pending2 (transition 9). Here 6 Process Control System Modelling TEPC waits for TECE's response, which can be either servRej (transition 10) or servReq (transition 11). If the latter, TEPC must respond with a servRef. The CNW might be congested, in which case transition 12 will occur after tran- sition 2. Congestion can be caused by congestion in any of the routes and switches that are needed to set up the connection. The object resource model in CNPC defines the exact reason for congestion, but TEPC does not have to know the details. The resource states of the behaviour, see Fig. 6.26, can be described as instance pictures of the object resource model (according to Fig. 6.21). The four most important object resource states are depicted here. AnotInCall {TE-SAPI A} {TE-SAPI E} Entities: TE-SAPI,TR-OSAPI {TR-SAPI CNU1} Identifier values: A, E, CNU1, CNU2 Relations: has {TR-SAP CNU5} Epending1 *{statelnCall inCall} {callId y} *{statelnCall pending} Add entity: calld Identifier value: y Add relations: has Add property: statelnCall {CNU1} Property values: inCall, pending {CNU5} conPending *{inCall} {inCall} dependsOn Add entity: conld Identifier value: conld15 {CNU1) conTo Add relations: dependsOn, connectsTo {conld conid15} Add property: conState Property value: pending {CNU5} hasProp Change property: stateInCall to inCall for E *{conState pending} AandEinCall *{inCall} *{incall} {CNU1} {conld15} {CNU5} Change property: conState to connected for conld15 *{connected}< Fig. 6.26 Object resource states, specified as instance pictures of the object resource model 6.2 A Service Network Modelling Case 6.2.7 Centralized and Decentralized Control So far we have designed our service network as a centralized control system, see Fig. 6.27. There are only two objects that have control responsibility on the net- work level: the CNPC for the connections and the TEPC for the calls (i.e. for tele- service connectivity). controllers AR and AccRoute CoreRoute Controller Manager Manager Controller Fig. 6.27 A centralized control syste