o their with one rather important exception dealt with in Section 42.4, SDH various destinations together with an accurate indication of the currently focuses on the transport of 2.048 Mbit/s, 34Mbit/s and 140 quality of the delivered VC payload. 42/6 The Synchronous Digital Hierarchy (SDH) Synchronous container (C12) = 35 bytes Small amount of 2Mbit/s plesiochronous bitstream capacity within the C12 is unoccupied by the 2Mbit/s plesiochronous Path overhead byte(s) circuit for synchronous container Container + Overheads = Virtual Container (VC12) Figure 42.4 Example of the creation of a VC This process of loading containers and then attaching POHs is 42.3.1 The concept of pointers repeated at several levels in SDH, resulting in the nesting of smaller VCs within larger ones. (See Figure 42.6.) The nesting hierarchy The solution adopted by SDH is to associate a pointer with each VC stops when the largest level of VC is loaded into the payload area of SO that when it is multiplexed, along with others, into a larger VC, a Synchronous Transport Module (STM). These logical STM sig- its phase offset in bytes can be identified relative to some reference nals are seen at the interface between any two pieces of SDH point in this larger VC. (See Figure 42.7.) Furthermore, there is also equipment, where they can be presented either electrically or, more a mechanism for allowing the value of this pointer to change if, for usually, optically. (See Fig 42.2.) Such an interface is referred to as some reason, there is a loss of synchronisation, and the smaller a Network Node Interface (NNI), because it is usually confined to capacity VC is running either slightly slower, or slightly faster than the internal interfaces within the network, rather than any interface the larger VC. In fact, each of the smaller capacity VCs has its own presented to a network user. A User Network Interface (UNI) has pointer, which can change independently of any of the others. also been defined, however, the way in which the payload informa- Although t