ll allows us to easily locate each traffic channel (VC), together multiplex and demultiplex many individual VCs, which, because with its associated management and control information i.e. POH, they have been created in disparate parts of the same, or even but without incurring large penalties in transmission delay. It could different SDH networks, may have slightly different short term bit be argued that SDH networks are not really synchronous at all, but rates. are actually very tightly controlled asynchronous networks. How- The basis of SDH 42/7 Master OSC. Master oscillator is independent and asynchronous with rest of network 2Mbit/s Stuff Plesiochronous Operation High rate bit stream Timing generator Timing generator is synchronised to rest of network Synchronisation feed from network 2Mbit/s Stuff containers High rate Plesiochronous Operation etc and bit stream Figure 42.5 Difference in synchronisation between PDH and SDH: (a) PDH; (b) SDH ever, the fact that we have quantised the slips due to this asynchron- any of the traffic paths within an SDH network. This, together with ism means that it is now possible, at any time, to locate and route network management software, gives us the traffic routing flexi- Level 1 Payload pointer e.g. VC12 Level 2 Payload pointer e.g. VC4 Level 3 Payload e.g. STM-1 Figure 42.6 General representation of a 3 layer synchronous multiplexer structure 42/8 The Synchronous Digital Hierarchy (SDH) LARGE VC PATH OVERHEAD PAYLOAD PATH OVERHEAD SMALL SMALL SMALL SMALL SMALL SMALL SMALL VC#1. Because of its phase relative to the PAYLOAD LARGE VC POH, it is split between one LARGE VC and the following one. Figure 42.7 Pointers allow small VC to have arbitrary phase with respect to large VC bility that was very difficult to achieve using PDH based equipment. standards in the group deal with such things as the functionality of In terms of actual network hardware, it opens the way to the multiplexers (G.781/2/3), the management requirements of such production of economically viable drop and