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Telecom Transmission&

Switching System

Najam ul Hasan

Email:najam.hassan@nu.edu.pk

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Overview

Time Division SwitchingHybrid of Time and Space Switching

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Space Time Space Switch

Time Space Time Switch

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Second type of circuit-switching is called

time-division switching. It uses time-division

multiplexing to achieve switching i.e. in this

type of switching, a single link can

accommodate transmissions by multiple

devices by allocating individual time-slots to

the devices for data transfer.

There are two popular methods used in time-division multiplexing.

1. Time-slot interchange

2. TDM Bus

TIME DIVISION SWITCHINGTIME DIVISION SWITCHING

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TIME DIVISION MULTIPLEXINGTIME DIVISION MULTIPLEXING

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In the previous diagram, input A wants

to send some data to output C, input

B to output D, input C to output A and

input D to output B. In the caseordinary TDM, the desired task can

not be accomplished as data is output

in the same order as they are input.Data from A goes to A, B to B, C to C

and D to D.

ORDINARY TDMORDINARY TDM

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However if we insert a device called

TSI into the link, it will change the

ordering of the slots based on the

desired connections. In the previouscase, it will change the order of the

data from A,B,C,D to C,D,A,B. Now,

when the demultiplexer separates theslots, it passes them to the proper

outputs.

TIME SLOT INTERCHANGETIME SLOT INTERCHANGE

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TDM AND TSITDM AND TSI

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TSI consists of random access memory (RAM)

with several memory locations.

The RAM fills up with incoming data from time-

slots in the order received. Slots are then sent

out in an order based on the decisions of a

control unit.

TIME SLOT INTERCHANGETIME SLOT INTERCHANGE

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Advantage of space-division switching is that it isinstantaneous and scalable. Its disadvantage is

the number of cross-points required to make

space-division switching acceptable in terms of

blocking. The advantage of time-division switching is that it

needs no crosspoints. Its disadvantage, in the

case of TSI is that processing each connection

creates delays. Each time slot must be stored bythe RAM, then retrieved and passed on.

We combine space-division and time-division

technology to take advantage of the best of both.

HYBRID OF TIME AND SPACE DIVISION SWITCHINGHYBRID OF TIME AND SPACE DIVISION SWITCHING

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T i m e s w i tc h

( n x n )

T i m e s w i tc h( n x n )

T i m e s w i tc h

( n x n )

T i m e s w i tc h( n x n )

m in c o m in g

h i g h w a y sm o u t g o i n g

h i g h w a y s

m x m

B s w i tc h

TST SWITCHTST SWITCH

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STS SWITCHSTS SWITCH

T i m e s w itc h

( n x n )

T i m e s w itc h

( n x n )

m

o u t g o i n g

h i g h w a y s

m

i n c o m i n g

h i g h w a y s

A s w itc h C s w itc h

m x k m x k

k l in k s

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The architecture of TST is more complex

TST becomes more cost-effective because

time expansion can be achieved at less cost

than space expansionTST has edge over STS when a large

amount of traffic must be handled

TST/STS COMPARISONTST/STS COMPARISON

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Chapter 8 :Data Communication and Networking, 4th Editionby Behrouz A. Forouzan

RECOMMENDED READINGRECOMMENDED READING