11.5 Functional requirement of Relay-MSC

3GPP43.069Release 17Stage 2TSVoice Broadcast Service (VBS)

The VBS handling process in the relay MSC is shown in figure 7.

Successful call set-up initiated by a service subscriber

When the VBS handling process in the relay MSC receives a VBS call set-up request from a service subscriber currently located in a relay MSC’s area, it interrogates its associated GCR to retrieve the anchor MSC address and waits for a response.

If the GCR returns a positive response containing the anchor MSC address, the relay MSC sets up a dedicated connection for the initiating service subscriber to the anchor MSC by constructing an IAM with calling party number set to the VBS prefix plus broadcast call reference, and with a generic number parameter with the number qualifier indicator set to "additional calling party number" and address signal set to the address of this relay MSC, sending it to the anchor MSC, and waits for call release.

Negative response received from the GCR

If the GCR returns a negative response to the relay MSC indicating that the call is already on-going locally, i.e. at the associated MSC, the relay MSC sends a Release message indicating ‘user busy’ to the service subscriber in order to force the mobile station of the service subscriber to look for notifications of the respective group ID on the NCH and join the broadcast call.

If (in a RANflex configuration with group call redundancy) the GCR returns a negative response to the relay MSC indicating that the call is already on-going at another MSC within the group call relay MSC redundancy pool, the selected relay MSC interrogates the relay MSC where the broadcast call is ongoing by means of the MAP service SEND_GROUP_CALL_INFO and waits for a response. If the selected relay MSC recognizes that the relay MSC where the broadcast call is ongoing is out of service or if that relay MSC responds with a positive SEND_GROUP_CALL_INFO response (i.e. the GCRs are out of synch and the broadcast call at the other relay MSC is not ongoing), then the selected relay MSC repeats the GCR interrogation including "ongoing call override indication". If the relay MSC where the broadcast call is ongoing responds with a negative SEND_GROUP_CALL_INFO response (ongoing call), then the selected relay MSC returns a Release message indicating "user busy" in order to force the mobile station of the service subscriber to look for notifications of the respective group ID on the NCH and join the broadcast call.

If the negative response from the GCR indicates any other reason than ‘on-going call’ the VBS call set-up request is rejected by sending a release message back to the initiator and the process returns to the idle state.

Successful call set-up initiated by the anchor MSC

When the VBS handling process in the relay MSC receives a PREPARE_GROUP_CALL message from the anchor MSC, it interrogates its associated GCR to retrieve the list of cells inside the relay MSC area into which the call is to be sent.

If the GCR returns a positive response, the relay MSC requests a Group Call number from its VLR.

If the VLR returns a Group Call number, a PREPARE_GROUP_CALL acknowledgement containing the Group Call number is returned to the anchor MSC and the relay MSC waits for the incoming call.

If the incoming call identified by the Group Call number is received, the relay MSC releases the Group Call number and sets up the downlinks to the cells inside the relay MSC area into which the call is to be sent.

If the call was originated by a service subscriber from the relay MSC area, the relay MSC shall send a SEND_GROUP_CALL_END_SIGNAL message including the IMSI of the originator to the anchor MSC when the downlink has been set up successfully to the originating cell.

If the call was not originated by a service subscriber from the relay MSC area, the relay MSC shall send a SEND_GROUP_CALL_END_SIGNAL message without IMSI information element to the anchor MSC as soon as the downlink has been set up successfully to any cell.

Then the relay MSC waits for the call to be released.

Negative response received from the GCR II

If the GCR returns a negative response to the relay MSC, the relay MSC returns a PREPARE_GROUP_CALL negative response to the anchor MSC and returns to the idle state.

No Group Call number received from VLR

If the VLR could not allocate a Group Call number, the relay MSC returns a PREPARE_GROUP_CALL negative response to the anchor MSC, informs the GCR that the call is no longer on-going and returns to the idle state.

Abort received from VLR

If the VLR indicates that the Group Call number supervision timer has expired, the relay MSC sends an ABORT message to the anchor MSC, informs the GCR that the call is no longer on-going and returns to the idle state.

Abort received from Anchor MSC

If, after returning a PREPARE_GROUP_CALL acknowledgement containing the Group Call number, an ABORT is received from the anchor MSC the relay MSC releases the Group Call number, informs the GCR that the call is no longer on-going and returns to the idle state.

Abort initiated by Relay MSC

The relay MSC may abort the dialogue by sending an ABORT message to the anchor MSC (e.g. if the relay MSC fails to establish any downlinks in its area). The relay MSC then also releases any resources, informs the GCR that the call is no longer on-going and returns to the idle state.

Unsuccessful call set-up

Unsuccessful call set-up is determined in the anchor MSC (as per subclause 11.3.1.1.2). The relay MSC follows the procedures specified for ‘ABORT received from Anchor MSC’ and ‘Call release’.

Call release

When receiving a release message from the anchor MSC for the dedicated connection which was set-up to for the initiating service subscriber located in the relay MSC area, the relay MSC releases the connection to the service subscriber and the process returns to the idle state.

When the initiating service subscriber terminates the call, the relay MSC sends a release message for the dedicated connection to the anchor MSC and the process returns to the idle state.

When receiving a release message from the anchor MSC for the connection which was set up using the Group Call number or an ABORT message, the relay MSC releases all downlinks to cells inside the relay MSC area, informs the GCR that the call is no longer on-going and the process returns to the idle state.

Figure 7: VBS handling process in the relay MSC (sheet 1 of 4)

Figure 7: The VBS handling process in the relay MSC (sheet 2 of 4)

Figure 7: VBS handling process in the relay MSC (sheet 3 of 4)

Figure 7: VBS handling process in the relay MSC (sheet 4 of 4)