amount importance to preserve a smooth interworking with tainer, this Container has a defined set of POH bytes appended to it, the existing PDH networks, most of which were designed to serve which remain completely unchanged until the Synchronous Con- the PSTN above all else. At the same time, it was also apparent that tainer arrives at its destination. The combination of Synchronous the increasingly widespread deployment of optical fibres would Container plus POH is known as a Virtual Container (VC). The VC rapidly reduce the cost of raw transmission bandwidth, hence there POH bytes allow a PTO to monitor several parameters, the most was relatively little pressure to restrict the proportion of bandwidth important of which is the error rate of the VC between the points at devoted to multiplexing overheads, as opposed to the traffic pay- which it was loaded and unloaded with its plesiochronous payload. load. Beyond the transport of isochronous traffic channels, and This provides a PTO with the much sought after end to end perform- smooth interworking with the existing PDH, the developers of SDH ance monitoring that was SO difficult using conventional PDH also addressed those weaknesses of the PDH that were identified techniques. above. They recognised that it was necessary to adopt not only a Most plesiochronous channels are bi-directional, hence there are synchronous frame structure, but one which also preserved the byte usually two continuous streams of VCs travelling in opposite direc- boundaries in the various traffic bit-streams. The basic SDH frame tions between the two end points at which the plesiochronous structure is one that repeats at intervals of 125us i.e. it is tailor made channel enters and leaves the SDH portion of a network. Viewed in for the transport of 64kbit/s channels, or any higher rate channels this way, the job of an SDH based network is to load its VCs with which are an integer multiple of 64kbit/s. (See Figure 42.3.) In fact, (usually) convention PDH channels, and then transport these t