ntroducing multiplexers in the switched transport network, the network can also make use of DCCs that switch higher bitrates, up to 140 Mbps. Figure 3.51 shows the DCC symbols. The digital cross connect relay-symbol Type: Instance: DCC Switch DCC Switch Controller Controller 2 Mbps M Number of transmission links C Bitrate per link (>2 Mbps) Fig. 3.51 The cross-connect switch symbol 3.4 Modelling Telecom Resource Systems By introducing DCCs in transport networks, one not only lowers transmission costs, but also creates more flexible transport networks, since transport capacity can easily be adapted to user's needs by DCC switching. The transport links are no longer permanent, but semipermanent. For example, a pair of switches (TRUs) in a network that provides teleservices (as the ISDN network) may at first cope quite well with a single 2-Mbps transport link, but may later need additional transport capacity to meet increasing traffic needs in the teleservice network. Note, however, that the DCC switch controller is part of the transport network and is not in the hands of the teleservice network. A physical network mapped on a switched transport network typically looks as shown in Fig. 3.52, i.e. the network service interfaces between transport networks and their users are normally also network interfaces. Transport protocol (frames) Switch TR-TS TA-TS Controller TA-TS TR-TS 2 Mbps TA-TS TA-TS TR-TS TR-TS TR-TS 2 Mbps TR-TS Fig. 3.52 Mapping physical nodes on a switched transport network Figure 3.39 showed the logical network and node structure of the ISDN network for circuit-switched services. This network is a teleservice network, defined through its TE-SAPs. Other types of teleservice networks are PSTN and GSM. The major differences between transport and teleservice networks are: Transport networks provide connectivity services from 2 Mbps and up. The service is a pure bitstream connectivity service. A teleservice provides connec- tivity for a specific type of information. For example, voice communication 3 Logic