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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

Two Channel

• 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.

Four Channel

• 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.