11.4 Functional requirement of Anchor-MSC
3GPP43.069Release 17Stage 2TSVoice Broadcast Service (VBS)
The VBS handling process in the anchor MSC is shown in figure 5.
Successful call set-up
When the VBS handling process in the anchor MSC receives a VBS call set-up request from either a dispatcher or a service subscriber currently located in the anchor MSC’s area or a service subscriber currently located in a relay MSC’s area, or – in a RANflex configuration or in a RANflex configuration with group call redundancy – from a service subscriber currently registered in a VMSC, it interrogates its associated GCR to retrieve the broadcast call attributes, and waits for a response.
If the GCR returns a positive response containing the broadcast call attributes, the anchor MSC sets up the downlinks to the cells inside the MSC area of the group call anchor MSC into which the call is to be sent, sets up the connections to the dispatchers to which a dedicated link is to be established , sets up the connections to the relay MSCs or relay MSC redundancy pools into which the call is to be sent and waits for call release.
Procedure Setup Connections to Relay MSCs and BSCs
The procedure is shown in figure 6.
The procedure
– sets up the downlinks to the cells inside the MSC area of the group call anchor MSC into which the call is to be sent;
– sends PREPARE_GROUP_CALL messages to all relay MSCs (in a RANflex configuration with group call redundancy: to all group call relay MSC redundancy pools) and waits for the responses.
If a positive response containing a Group Call number is received from a relay MSC, the anchor MSC constructs an IAM using the Group Call number as called party address, sends it to the relay MSC and waits for the SEND_GROUP_CALL_END_SIGNAL message.
If a SEND_GROUP_CALL_END_SIGNAL message without IMSI is received from a relay MSC, this indicates to the anchor MSC that at least one downlink to a cell has been successfully connected in the relay MSC.
If the call was originated by a service subscriber in a relay MSC area and a SEND_GROUP_CALL_END_SIGNAL message with the IMSI of the originator is received from this relay MSC, this indicates to the anchor MSC that the downlink to the originating cell has been successfully connected.
Relay MSCs (in a RANflex configuration with group call redundancy: relay MSC redundancy pools) that do not send positive responses to the PREPARE_GROUP_CALL message are no longer considered to belong to the list of relay MSCs (or relay MSC redundancy pools) for this VBS call.
If, after receipt of a positive response containing a Group Call number from a relay MSC, the anchor MSC receives an ABORT from the relay MSC, the relay MSC or relay MSC redundancy pool will no longer be considered to belong to the list of relay MSCs or relay MSC redundancy pools for this VBS call.
Optionally in both cases the anchor MSC may retry to establish the connection to the relay MSC or relay MSC redundancy pool, instead of no longer considering the relay MSC or relay MSC redundancy pool to belong to the list of relay MSCs or relay MSC redundancy pools.
If an ABORT message is received from the relay MSC or relay MSC redundancy pool the anchor MSC releases the connection, established with the Group Call number, to the relay MSC or relay MSC redundancy pool and the relay MSC or relay MSC redundancy pool is no longer considered to belong to the list of relay MSCs or relay MSC redundancy pools for this VBS call.
If a Release message for the connection established with the Group Call number is received from a relay MSC or relay MSC redundancy pool, then the relay MSC or relay MSC redundancy pool is no longer considered to belong to the list of relay MSCs or relay MSC redundancy pools for this VBS call. If the negative response on the PREPARE_GROUP_CALL message is received from originating relay MSC, the call may be released.
Unsuccessful call set-up
If the call set-up is unsuccessful (i.e. conditions for call establishment have not been met as per subclause 11.3.1.1.2) the anchor MSC releases the connections to the relay MSCs, aborts any dialogues to relay MSCs, releases all connections to dispatchers, all downlinks to cells inside the anchor MSC area are released, the GCR is informed that the call is no longer on-going and the process returns to the idle state.
Negative response received from the GCR
If the GCR returns a negative response to the anchor MSC indicating that the call is already on-going locally i.e. at the associated MSC, the anchor MSC checks whether the new call was initiated by a dispatcher. If so, the dispatcher is connected to the on-going call and the process returns to the idle state. If the new call was initiated by a service subscriber, a Release message indicating ‘user busy’ is returned 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 selected anchor MSC indicating that the call is already on-going at another MSC within the group call anchor MSC redundancy pool, the selected anchor MSC checks whether the new call was initiated by a dispatcher or a service subscriber served by another VMSC. If so, the selected anchor MSC forwards the call setup request to the anchor MSC where the group call is ongoing (possibly using a special prefix for the called party number). The selected anchor MSC then acts as a transit node unless it recognizes that the anchor MSC where the broadcast call is ongoing is out of service. In the latter case the selected anchor MSC repeats the GCR interrogation including "ongoing call override indication". If the call was initiated by a service subscriber served by the selected anchor MSC, the selected anchor MSC interrogates the anchor 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 anchor MSC recognizes that the anchor MSC where the broadcast call is ongoing is out of service or if that anchor 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 anchor MSC is not ongoing), then the selected anchor MSC repeats the GCR interrogation including "ongoing call override indication". If the anchor MSC where the broadcast call is ongoing responds with a negative SEND_GROUP_CALL_INFO response (ongoing call), then the selected anchor 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.
Call release
If the anchor MSC receives the specific DTMF message sequence or the specific DTMF tone sequence for call termination from a dispatcher who is allowed to terminate the voice broadcast call (see figures 4b to 4d), or a Termination Request message from the service subscriber who has initiated the VBS call or a Release message from the relay MSC for the calling service subscriber’s dedicated connection to the anchor MSC, the connections to the relay MSCs are released and any MAP dialogues to relay MSCs are closed by sending SEND_GROUP_CALL_END_ SIGNAL_ACK, all connections to dispatchers are released, all downlinks to cells inside the anchor MSC area are released, the GCR is informed that the call is no longer on-going and the process returns to the idle state.
Figure 5: VBS handling process in the anchor MSC (sheet 1 of 3)
Figure 5: The VBS handling process in the anchor MSC (sheet 2 of 3)
Figure 5: VBS handling process in the anchor MSC (sheet 3 of 3)
Figure 6: Set-up connection to relay MSCs and BSCs (sheet 1 of 1)