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What is switching ? Let's see this example to find the answer.
Say there are 8 people with telephones. If we were to use direct lines between all the people, we would need 28 duplex ( wires that allow simultaneous two-way conversation ) lines. The arithmetic is pretty simple - to connect n subscribers directly, we need n(n-1)/2 lines. This is alright as long as the number of subscribers is less and the distances are also small. We are talking about connecting the entire world - obviously direct connections are not the answer.
----------------- 1---------------| |--------------------- 5 | | 2---------------| |--------------------- 6 | switch | 3---------------| |--------------------- 7 | | 4---------------| |--------------------- 8 -----------------Now, if we were to use a switch instead, we could reduce the number of lines needed to just 4, because with 8 subscribers, there would at the most be just 4 conversations simultaneously. The switch would have 4 lines internally and it would use the each line to connect a pair of subscribers.
A very important point here is to understand the difference between switching and multiplexing. Multiplexing is done to maximise the use of a communications channel. But switching is the manipulation of the ends of the communications channel, put very simply. Let us assume the switch in the above diagram has 4 internal lines A, B, C and D. Say A is being used to connect 1 to 7 and B to connect 4 to 5. Now if 3 were to wish to get connected to 8, the switch would 'patch' the ends of C so that 3 and 8 are connected. Instead, if 6 had lifted the phone BEFORE 3 DID and tried to get connected to 2, the switch would use C to 'patch' a connection between 6 and 2. We assume that the order in which the lines A, B, C and D are used is in accordance with their alphabetical order. This assumption is valid and any other order would not have any bearing upon the concept of switching. The fact remains that the lines A, B, C and D are not fixed. Their end-points change from time to time. Thus they are switched circuits.
------------- ---------- 1 --------------| | | |----------9 . | | | | . . | switch a |---------| switch b | . . | | | | . 8 --------------| | | |---------16 ------------- ----------Consider 2 such 8-subscriber switches. They are seemingly connected by just one line. But this line is a multiplexed line, and is not switched. It is called a trunk. It is not switched because it always carries traffic from switch a switch b. Let us assume the multiplexer is capable of sending 4 simultaneous conversations over one line. Then the trunk could be carrying conversations between 1-15, 4-9, 8-11 and 5-12. The lines internal to switch a would connect 1,4,8 and 5 to the multiplexer/demultiplexer ( remember the line is duplex ) and therefore the trunk. Similarly, the lines internal to switch b would connect 15, 9, 11 and 12 to the multiplexer/ demultiplexer. The switched circuits are inside the switches a and b. But the trunk between a and b is multiplexed with 4 conversations. So in a sense, the trunk is not switched. But if you had more than one trunk between switches a and b, then the trunks would also be switched. Why, because a call from 1 to 15 could go on either trunk 1 or trunk 2 ( assuming there are two trunks each capable of carrying 4 conversations ). Thus, the trunks are now switched, in addition to being multiplexed.
Time switching
Time-Space switching
Stored Program Control
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