e dumb. By means of H.248 a suitable number of control nodes can control switching nodes in the connectivity network. The control nodes could be arranged in pairs for increased reliability. See Figure 3.48. Alternatively, one switching point can be controlled by more than one server, provided that the networks utilize the same technology in the connectiv- ity layer. The control nodes do not serve all nodes in the connectivity network. The served nodes are edge nodes (towards access) and interworking nodes (towards other networks). Especially in tele-centric networks the nodes are called media gateways. For a call set up the communication control part handles the 'call' phase and the connectivity part handles the 'connect' phase. By such a separation the connectivity becomes more independent of the network type. H.248 defines a connection model, which is a central concept for describing the logical entities within the media gateway that can be controlled by the server. Thanks to this model, different transmission media can coexist, and media streams can be processed in the connection. H.248 allows an authenticated server to establish, move, modify, remove and obtain events that have been reported on a connection or group of connections. A server can audit the media gateway to determine the extent of its capabilities. Understanding Changing Telecommunications - Building a Successful Telecom Business New switching architecture, with some common protocols Access signalling (ISUP, V5.2, DSS1, SIP, H.323 (IP) for PSTN/ISDN and PLMN) Call control (many types) Trunk signalling BICC (for PSTN/ISDN and PLMN) H.248 PNNI (ATM) "Connection" OSPF (IP) Converged LDP (MPLS) Command (PVC) (Leased lines) or IP or combinations such as MPLS Figure 3.47 By means of H.248 call and connect functions can be split. A horizontal inter- face appears Target network: Whole network controlled by a few server pairs Connectivity network plane Figure 3.48 Physical separation of call and connect nodes Communication control can comprise both peer-