al Network Modelling between two teleservice users requires that the network uses PCM-encoders, which are resources that belong to the teleservice network. Special teleservice resources are needed for each type of teleservice. Users of transport networks are interconnected over TR-SAPs. A TR-SAP comprises many channels that are all available simultaneously as soon as the transport link exists. The capacity of transport links (2 Mbps and up) are adapted for interconnecting switches in teleservices networks, not for providing teleserv- ices. Teleservices that are provided in TR-SAPs are based on transport SAPs with capacities that correspond to the capacity of a channel. Teleservice terminal systems (TE-TS) may have access to multiple CN connections, but most tele- services (as, e.g. voice communication services) are based on using one CN con- nection at a time. Users of transport SAPs cannot order transport links over their network service interface. There is no CSI in this interface. If a TRU needs more capacity or wants to reroute a transport link to some other TRU, the TRU must contact the transport network operator by other means. A teleservice user (TEU), on the other hand, can order teleservices on demand over a CSI that exists within the terminal system in which the TEU exists. The transport operator sets up transport links between units in teleservice net- works (normally between circuit switches that belong to the teleservice net- work). A transport link comprises many channels. Establishing, releasing and rerouting transport links always comprises the whole link, not individual chan- nels. Establishing and releasing the use of circuit switched teleservices is based on establishing, releasing and rerouting CN connections, which is the responsi- bility of a teleservice network. A CN connection is basically the same concept as the OSI connection, though produced by circuit switching, see Fig. 3.53. A CS switch CS switch TE-TS controller TE-TS TE-UDT TE-UDT) TE-TS TE-TS TE-UDT Teleservice protocol TE-UDT CN 