al process controller SAP service access point TE teleservice UDT user-data termination Fig. 3.29 The logical node structure of an ISDN terminal system that provides services based on 64-kbps circuit-switched connections 12 Note, however, that the correct way to model behaviour in AML is to consider units as state machines and use either the graphical notation (state diagram) or the textual notation (event specification), as defined by AML, see Sect. 7.5. 3.3 Network Service Interface Scenarios A short explanation of the model in Fig. 3.29 follows: The ATN user (CNU), the Terminal Process Controller (TE-TPC) and the tele- service controller (TEC) have a permanent existence in the model (from the CSPs point of view). Teleservice users (TEU) and teleservice protocol handlers (TE--UDT) have a temporary existence in the model. TE-UDTs use 64-kbps connections that are established over the ATN. There can exist several TE-UDTs in parallel. The ATN-SAP for 64-kbps connection end- points is therefore terminated in a logical node that is the connection user (CNU). Conceptually, the CNU handles instantiation of TE-UDTs on request from the TE-TPC. Before the TE-SAP can be used, a TEU first requests a teleservice from the The TEC then asks the TE-TPC to establish the service (i.e. the TE-SAP). The TE-TPC then asks the operator network (over the teleservice access proto- col) to use one of the B-channels of the ATN to establish the requested teleserv- ice between this terminal system and another terminal system. When that is done, the TE-TPC first orders the CNU to connect the actual TE- UDT to the selected B-channel. The TE-TPC then verifies the establishment of the service to the TEC, and the TE-UDT and the TEU become connected (logically over the TE-SAP) by local means that are not depicted in the model. Figure 3.30 is a scenario that describes the interactions between elements in this model for establishing a teleservice called tes. Events and messages in the scenario: serviceReq(servid destSAPI) 2 serviceCsReq(servid dest