Multiplexing

Time division multiplexing

Time division multiplexing is a technique used to carry several signals from different transmission channels on another single transmission channel.

It is primarily used with digital signals.

A switching circuit is used to select the signal from each transmission channel in turn, and these are pushed down the single channel at a much higher transfer rate.

Each of the incoming single transmission channels feeds the data into a store, with a gated output - the switching circuit opens each gate in turn at the higher transfer rate, and the data is read out and sent to the output transmission channel.

The output transmission channel can be thought of as containing time slots, with each time slot containing data from one of the input transmission channels.

Time division multiplexing is used in the formation of the bit streams which carry several telephone channels over the trunk networks.

In Europe, 30 PCM channels, each running at 64 Kb/s, are time division multiplexed up to a data stream running at 2.048 Mb/s.

Several of these 2.048 Mb/s data streams are again time division multiplexed up to a higher level data stream running at 155.520 Mb/s. This hierarchy of different levels of bit stream data rates is called SDH.

Japan and North America have different hierarchy structures.

Frequency division multiplexing

Frequency division multiplexing is a technique used to carry several signals on the one transmission channel.

It is normally applied to analogue signals, but can also be used for digital signals, especially higher rate data streams which are the result of time share multiplexing.

Each signal is used to modulate a different frequency of carrier, and then all the modulated carriers are summed together, and sent along the transmission channel.

This is the multiplexing process.

At the receiver, narrow band pass filters are used to seperate out all the seperate carriers, each carrier goes to a demodulator, and the original signals are recovered.

This is the demultiplexing part of the process.

The early telephone trunking system in the UK was based on frequency division multiplexing.

In this system, 12 telephone channels were each used to modulate a different carrier in the range 60 to 108 Khz - the modulation used was single side band. The modulated carriers were then combined into one composite signal.This formed what was called a basic group.

Five of these basic groups were then used to modulate another set of carriers from 420 to 612 Khz, again with single sideband modulation, and the carriers were combined to form a supergroup. A supergroup therefore contained 5 x 12 channels.

Similarily, 15 supergroups were combined to form a hypergroup, which carried 900 channels.

These hypergroups were sent over a single long distance wide band transmission channel.

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