ram for the PCS provider port in Fig. 6.16 shows that the communication events in the diagram must sometimes have a service relation prefix in order for the diagram to be unambiguous. This pre- fix is a kind of object identifier that identifies the other party. The prefix can either be the name or identity of the other object, or a negotiated, temporary reference between the objects. The latter is called service relation identifier (SRid). In Fig. 6.16 the latter is assumed. To use some type of service relation prefix is necessary in this case, because the PCS is supposed to handle many TECs simultaneously. 6.2 A Service Network Modelling Case S1 operations: servREQUEST PCS userServRelORDER TE-SAPI=A inwServRelC_REPORT (service relation identifier) PCS provider-port behaviour: TEC user-port behaviour: SRid-UserServRelOrd SRid-servRef(...) SRid-nwServRelRepCnf) active state EFSM- state (AnotInCall) (AnotInCall) iie: startCall SRid-servRef(...) SRid-servReq(D... SRid-servReq(D,... pre: problems pre: noProblem SRid-servReq(D,...) pending state SRid-servCnf(...) SRid-servCnf(...) SRid-userServRelOrd SRid-nwServRelRepCnf post: AandDinCall EFSM-state interrupt state (AandDinCall) active state iie: relFromOtherParty (AandDinCall) iie: interrupt iie: endCall TNW service states, provider side: TNW service states, user side: AnotInCall AandDinCall noCall pending inCall (with D) Fig. 6.16 A possible service behaviour between a calling TEC and the PCS. The state diagrams make use of the following concepts: service relation identifier, communication event, initiating event, internal initiating event (iie), result event, precondition (pre), postcondition (post), reactive state, resource state, EFSM state, active state, pending state, Moore state, interrupt state The state diagram for the PCS provider port: The structures of these state diagrams are defined by reactive states of the type EFSM state, interconnected by transitions between reactive states: A transition has an originating state and a next state. Next state is defin