container has a defined set of POH bytes appended to it, which remain completely unchanged until the synchronous container arrives at its destination. The combination of synchronous container plus POH is known as a VC. The VC POH bytes allow a PTO to The Basis of SONET and SDH Master OSC Independent and Asynchronous with Rest of Network 2-Mbit/s Plesiochronous Stuff Operation High-Rate Bitstream Timing Generator Synchronized to Rest of Network Synchronization Feed from Network 2-Mbit/s Plesiochronous Stuff Containers High-Rate Operation etc. and Bitstream Fig. 10.3 Difference in synchronization between PDH and SDH: (a) PDH; (b) SDH. monitor several parameters, the most important of which is the error rate of the VC between the points at which it was loaded and unloaded with its plesiochronous payload. This pro- vides a PTO with a capability to monitor end-to-end performance. Most plesiochronous channels are bidirectional, hence there are usually two continuous streams of VCs traveling in opposite directions between the two endpoints at which the plesiochronous channel enters and leaves the SDH portion of a network. As a result, the job of an SDH-based network can be considered to be to loading its VCs with (usually) conven- tional PDH channels and then transporting these to their various destinations together with an accurate indication of the quality of the delivered VC payload. This process of loading containers and then attaching POHs is repeated at several levels in SDH, resulting in the nesting of smaller VCs within larger ones (see Figure 10.4). The nesting hierarchy stops when a VC of the largest defined level is loaded into the payload area of a Synchronous Transport Module (STM). These logical STM signals are what is seen at the interface between any two pieces of SDH equipment, and they can be presented either electrically or, more usually, optically (see Figure 10.5). Such an interface is referred to as a Network Node Interface (NNI) because it is usually confined to the internal interfaces within the 