cient form of communications, so the design of multi- secondly, allocate all the available bandwidth to each channel for a plexers was adapted for handling this type of traffic. It was in this fixed discrete time period. The first of these methods would be period that many of the design techniques to overcome specific Frequency Division Multiplexing (FDM) and the latter Time Divi- engineering and networking problems evolved. sion Multiplexing (TDM). FDM is used primarily as an analogue In common with all multiplexing techniques the primary require- solution to multiplexing and, for example, has been used exten- ment was for the two multiplexers to have some form of synchroni- sively in telephony; indeed many of the FDM standards and tech- sation procedure whereby they could calculate the location of each niques such as the multiplexing ratios dictated by the early designs channel. This was done by defining a fixed length composite frame of telephone exchange multiplexers (such as 24:1) are still in evi- format, as in Figure 40.2, that had a unique synchronisation code at dence in some of the latter digital exchanges. the start. When the link was first started the multiplexers would This chapter is concerned however with the use and applications enter into a 'training' period, with only empty frames plus the of latter type of multiplexing technique, Time Division Multiplex- synchronisation 'header' code being sent to each other to determine ing which can be used in one of two modes, deterministic or the frame boundaries. The composite overhead or bandwidth non-deterministic Deterministic TDM (which is the more com- needed could be taken by reducing the available aggregate from the monly known just as TDM) allocates the available transmission input channels. channel bandwidth of a fixed regular basis to the input channels, Starting with the first commercially available asynchronous whether they have data to send or not. Non-deterministic TDM or multiplexers the initial multiplexing technique was bit interleaved. st