 insert multiplexers and equipment (G.784), and the equivalent recommendations for line cross connects. systems (G.958). Optical interfaces for all types of SDH equipment are covered in G.957. Further standards (G.sdxc1/2/3, still in the draft stages) address the functionality of synchronous cross con- The SDH standards nects. Finally there are two more standards (G.snal/2), which ad- dress the way that entire SDH based networks should be constructed There are now a dozen or more CCITT recommendations describing in order that they can interwork successfully with other such net- various aspects of SDH, but the centre-piece of the whole group is works and, even more importantly, so that the management of these undoubtedly CCITT G.707/8/9. These three standards define the networks can be brought under software control. Nevertheless, SDH multiplexing structure and the ways that non SDH traffic because of their central role in SDH, we shall concentrate mainly on channels can be mapped into the SDH Virtual Containers. Other explaining G. 707/8 & 9. VC12 path termination points VC12 path SDH NETWORK Figure 42.8 2Mbit/s plesiochronous circuit from A to D, which is transported by an SDH network for part of its journey The SDH standards 42/9 2Mbit/s MUX 1 Site A VC12#1 Site D 2Mbit/s VC12#2 MUX 2 MUX 4 VC4 containing Site B VC12#1,2 and 3 2Mbit/s MUXES 2, 3 and 4 are well 2Mbit/s VC12#3 synchronised to each other. MUX 1 MUX 3 has temporarily lost network synchronisation and has fallen back Site C to using its own internal source. MUX 4 must now deal with an asynchronous VC12#1 Figure 42.9 Synchronous multiplexing of VC12s into a VC4 when VC12s are created in various places Bearing in mind the nesting of smaller VCs within larger ones, and the receiving path termination detects any discrepancy between the thence into STMs, the best way to appreciate the details of the SDH POH and the content of the VC12 payload (i.e. the C12), this multiplexing standards G707/8/9, is to follow the progress of a indicates that the VC12 paylo