atistical TDM (which is more commonly just statistical multiplex- In this method, the incoming data on each channel is sampled bit by ing) allocates the available bandwidth to input channels only on bit and stored in a transitory central buffer before being assembled demand when data is present. on to a composite. This is the simplest technique that can be employed as it only requires that the multiplexer recognises the bit boundaries of the incoming data. Providing that the sequence of data bits is then forwarded to the remote end and demultiplexed without Time Division Multiplexers losing integrity, then any type of data format can be handled. However, as this meant sending all data bits for an asynchronous 40.2.1 Principles character including the stop, start and parity bits, the multiplexing technique adopted quickly moved to byte interleaved. In this Time division multiplexing is the earliest and simplest form of method the ancillary bits could be stripped off and only the data digital multiplexing. It was developed to solve the communications sent. At the receiving end they could be reconstituted. The byte was Total Total = Nbit/s Total = Nbit/s Nbit/s Figure 40.1 Basic model for a multiplexer 40/4 Multiplexers Sync. pattern Nbit/s fixed Frame Figure 40.2 The basic operation of framing 8 bits with data requiring 7 bits and the 8th bit in each byte Figure 40.3 shows the ideal model of time division multiplexing indicating whether data or controls were being sent. Although it operation, where all eight channels are being efficiently multiplexed meant that 5 bit data such as Baudot had 2 bits per byte wasted and onto a composite line. At any one instant in time there is always one that 8 bit codes could not be sent, as the majority of traffic was based of the channels using the available bandwidth. Figure 40.4 shows on 7 bit code (ASCII), the overall gains made it worthwhile. the realistic utilisation of the line. Note that only channels 1, 2 and As stated above, the original design was for handling asyn- 6 ha