refuses to receive the message, he responds with recReject, otherwise the response is srecRequest. Using this type of receive operation might be a way to prevent overflow of mes- sage buffers in the CSP, although the user is still the one who decides if he is prepared to deal with the message. The primary reason for using these naming conventions is to make models more expressive and uniform, when they are annotated with operation names or abstract service primitives. A secondary reason is that tools can be developed that perform logical control of models and give support in code generation from models, based on these naming conventions. The latter subject is discussed in more detail in Sect. 4.5.2. 3 Logical Network Modelling Network Service Interface Scenarios This section is to demonstrates how ASPs can be used to describe behaviour in log- ical networks. We will provide some models that show the logical node structure of terminal systems for circuit-switched and packet-switched services. We will use these models to discuss some scenarios for the sequences of events when these interfaces are used to establish connectivity and teleservices ¹². The models in this section also give an understanding of how to use the symbols when modelling logical networks. 3.3.1 Circuit-Switched Services We will start by discussing how a teleservice that requires a single 64-kbps CN connection is established in ISDN. Figure 3.29 shows the structure of logical nodes in an ISDN terminal system. The physical node (terminal system, Case A) is shown only for orientation purposes. It has otherwise no bearing on this discussion. Terminal System Case A The ISDN-CSP TE-SAP TE-SAP (for speech) Teleservice Access Network (for tes) Protocol Process Control System TE-UDT tes protocol TE-UD protocol (tes) TE-UDT (tes) element TE-UDT (speech) speech protocol (speech) logical {64 kbps} SAPI 0 ATN access transport network TEC orderer of a teleservice TEU user of a teleservice CNU user of circuit-switched connections SAPI SAP identifier TPC termin