Time Division Multiplexing
Passing multiple communication streams down a single cable, fibre or radio link.
Multiplexer: Many Devices Share One Channel
Four telephones share one cable.
In real life, this technique works for hundreds of phones.
For intelligible audio, 300 to 3000 Hz must be sent down the line.
The switch must visit each phone at least 6000 times per second (Double the highest audio frequency).
This is called BASEBAND SIGNALLING because the original signal is sent down the cable without a carrier signal.
Ethernet, used in most local area networks, uses baseband signalling.
This is somewhat simplified because it's not a good idea to try to multiplex an analogue signal.
The audio from the phone is passed through an ADC, analogue to digital converter, and the digital signal is then multiplexed.
The animation and circuit diagrams show one way links. All this has to be repeated for the other direction to give full duplex two way communication.
Multiplexer: Circuit Diagrams
If S0 is high, the top AND gates are open and D0 is copied to Q0.
If S0 is low, the bottom AND gates are open and D1 is copied to Q1.
The clock signal S, must be synchronised at both ends of the link.
This is an example of a synchronous communication link.
S0 and S1 count repeatedly from 0 to 3. This is address information.
The AND gates are connected as address decoders.
Each AND gate is opened by one of the numbers between 0 and 3.
At the receiver, similar address decoding happens, opening the correct AND gate.
In this way, each channel is copied to the correct output while sharing the connecting cable.
This technique can scale up to hundreds of channels.
The clock signal S0, must be synchronised at both ends of the link.
This is an example of synchronous communication where identical clock signals are used at both ends of the link.
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