arising from an it, and is, in turn, a client of the layer below it, in much the same element manager, which is being relayed by an intermediate net- way as the ISO seven layer information transfer model consists of work element to a more remote element. (See Figure 42.24). This layers which participate in client-server relationships with their flow of management information amongst network elements, ele- vertical nearest neighbours. (See Figure 42.26.) This layering within 42/20 The Synchronous Digital Hierarchy (SDH) Multiple Service Networks, which require a high performance transport layer N-ISDN B-ISDN WBLLN Transport Network : Multiplexing, Transmission, Grooming, Consolidation, Routing, Capacity Allocation, Network Management Figure 42.25 Managed transmission network; SDH network as a bearer for other services an SDH network could be viewed as a subdivision of ISO layer 1. example is slightly more complicated in that the transport layers are As an example of a non-SDH client-server relationship in a trans- divided between those concerned with end to end networking (i.e. port network, consider Figure 42.27, which shows a 64kbit/s circuit the Path Layers) and those concerned with transport between each being a client of the 2Mbit/service layer i.e. the 2Mbit/s layer, which pair of SDH network elements along the route (i.e. the STM section can be viewed as a 2Mbit/s network, transports the 64kbit/s circuit layer). As an additional complication, there are two path layers, the between its desired end points. In order to do this, the 2Mbit/s layer lower order paths consisting of VC1s, VC2s or VC3s, and the higher will probably call upon the services of the 8Mbit/s layer, and SO on order paths, which are VC4s in CEPT countries, but could also be up to the 140Mbit/s layer. The SDH counterpart of this simple PDH VC3s. (See Figure 42.28.) 64 bit/s data 3.4 Hz 64 bit/s voice 3.4kHz voice digital data network network 2048 bit/s 2048 bit/s transmission network VC12 network Inter layer adaptation VC4 network PT STM-N ST