tt&s lec3
TRANSCRIPT
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Telecom Transmission&
Switching System
Najam ul Hasan
Email:[email protected]
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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
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