al system and its TEUs have one and the same identity-a TE- SAPI. The properties of a TNW are quite different from a CNW: The congestion state property (congState) is not the same as for the CNW. CNW might be congested because no idle channels are available in routes, or switch paths in switches. The TNW might be congested because a physical node is overloaded with service requests. The SET OF callParty property consists of an attribute triplet for each party in a call. Each party is identified with its TE-SAPI, a temporary call reference, call- Ref, and its state in the call. A party is either pending the call setup, inCall or temporarily suspended from the call. A suspended party can be inCall in another call. 6.2 A Service Network Modelling Case The subscription property refers primarily to the particular teleservices a user is allowed to use. This includes different features and grades of barring, e.g. com- pletely barred from service access (for instance, when a bill is unpaid). The capability property identifies the teleservice resources that are available in terminal systems. The TEPC will use this information to verify that two users can communicate. For example, the calling user may be able to handle 64-kbps synchronous data service. TEPC sees, however, that the other user can only cope with 9600-kbps asynchronous data (probably because he has an analogue access). If the users accept it, TEPC can use this information to insert suitable protocol conversion elements (TRs) in the connection. TR types that may be used are rate adaptors, analogue to digital conversion elements, adaptors between different fax-standards and echo cancellers. Our next task is to decide how the knowledge defined by these models is to be distributed over the service network. Both parties in a service relation must know about their mutual service resource model, but, as has already been indicated, both parties do not need to have complete knowledge about the model. The model can be used by the parties for two things: To control the 