etwork atomic functions are identical. Chapter 10 Synchronous Optical Networks and Synchronous Digital Hierarchy Isochronous Plesiochronous Plesiochronous 64 kbit/s Primary 34- and 45-M Broadband Layers Layers Layers Services (1.5 and 2M) VC-1 sync VC-1 async VC-3 async VC-2.nc 140-Mbit/s Lower-Order Path Layer Network 149.92-Mbit/s Layer Cellstream Adaptation: VC-1, VC-2, and VC-3 into VC-4 VC-4 async VC-4 byte async Higher-Order Path Layer Adaptation: VC-4 into STM-1 STM Section Layer Adaptation: Optical Interface Parameters Optical Section Layer Fig. 10.27 Layering in an SDH transport network. Link Connection in Client Layer Client Layer Server Layer Adaptation Function (i.e., server layer adapts client-layer information to server-layer format) Path Terminations (i.e., server layer adds overheads to adapted information) Fig. 10.28 Handling of a link connection in a client layer by a path in a server layer. Path in Server Layer G.805 not only describes a series of vertical client-server relationships for an SDH transport network; it also gives a structure to each of the individual layers. Each layer can be viewed as a network in its own right, which can be partitioned into a series of subnet- works (see Figure 10.29). These subnetworks are connected together by "link connections" and can, if necessary, be further subdivided into yet smaller subnetworks. The logical place to stop this process is where a whole subnetwork is completely contained within one network element. As an example, consider a VC-12 cross-connect. The switching matrix (connectivity function) within this element can be considered as a subnetwork, which is capable of making VC-12-layer subnetwork connections from one port to another (see Figure 10.30). SDH-Based Networks Link Connections in Client Layer Path in Client Layer Client Layer Server Layer Adaptation Server Layer Network Adaptation Subnetworks Termination Termination Link Connections Subnetwork Connections Path in Server Layer Fig. 10.29 Partitioning of a transport-layer network into 