The application initiates a circuit switched connection with continuity check required by invoking ISUPConnectReq with the Nature of connection indicators IE field contChkInd set to CONTCHK_REQ. This results in the generation of an ISUP initial address message (IAM) to the far exchange.
When the initial address message has been sent, the ISUP layer issues a status indication (EVTSITSTAIND) with event type CONTCHK to the application. This is an indication to the application to perform the continuity check on the circuit.
The application then reports the results of the continuity check by calling ISUPStatusReq with event type of CONTREP. Assuming that the continuity test is successful, call processing resumes normally. If the continuity test fails, the application can request a recheck of the circuit as described in Continuity recheck. Optionally, the application can release the connection as described in Clearing connections.
Note: The BICC variant does not support establishing connections with continuity check required. To establish connections with BICC, see Establishing connections.
The following illustration shows the functions and events used to establish an outgoing connection with continuity check required:
Alternatively, the far exchange can initiate the connection with continuity check required by sending the IAM message. The application receives a connect indication (EVTSITCONIND) event with the Nature of connection indicators IE field contChk set to CONTCHK_REQ. This is an indication to the application to set up a loopback condition on the specified circuit.
The far exchange then performs the continuity check and sends a continuity message (COT) to report the results of the check. This results in the generation of a status indication (EVTSITSTAIND) with event type CONTREP to the application.
Regardless of the results of the check, the application can remove the loop back condition on the circuit. If the continuity check was successful (status event structure contInd IE set to CONTCHK_SUCC), the application can continue with normal call setup. In the case of failure (status event structure contInd IE set to CONTCHK_FAIL), the application can wait for a continuity recheck on the indicated circuit.
The following illustration shows the functions and events used to establish an incoming connection with continuity check required: