cally network elements from several manufacturers. The pairing of path controllable connectivity function. termination functions on opposite sides of a network, together with a similar operation on the same functions in the STM layer, allows 42.6.1 SDH network topologies a PTO to accurately monitor the service which the SDH network is delivering, and to pin-point those network elements responsible for Once again traffic routing flexibility, its control and physical dis- any poor performance. Beyond that, the SDH network model tribution within an SDH network, is one of the most important Link connections in client layer Path in the client layer Client layer Server layer Adaptation Adaptation Server layer network Subnetworks termination termination Link connections Subnetwork connections Path in the server layer Figure 42.30 Partitioning of a transport layer network into a series of subnetworks SDH based networks 42/23 2 Mbit/s 2 Mbit/s 2 Mbit/s layer VC12 LAYER Subnetwork Subnetwork connections Subnetwork in VC12 DXC in VC12 DXC Link connection VC12 path VC12 path termination termination Path in VC12 layer VC4 LAYER STM LAYER Path in STM layer - known as a 'section' Figure 42.31 Use of two VC12 cross connects to produce a path within the VC12 layer influences on the topologies proposed for the deployment of SDH limitation on the re-routeing possibilities for any traffic affected by equipment. The most obvious manifestations of this are the drop and a cable break. Anything more complicated than a simple clock- insert ring topologies that are finding favour in the former junction wise/counter-clockwise routing decision requires up to date knowl- areas of PTO networks. (See Figure 42.32.) edge of a rather more extensive portion of an SDH network than just The idea behind a drop and insert ring is that the ring structure can a simple ring. The nodes of such a ring are populated by drop and give a high degree of protection against cable cuts etc. due to its insert multiplexers which have a restricted traffic routing fl