for crE. TEPC then offers TECE the call with servinv and enters the pending state Epending1. Activities in transition 2: TECE responds positively with servReq. TEPC changes the stateInCall attribute for crE to inCall. TEPC now has to find the TR-SAPIs for TECA (CNU1) and TECE (CNU5). TEPC can access these identifiers by reading the abstraction relations between terminal systems and CNUs in the object resource model TCPC then reserves a connection identifier conld=conId15 for a connection between A and E. TEPC relates calld=y to conld15 by a temporary abstraction relation dependsOn in the object resource model. TEPC sets the conState attribute for conld15 to pending. TEPC then requests the connection with conReq from CNPC and enters the con- Pending state. Activities in transition 3: CNPC verifies the establishment of the connection between CNU1 and CNU5. Note that the conCnf contains a chanld=B1 for CNU1. This is because that CNPC saw that CNU1 terminates an ISDN access route with two channels. CNPC assigns one of these to the call. No channel was selected for CNU5, since there is only one channel in that access route. TEPC changes the conState attribute for conld15 to connected in the object resource model. TEPC then verifies the call to both TECA and TECE. Note that the selected channel in TECA's access is just relayed in the servCnf to SECA, i.e. it is not stored in the object resource model. Transitions 4, 5 and 6 describe the three ways a call can be released. Transition 4 and 5 are regarded as normal ways. Transition 6 represents a fault situation, where the CNPC detects that it has lost the connection within the call. A number of problems might lead to the fact that the requested call cannot be set For example, if the preconditions for transition 1 are not fulfilled, transition 7 will occur instead. If TECE is not willing to take the call, transition 8 will occur after transition 1. If TECA sends an interrupting event, servIntOrd, before TECE has answered in state Epending1, TEPC enters the pending state E