bly easily from the definition An SDH based network can be viewed as the transmission bedrock of the objects themselves. However, the existence of such a message which supports all other terrestrial telecommunications services. set, then leads to the requirement for an agreed information protocol (See Figure 42.25.) As already mentioned, the main advantages of stack with which to transport it. For information transferred be- such a network are the ease and precision with which the available tween a network element direct to its element manager, CCITT network bandwidth can be partitioned amongst the higher layer G.773 details several allowed protocol stacks, which split into two services, together with accurate monitoring of the quality of the groups, those which have a full 7 layer structure and those which transmission links. Despite this, the control and management of have a 'short stack', where layers 4, 5, and 6 are absent. Most complete networks of the size operated by BT, France Telecom, or observers favour the use of the heavier, but more flexible, seven DBP is a difficult problem, which requires some degree of stand- layer protocol stacks for SDH networks, while the 'short' stacks are ardisation in the way such networks are functionally decomposed, more appropriate for PDH equipment for which it is more difficult in much the same way as the individual network elements have to justify the burden of the additional layers. already been functionally decomposed into their atomic elements by Not only are there defined protocol stacks for information transfer CCITT G.783. direct from element manager to network element, but there is also a The basic idea behind the functional decomposition described in defined protocol stack for information transfer between individual CCITT G.snal is that a transport network can be stratified into a network elements. In general, the majority of information transfer number of layers. Each layer provides a service to the layer above between network elements is actually information 