Call Setup Procedures

Call setup is triggered when a message from the application is received by the ISUP library containing the SETUP_REQ primitive and IAM. The request to setup a call may be unsuccessful due to various factors, such as failure to receive a specific ISUP library or dual seizure. The following scenarios illustrate the behavior of the ISUP library in these various circumstances.

SETUP Request Locally Refused by HP OpenCall SS7 ISUP

The application issues a SETUP_REQ to HP OpenCall SS7 ISUP. The primitive is locally rejected. Refer to the HP OpenCall SS7 ISUP API man pages for details on how the failure condition is reported to the application.

This situation occurs in the following cases:

the (DPC, CIC) does not correspond to a valid configured circuit
the circuit is reserved for inbound traffic
the circuit has already been seized (inbound seizure)
the circuit is under reset
the IAM message cannot be encoded
the IPC flow control is active
the SS7 Stack underneath is unavailable or congested

Figure 16-1 SETUP_REQ Locally Refused by HP OpenCall SS7 ISUP

SETUP Request - Dual Seizure Case 1

This scenario corresponds to the case where a circuit is seized on both sides at the same time. As a result of the SETUP_REQ, HP OpenCall SS7 ISUP issues an IAM (IAM-1) to the network. The issued IAM crosses the IAM (IAM-2) received from the peer, on its way to HP OpenCall SS7 ISUP. Consequently, HP OpenCall SS7 ISUP receives this IAM when already engaged in the processing of an outbound call (CPCO).

In such a case, HP OpenCall SS7 ISUP instances determine which of the inbound and outbound call attempts must be abandoned as follows. If the Signaling Point Code of HP OpenCall SS7 ISUP is greater than the Signaling Point Code of the peer, then HP OpenCall SS7 ISUP controls all even CICs and the peer controls all odd CICs.

Here, HP OpenCall SS7 ISUP gives up and issues a SETUP_FAILURE_IND primitive with a DUAL_SEIZURE cause. The application may use this information to re-attempt the call setup on another circuit.




	NOTE: There is no automatic call setup re-attempt as another circuit has to be selected.

Figure 16-2 Dual Seizure

SETUP Request - Dual Seizure Case 2

In this second scenario of incoming seizure, the IAM from the peer has already been received by HP OpenCall SS7 ISUP at the time the SETUP_REQ is issued by the application. Most likely the SETUP_IND is queued in the HP OpenCall SS7 ISUP API’ s Up List, waiting for the application to read it.

In this case, HP OpenCall SS7 ISUP is already engaged in the processing of an inbound call (CPCI) for the circuit at stake. The SETUP_REQ is therefore refused directly (that is, synchronously) without delivery of any asynchronous indication. Refer to the HP OpenCall SS7 ISUP man pages for details on how the refusal is communicated back to the application.

The application may re-try a call setup on another circuit.

Figure 16-3 Incoming Seizure

SETUP Request - Failure to Receive ACM

In this scenario, an ACM must be received as a response to the IAM within T7. Failure to receive ACM within T7 makes HP OpenCall SS7 ISUP abandon the call attempt and issue a START_RELEASE_IND to the application.

Figure 16-4 Failure to Receive ACM

Incoming Call Setup with Immediate Release

In this scenario, an REL message is received immediately following the incoming IAM message.

Successful Call Setup for Incoming Side

Call Setup Including ACM Message Use

The following figure presents a successful call setup scenario where the application sends ACM messages.

Figure 16-5 Successful Call Setup - Incoming Side

Call Setup Without ACM Message

The following figure presents a successful call setup scenario where the application directly sends an ANM message to complete the call.

Figure 16-6 Successful Call Setup - Incoming Side