scrib. Switching B-channel resource data man. machine manager Many LAPD Single LAPD layer interfaces layer interface Fig. 2.22 An ESM sees resources as abstract machines By separating an ESM from its "containment" in an actor, as depicted in Fig. 2.23, we actually define a layer in two separate models: an LSM and a layer proto- col machine (LPM), related through containment relations that we call ESM_in. DSS1 L3 Terminal user abstract Call-handling protocol Call-handling ESM in LE ESM in TE ESM_in Subscrib. Switching B-channel Terminal ESM_in data man. machine manager resource Call-handl. Call-handl. actor in LE actor in LE DSS1 L3 message protocol DSSI L3 agent agent Many LAPD Single LAPD layer interfaces layer interface Fig. 2.23 Layer state machines (LSM, an abstract machine system) are contained in layer protocol machines (LPM). SH=state-machine handler The DSS1 L3 LPM is a machine that supports distribution of the DSS1 L3 LSM in the network. The LPM can be designed SO that it becomes independent of what the LSM is about, i.e., all functions that are unique to the DSS1 L3 layer are then defined by the LSM. 2 Layer Structures Let's therefore analyse how the DSS1 L3 protocol can be described on the (much simpler) LSM level, where interactions are in terms of events instead of message exchange. In Fig. 2.24 we compare the message exchange between DSS1 L3 agents with the events between DSS1 L3 ESMs for the most common messages that are used in the setup procedure for an originating call. Events in LSM: Terminal user callCnf(...) callReq(...) connectReq(...) Call-handling connectRef(...) Call-handling ESM in LE callStateRep(...) ESM in TE connectCnf(...) LE resources TE resources Message exchange in LPM: SETUP RELEASECOMPLETE DSS1L3 DSS1L3 CALLPROCEED agent agent in LE in TE SETUPACK Many LAPD Single LAPD layer interfaces Tayer interface Fig. 2.24 The translation between LSM-ASPs and messages in DSS1 L3 These messages are: SETUP from the TE that requests a call. A lot of message parameters are included, e.g., the 