a series of subnetworks. 2 Mbit/s 2 Mbit/s 2-Mbit/s Layer VC-12 Layer Subnetwork Subnetwork Subnetwork Connections in VC-12 DXC in VC-12 DXC Link Connection VC-12 Path VC-12 Path Termination Termination Path in VC-12 Layer VC-4 Layer STM Layer Path in STM Layer, Known as a "Section" Fig. 10.30 Use of two VC-12 cross-connects to produce a path within the VC-12 layer. Chapter 10 Synchronous Optical Networks and Synchronous Digital Hierarchy To connect one of these VC-12s to another cross-connect, a link connection is now required, which will need to make use of the VC-4 and STM layers. This concatenation of subnetwork and link connections across the VC-12 path layer network begins and ends at the points where the VC-12 is created and destroyed-namely, the path termination points. It is at these points that the VC-12 POH is added or removed, exposing only the C12 syn- chronous container. This same path termination function is recognized in both G.783 and G.803, and it is located immediately next to the adaptation function previously described. On the assumption that the C12 in question is carrying a 2-Mbit/s circuit, the combination of adaptation functions, termination functions, and the chain of subnetwork and link con- nections in the VC-12 layer has succeeded in transporting this 2-Mbit/s circuit between the two end points of a single 2-Mbit/s link connection (see Figure 10.30). From this point, the preceding analysis can now be repeated in the 2-Mbit/s layer, where the particular 2-Mbit/s circuit previously cited will probably be found serving as part of a link connection for a number of 64-kbit/s circuits (see Figure 10.26). This formal description of an SDH network opens up the prospect of real control and management of large networks consisting of network elements from several manufacturers. The pairing of path termination functions on opposite sides of a network, together with a similar operation on the same functions in the STM layer, allows a PTO to accurately monitor the service that the SDH network is de