1. Subject Name: DIGITAL SWITCHING SYSTEMS Subject Code: 10EC82
Prepared By: Farha Kowser, Aparna P
Department: Electronics and Communication Engineering
Date:
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2. UNIT 5 TIME DIVISION SWITCHING
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3. Time division switching networks Synchronization
Contents:
Introduction
Space switches
Time switches
Time division switching networks
Synchronization
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4. Introduction:
Time division switching makes use of time division multiplexing in order to perform switching. TDM is a digital multiplexing technique which is used to combine signals.
There are two popular methods used in TDS
Space switch
Time switch
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5. Space Switch:
A space switch is shown in figure. It consists of M incoming and N outgoing PCM highways.
The connection store for each column of cross points is a memory with an address location for each time slot which stores the number of cross points to be operated in that time slot
In each time slot, the number stored at the corresponding store address is read out and decoding logic converts this into a pulse on a single lead to operate the relevant crosspoint
Fig: Space Switch
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6. Figure below illustrates the working of a space switch
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7. Time Switch:
A time switch is also called as time slot interchange(TSI).
A limitation of space switch is that a particular time slot t0 can be move to t0 of a different trunk that is order of channels cannot be changed.
A time switch can interchange the time slots as shown in fig. To perform its operation, a time switch contains a speech store apart from connection store.
The limitation of a time switch is it can allow change of channels only on the same line or trunk but not between different trunks.
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8. Figure below illustrates the working of a time switch
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9. S-T-S Switching network:
To overcome the limitations of time switch and space switch, we go for combination switches.
STS switching network
TST switching network
To establish a connection between X time slot of an incoming PCM highway and Y time slot of an outgoing PCM highway, it is necessary to select a link having address X free in its speech store and address Y free in its connection store.
Fig: STS Switching Network
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10. T- S -T Switching network:
Fig shows a TST switching network. Each of the m incoming and m outgoing highways is connected to a time switch. To establish a connection between X and Y time slots, it is necessary to choose a time slot Z which is free in the connection store of the incoming highway and the speech store of the outgoing highway. The connection is established by setting the incoming time switch to shift from X to Z switch to shift from Z to Y and appropriate crosspoint at time Z in each frame.
Fig: TST Switching Network
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11. Grade of service of Time Division Switching Networks:
GOS is calculated in two ways
Mode 1: Connection is required to a particular free channel on a selected outgoing highway
Mode2: Connection is required to a particular outgoing highway, but any free channel on it may be used.
STS switching Network
B1 =[1-(1-b)2] k
B2 = [ B1 + c(1-B1)] n
TST switching Network
B1 =[1-(1-b)2] n
B2 = B1
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12. Frame synchronization:
Synchronization is one of the most critical functions performed at the receiver of a synchronous communication system. To some extent, it is the basis of a synchronous communication system.
Carrier synchronization and symbol synchronization needs to estimate the phase of synchronous signal which can be realized by using a PLL.
Frame synchronization is realized in a different way inserting frame alignment signal (distinctive bit sequence). Therefore, the basic task of frame synchronization is how to detect the alignment symbol. Besides add frame alignment bits, some code such as self-synchronizing code can be synchronized without add extra bits. In this section, we only focus on the first method inserting frame alignment signal.
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13. Fig: Frame Alignment of PCM signals
To solve the problem of frame misalignment, the line terminating unit of a PCM junction stores the incoming digits in a frame alignment buffer as shown in figure.
Digits are read into this buffer at rate fa of the incoming line and read out at the rate fb of the exchange clock.
The minimum size of the buffer should be at least equal to one frame.
Fig: Frame Alignment of PCM signals
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14. Synchronization Networks:
To maintain the same clock frequency in all the exchange, a synchronization network is used in all the exchanges.
We have two types of networks
Unilateral
Bilateral
In unilateral network, one exchange acts as master and one exchange acts as a Slave whereas in bilateral there is mutual relationship ,each exchange influences the frequency of the other
And in each type, we further have Single ended and Double ended networks.
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15. Synchronization hierarchy of an integrated digital network:
A synchronizing network for an IDN is shown in fig . The sync links are provided by PCM systems that carry normal traffic between exchanges.
Frequency control is exerted downwards from the national reference standard by unilateral links from each exchange to those in the next lower level.
However bilateral links are used for exchanges in the same level of the hierarchy.
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17. END
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