e the PDH restricts them to use only space switching, as to some extent they could afford to resist these demands. Now, opposed to the much more efficient Time-Space-Time tech- however, liberalisation and deregulation have introduced signifi- niques that SDH allows. SITE A SITE B SITE C 'MUX MOUNTAIN' OPTICS OPTICS OPTICS OPTICS OPTICS 34 - 140 34 - 140 34 140 34 - 140 34 140 8 - 34 DROP TRAFFIC DROP TRAFFIC THROUGH TRAFFIC Figure 42.1 The difficulty of accessing a passing 2Mbit/s stream using PDH equipment 42/4 The Synchronous Digital Hierarchy (SDH) 2. Lack of performance. The PDH does not currently have any interwork with each other Without an agreed Mid-Fibre Meet internationally standardised ways of monitoring the perform- standard, this interworking becomes rather clumsy, as both ance of traffic channels at 8Mbits and above. Even at 2Mbit/s, ends of an interconnecting line system have to be bought from where there is a mechanism (CRC4), it only works when the the same manufacturer, and will probably need to be controlled 2Mbit/s channel has a G.704 frame structure imposed on it. as a pair, rather than independently by each operator. It is Furthermore, the PDH does not have any agreed management interesting to note that Mid-Fibre Meet was one of the original channels or protocol stacks etc. Although some of the spare bits targets for the SDH standard, when it was first proposed in the in the frame alignment words can be, and already are, used to US as part of the early work on SONET, back in 1984. (SONET, carry management information, the information capacity is Synchronous Optical NETwork, is the North American somewhat limited (e.g.22kbit/s@34Mbit/s). Worse still, when counterpart to SDH. It preceded SDH by some years.) operating at one of the higher order rates, the PDH makes it difficult to retrieve any management information associated with lower order channels, without a full demultiplexing oper- Of the above three problem areas, it is reasonable to suppose that ation. Finally, even at 2Mbit/s, there i