut9

PCMOut12

PCMOut13

PCMOut16

An Example Of Large Capacity Switch


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MaGee Academy 89

C-DOT has designed large capacity switches of upto

16000 lines Design is modular

Basic building block is switch of 512 lines

This is called Base Module (BM)

This can be used as a small capcity 512 line

switch (for smaller towns)

BM can do all the functions of an exchange

It has a time switch unit

Original design of C-DOT was non-blocking

Can support all users being in active call

Total of 32 BMs can support 16000 lines

n xamp e arge apac y w c(Contd.)


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MaGee Academy 90

Each BM can support various types of terminations

It could be subscribers call line circuits It could be analog trunks / digital trunks

E&M Trunks, Ring Down Trunks, Two way trunks, PCM

trunks, etc.

Each of these can be configured for different signallingschemes

Various forms of CAS

SS7 (a form of CCS)

Calls within the same BM (intra BM) are switched by Time

Switch

Calls between 2 BMs (inter BM) are switched by Time Switch

and Space Switch

C-DOT SBM Configuration


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C-DOT SBM Architecture


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C-DOT 16,000 Line Architecture


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Redundancy in Digital Exchanges


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MaGee Academy 94

Telecom networks require very high reliability

Availability should be 4 9s or 0.9999 Down time of less than 1 hour in a year

Local exchange (Class 5 switch) is the most critical link

for network performance

When a large capacity local exchange fails, it is ofserious consequence

Redundancy of critical sections of exchange are

duplicated

Duplicated section remains in hot standby

Ready to take over at any time failure occurs

Redundancy in Digital Exchanges (Rady)


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MaGee Academy 95

y g g ( y)

Modern digital exchanges have a large switching matrix

concentrated in a few devices

Call control software on one (or a few) powerful

processor(s)

Failure of a device or card can cause exchange to go

down

Unacceptable

All active subsystems/cards performing functions that can

affect more than a small group of subscribers, or which

cause more than a marginal degradation of service, are

duplicated in hot standby mode

Redundancy in Digital Exchanges (contd.)


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MaGee Academy 96

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Switch card, CPU cards, tone generator cards,database storage, etc.

duplicated

Subscriber line interface cards, trunk interface cards,etc., not duplicated

failures affect a few subscribers or calls

Passive subsystems (cables, motherboard / backplane)are much less failure-prone, and are not duplicated

both copies of a card may be connected to samebackplane

General philosophy is to make system robust to a

single-point failuremaintenance philosophy is quick replacement of

faulty card

Good maintenance/health monitoring software is amust

Copy Assertion


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MaGee Academy 97

How does passive copy cut in when failure occurs?

Copy 0 writes on line 0 if it ishealthy

Similarly, copy 1 writes on line1

Each copy can know the others health

On power-up, some logic necessary for deciding which copyshould be active

If fault occurs, passive copy takes over

When faulty copy is replaced, it comes up as passive

copy

Forced copy-switch (from console) usually provided for

To exercise passive copy

To update software without bringing system down

Copy

0Copy

1Line 1

Line 0

Health

Lines

Recap Of Exchange Functions


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What are BORSCHT functions?

Explain 3 functions of exchange

Explain 3 more functions of exchange

Explain analog space switching

What is concentration and why is it done?

Explain how time slot interchanger works?

Explain functioning of space switch

Why multistage switching is required?

basics of pstn

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