 properties of channels in routes and switch paths are abstracted in this model, compared to the resource system model. Connection endpoints are grouped into TR-SAPs. The concept connection is introduced as an abstraction of a con- catenation of channels in routes and paths in switches. A connection has (in this model) two connection endpoints. The CNW congestion state is defined as optional, since the TEPC need not know about these states. For example, if the TEPC requests a connection to a CNU that has a blocked access route, the CNPC 6 Process Control System Modelling refuses to set up the connection, provided that the CNPC knows that the access route is blocked. The same thing will happen if the core route has no idle channel or is blocked. The TNW model represents the common knowledge between TECs and the TEPC. TECs do not have to know anything about the connection network. Instead address information (TE-SAPI) is required, as are other service related properties. Figure 6.11 depicts the TNW-model. General notation Intuitive notation and instance This TNW: Teleservice/ o*congState Network acronym TNW properties are: *congState #*callld Call properties are: *SET OF CallParty calls #*callld CallParty{TE-SAPI,callRef, *SET OF CallParty stateInCall{pending, TS properties are: inCall, #*TE-SAPI{A,B,C,D,E,F} suspended}} Teleservice Teleservice *subscription User (TEU) Users (TEU) *capability #*TE-SAPI Terminal *subscription System *capability Fig. 6.11 The service resource model for the teleservice network (TNW) The model introduces the concept call as an abstraction of all connections and teleservice resources that are allocated to communicating users (TEUs). A call may exist between just two users, or between many users (conference calls, etc.). A TEU uses a terminal system. It is also assumed that each terminal system has a sin- gle TE-SAPI. We also could have defined a TNW that distinguishes TEU identities from terminal system identities. However, to avoid unnecessary complications, it is assumed that a termin