 bandwidth being allocated to it. low priority. High priority is dealt with as described above. Low Prone to errors; in the process of being transmitted, any errors priority control information is sent every N frames, where N could generated on the composite circuit are passed to the receiving be from 4 to 64 depending on the frame size and speed of the end without any indication to the end device (in fact the parity composite. recovery method makes this worse as it hides dual bit errors). Channel 1 Channel 1 Virtual Virtual channels channels Direction of data Channel 8 Channel 8 Bit interleaved data Figure 40.3 Ideal model of a TDM Time Division Multiplexers 40/5 Channel 1 Channel 1 Virtual Virtual channels channels Direction of data flow Channel 8 Channel 8 Bit interleaved data Figure 40.4 Line utilisation in a typical TDM Unless the link synchronisation actually fails the remote end 1. To designate one of the input channels as a 'vari-speed' input has no indication that there is a problem on the line. which may be attached to non-critical devices. This approach, for obvious reasons, has to be handled with caution. 40.2.2 Synchronous TDM By the preferred technique used by latter designs to perma- nently allocate a synchronisation overhead. As synchronous protocols evolved the need to multiplex them grew; networks with remote sites that had a mix of asynchronous and A mixture of bit and byte interleaving with a fixed synchronisation synchronous terminals needed to connect them to the central site header was also adopted by some high order multiplexers, where computing resource. multiple voice channels needed to be multiplexed after being digi- For synchronous TDMs, the bit-interleaved method was adopted tally encoded; the US Bell D1 system for example. The basic as the most superior technique. The main difference from asyn- technique employed is for an analogue sample of the voice signal to chronous multiplexing being that as well as keeping data integrity, be converted into an 8 bit code word, after being manip