ulated the multiplexers had to keep the associated clock timing synchroni- through a compander circuit operating on a logarithmic law to sation SO that the demultiplexed recovered data could have a re- reduce the signal to noise level. The Bell D2 system used bit robbing generated clock associated with it. This point will be referred to later for signalling purposes. The standards which has been adopted for as it imposes major restrictions on the design and operation of this type of high order multiplexing are discussed later in this networks and techniques have been developed to solve the problem. Chapter e.g. the CCITT standard G.711 for pulse code modulation. Apart from timing considerations, the synchronisation header in the composite frame has to be included in the overall bandwidth calcu- From the viewpoint of line efficiency between two point to point lation. multiplexers, the advantages and disadvantages of synchronous In asynchronous data the actual speed of transmission can vary, TDM compared to asynchronous TDM are almost identical. The according to the manufacturer's specification, by up to +15%, main exception is that error detection and correction is not an issue therefore devices running overspeed were a particular problem. with a synchronous data, as it will be enveloped in its own protocol Synchronous data has a much tighter specification and may only independent from the multiplexer and therefore will have its own vary by 0.1%, as recommended by CCITT V.28. This is particularly error recovery mechanisms via retransmission. However, from a important for synchronous modems which cannot tolerate any devi- networking viewpoint, the network timing recovery mechanisms ation from the specified speed. and the methods by which the nodes in a network can be syn- The technique of 'robbing' the low speed input channels and chronised together, is a major issue. allocating it to the composite was normally only applied within Figure 40.5 shows a more realistic model of a multiplexer, with these constraints. Tw