idering two PSTN switches that are interconnected by several groups of 30 channels that are transported over an SDH network (see Figure 10.19). It is easy for such a PSTN switch to operate as part of the SDH network by generating and byte-synchronously loading its own VC-12s, because it already knows the location of all 30 channels. It obviously does not need to search for frame alignment because this too is already known. At the receiving PSTN switch, because the VC-12s were mapped byte synchronously, there is also no need for a frame alignment operation because this follows automatically when the phase of the VC-12 is known. The removal of the G.704 frame Chapter 10 Synchronous Optical Networks and Synchronous Digital Hierarchy STM Signals within NNI Connection Function Plesiochronous Interfaces Fig. 10.20 Generalized representation of the traffic paths through an SDH network element. alignment process not only leads to a reduction in gate count (and hence cost) but also reduces the delay associated with this operation. These advantages apply not just to the PSTN example previously given, but, in fact, to any network where visibility and routing of individual 64-kbit/s channels is required; hence they could also apply to networks of PBXs or 64-kbit/s cross-connects. SDH Network Elements Because traffic-routing flexibility is one of the reasons for the existence of SDH, the simplest way of considering any SDH network element is as a group of transport termination func- tions (TTFs) surrounding a traffic path connectivity function (see Figure 10.20). The TTFs are essentially the conversion functions from the VC level to either the STM or plesiochronous signal levels, while the connectivity function allows VCs to be routed between the various TTFs. More than anything else, it is the size and flexibility of the con- nectivity function that distinguishes one type of network element from another. For example, a cross-connect might have a large connectivity function, capable of simultaneously routing a large number of