ee Figure 10.25). This layering within an SDH network could be viewed as a subdivision of ISO layer 1. As an example of a non-SDH client-server relationship in a transport network, consider Figure 10.26, which shows a 64-kbit/s circuit as a client of the 2-Mbit/service layer (i.e., the 2-Mbit/s layer), which can be viewed as a 2-Mbit/s network, transporting the 64-kbit/s circuit between its desired end points. To do this, the 2-Mbit/s layer will probably call on the services of the 8-Mbit/s layer, and SO on up to the 140-Mbit/s layer. The SDH counterpart of this simple PDH example is slightly more complicated in that the transport layers are divided between those concerned with end-to-end networking (i.e., the path layers) and SDH-Based Networks 64-bit/s Data 3.4-Hz 64-bit/s 3.4-kHz Voice Voice Digital Data Network Network 2048-bit/s Transmission 2048-bit/s Network VC-12 Network VC-12 Interlayer Adaptation VC-4 Network PT STM-N STM-N Network Fig. 10.25 Layering and client-server relationships between the layers of an SDH network. 64 kbit/s 64 kbit/s 2-Mbit/s 64-kbit/s/2-Mbit/s Muliplexers Fig. 10.26 A 64-kbit/s circuit making use of a 2-Mbit/s service to get from A to B. those concerned with transport between each pair of SDH network elements along the route (i.e., the STM section layer). As an additional complication, there are two path layers: the lower-order paths consist- ing of VC-1s, VC-2s, or VC-3s, and the higher-order paths, which are VC-4s in ETSI coun- tries, but could also be VC-3s (see Figure 10.27). Usually when a client layer makes use of a server layer, it is necessary to adapt the client signal to a format that can be carried by the server layer (see Figure 10.28). Plesiochronous "bit stuffing" and SDH pointer adjustments are examples of adaptation functions. The adaptation function is only one of the network atomic functions that have been described in G.803, and it is indeed fortunate that G.803 has been developed in full recognition of the contents of G.783, because many of the equipment and n