ithout incurring large penalties in transmission delay. It could be argued that SDH networks are not really synchronous at all but are actually very tightly controlled asynchronous networks. However, the fact that we have quantized the slips due to this asynchronism means that it is now possible, at any time, to locate and route any of the traffic paths within an SDH network. This, together with network management software, gives us the traffic-routing flexibility that was very difficult to achieve using PDH-based equipment. In terms of actual network hardware, it opens the way to the production of economically viable drop-and- insert multiplexers and cross-connects. SDH Standards There are now many ITU-T recommendations describing various aspects of SDH, but the centerpiece of the whole group is undoubtedly ITU-T G.707. This standard defines the basic SDH multiplexing structure and the ways that non-SDH traffic channels can be mapped into the SDH virtual containers. Other standards in the group deal with such things as the functionality of multiplexers (G.783), the management requirements of such equipment (G.784), and the specific recommendations for line systems (G.958). Optical interfaces for all types of SDH equipment are covered in G.957. Finally, there are two more standards, G.803 and G.805, which address the way that entire SDH-based networks should be constructed SO they can interwork successfully with other such networks and, even more important, SO that the management of these networks can be brought under software control. Neverthe- less, because of its central role in SDH, we will concentrate mainly on explaining G.707. Bearing in mind the nesting of smaller VCs within larger ones, and thence into STMs, the best way to appreciate the details of the SDH multiplexing standard, G.707, is to follow the progress of a bidirectional 2-Mbit/s plesiochronous circuit, which, for part of its journey, is transported across an SDH-based network (see Figure 10.7). Such a 2-Mbit/s circuit could be a channel betwee