Time-Division Multiplexing
A single transatlantic fibre-optic cable can carry millions of simultaneous phone calls and internet connections down one physical strand of glass. It manages this not by being infinitely fast for one signal, but by cleverly sharing its capacity between many signals at once — a technique called multiplexing.
Why share a channel at all?
Transmission media aren't free. Laying long-distance optic fibre, launching a satellite, or building a network of ground stations for a radio link all represent significant cost, as the previous lesson discussed. It would be hopelessly wasteful to build an entirely separate physical link for every individual signal that needs to travel between two locations — one dedicated cable per phone call, say. Multiplexing is the general term for techniques that allow several separate signals (channels) to share a single transmission path, getting far more value out of one expensive physical link.
Several multiplexing techniques exist, but the specification focuses on one in particular: time-division multiplexing (TDM).
The basic principle of TDM
In time-division multiplexing, each of several input channels is allocated its own short, repeating time slot on the shared transmission path, rather than being allowed to transmit continuously. A device called a multiplexer rapidly switches between the input channels in turn: it sends a brief sample or burst of data from channel 1, then switches to channel 2, then channel 3, and so on through however many channels are sharing the link, before cycling back around to channel 1 again. The result is a single stream on the shared path in which samples from all the different channels are interleaved in time, rather than all channels transmitting simultaneously and continuously.
At the receiving end, a demultiplexer reverses this process. Crucially, it must be synchronised with the same timing and cycle as the transmitting multiplexer — it needs to know exactly which time slot belongs to which channel in order to sort the single interleaved stream back out into the original separate channels. Without this synchronisation, the demultiplexer would have no way to tell which sample belonged to which original signal, and the whole scheme would fail. This synchronisation requirement is one of the genuine practical complexities of building a working TDM system.
TDM works particularly naturally with digital signals. As covered in the analogue-to-digital conversion content earlier in this module, a digitised signal is already broken up into a sequence of discrete samples, each represented as a short burst of binary data. Since these samples are already discrete in time, they can readily be interleaved with samples from other digitised channels without any loss of the underlying information — this is exactly why digital communication systems make such heavy practical use of time-division multiplexing, from telephone networks to internet backbone links.
Key takeaways
- Multiplexing allows several separate signals (channels) to share a single transmission path, avoiding the cost of a dedicated link for every signal.
- In TDM, each channel is allocated its own short, repeating time slot, and a multiplexer switches rapidly between channels, interleaving their data in time.
- A demultiplexer at the receiving end sorts the interleaved stream back into its original separate channels.
- Synchronisation between the multiplexer and demultiplexer is essential — without it, the receiving end cannot correctly identify which sample belongs to which channel.
- The data rate available to each individual channel is the total shared capacity divided by the number of channels sharing it.
- The time slot duration is the total cycle time divided by the number of channels.
- TDM suits digital signals particularly well, since digitised data is already a sequence of discrete samples that can be readily interleaved with other channels' samples.