d which is synchronous to the there is also no need for a frame alignment operation, because this SDH network or, more precisely, synchronous with the network follows automatically once the phase of the VC12 is known. The External External SDH Mux SDH Mux at site A at site B a) Plesiochronous 2 Mbit/s 2 Mbit/s plesioch STM-1 plesioch signal signal SWITCH SWITCH Frame align and plesiochronous stuff/destuff operations at these points STM-1 b) Byte sync Site A Site B No frame alignment plus easy loading of VC12s Figure 42.20 Comparison of plesiochronous and byte synchronous mappings of 30 channel groups between two PSTN switches The SDH standards 42/17 removal of the G.704 frame alignment process not only leads to a of the connectivity function that distinguishes one type of network reduction in gate count (and hence cost), but also reduces the delay element from another. For example, a crossconnect may have a large associated with this operation. These advantages apply not just to connectivity function, capable of simultaneously routing a large the PSTN example given above, but, in fact, to any network where number of VCs of a given type between a large number of TTFs, visibility and routing of individual 64 kbit/s channels is required, with very few restrictions on this connectivity. This, of course, is hence they could also apply to networks of PBXs or 64 kbit/s cross consistent with the normal role of a crossconnect, as a major traffic connects. routing node in a transport network, where electronically control- lable traffic routing flexibility is its raison d'etre. At the other end 42.4.7 SONET and SDH of the scale come Line System Terminals (LTEs), which normally incorporate a multiplexer as well. The connectivity between tribu- Several mentions have been made of SONET as being both the taries and aggregate ports of such a multiplexer is normally hard forerunner and the North American equivalent of SDH. The SONET wired, hence no flexibility whatsoever. Between these two extremes standards were designed around 