he use of these pointers still entails some input buffers, tion in a UNI is mapped into a standard NNI signal is not yet these are very much smaller than would be required if there were no completely defined, and hence it is not widely used. Finally, the mechanism for changing the phase of a small capacity VC within a reason for the name Virtual Container is that unlike the STM signals larger one, hence the problem of excessive delays can be contained. that appear at the NNI, VC signals are never presented to the outside We now have a picture of an SDH network as one in which the world. They exist only within pieces of SDH equipment or within majority of VCs, both large and small, are well synchronised to each STM signals, hence an SDH network element can have NNI and other, but, at the same time, there are a few which are not SO well PDH interfaces, but never VC interfaces. synchronised, and every SO often, the increasing strain of their Returning to the question of exactly what is synchronised to what, asynchronism has to be relieved by a byte sized slip relative to the the basic problem is that, as outlined in Section 42.2, it is very majority of the other VCs in the network. Nevertheless, we retain difficult to maintain a complete transmission network in rigid syn- our ability to locate the management and control information in the chronisation for all time. Even if we could tolerate the delays VC's POH bytes, because the pointer value associated with VC is introduced by the addition of the numerous 'wander buffers' necess- recalculated whenever a slip occurs. ary to accommodate the slow changes in transmission medium The pointer mechanism described above is at the very heart of the delay, there is no guarantee that different PTOs' networks would all SDH standard. It is this mechanism that enables us to construct be synchronised to the same master clock. For a network based on networks that are nearly, but not completely synchronous, and yet the SDH, this problem translates to that of how to synchronously sti