M-N network Figure 42.26 Layering and client server relationships between the layers of an SDH network SDH based networks 42/21 64 kbit/s 64 kbit/s 2 Mbit/s 64 kbit/s / 2 Mbit/s multiplexers Figure 42.27 64kbit/s circuit making use of a 2Mbit/s service in order to get from A to B Usually when a client layer makes use of a server, it is necessary making VC12 layer subnetwork connections from one port to an- to adapt the client signal to a form suitable for transport by the server other. (See Figure 42.31.) layer. (See Figure 42.29.) Examples of this 'adaptation' function are To connect one of these VC12s to another cross connect, a link the plesiochronous 'stuffing' which occurs when a 2Mbit/s channel connection is now required, which will need to make use of the VC4 is multiplexed into an 8Mbit/s one, or the progressive change in a and STM layers. This concatenation of subnetwork and link connec- VC12 pointer value to accommodate a small frequency mismatch tions across the VC12 path layer network begins and ends at the with the VC4 into which it is being loaded. The adaptation function points where the VC12 is created and destroyed, namely the path is only one of the network atomic functions that have been described termination points. It is at these points that the VC12 POH is added in G.sna1, and it is indeed fortunate that G.snal has been developed or removed, exposing only the C12 Synchronous Container. This in full recognition of the contents of G.782/783 because many of the equipment and network atomic functions are identical. same path termination function is recognised in both G.783 and G.snal and it is located immediately next to the adaptation function G.snal, not only describes a series of vertical client-server rela- described above. On the assumption that the C12 in question is tionships for an SDH transport network, it also gives a structure to each of the individual layers. Each layer can be viewed as a network carrying a 2Mbit/s circuit, then the combination of adaptation func- in its own right, w