11.5A Functional requirement of group call serving MSC (within a RANflex pool)

3GPP43.068Release 16Stage 2TSVoice Group Call Service (VGCS)

The process in the group call serving MSC is shown in figure 10A1.

The group call serving MSC is either the anchor MSC or a relay MSC.

Successful call set-up initiated by a service subscriber

When receiving a SEND_GROUP_CALL_INFO interrogation request from a VMSC, the group call serving MSC (in a RANflex configuration with group call redundancy: the selected group call serving MSC) interrogates its associated GCR to retrieve the Group Call Reference and the Anchor MSC address from the given cell Id / Group Id pair and return this information to the VMSC. The group call serving MSC shall temporarily store in its associated GCR the initiating subscriber’s talker priority, his subscribed additional information, his IMSI as received in the interrogation request from the VMSC and the originating cell id. The group call serving MSC shall then wait for the group call being set up:
If the group call serving MSC is the anchor MSC, it waits for an IAM from the VMSC; if the group call serving MSC is a relay MSC, it waits for a PREPARE_GROUP_CALL from the anchor MSC.
Waiting for the group call being set up shall be supervised by a timer T3.
When receiving IAM from the visited MSC, timer T3 is stopped and processing continues in the process VGCS_Handling_Anchor_MSC after reception of the VGCS call setup request in Idle state.
When receiving PREPARE_GROUP_CALL from the anchor MSC, timer T3 is stopped and processing continues in the process VGCS_Handling_Relay_MSC after reception of PREPARE GROUP CALL in Idle state.

Unsuccessful call set-up

In a RANflex configuration: If the group call reference could not be retrieved or the GCR returns a negative response indicating "on-going call" to the group call serving MSC, an error indication is returned to the VMSC and the process returns to the idle state.

In a RANflex configuration with group call redundancy: If the GCR returns a negative response indicating "on-going call" to the group call serving MSC, the group call serving MSC forwards the SEND_GROUP_CALL_INFO message within the same redundancy pool to the group call serving MSC where the group call is ongoing.

If the selected group call serving MSC recognizes that the group call serving MSC where the group call is ongoing is out of service or if that group call serving MSC responds with a positive SEND_GROUP_CALL_INFO response (i.e. the GCRs are out of synch and the group call at the other group call serving MSC is not ongoing), then the selected group call serving MSC repeats the GCR interrogation including "ongoing call override indication". If the group call serving MSC where the group call is ongoing responds with a negative SEND_GROUP_CALL_INFO response (ongoing call), then the selected group call serving MSC returns a negative SEND_GROUP_CALL_INFO response indicating "ongoingGroupCall" to the VMSC 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 group call.

At timeout of timer T3 the temporarily stored initiating subscriber’s talker priority, his subscribed additional information and his IMSI shall be deleted from the GCR.

Figure 10A1: The VGCS handling process in the group call serving MSC (sheet 1 of 2)

Figure 10A1: The VGCS handling process in the group call serving MSC (sheet 2 of 2)

Retrieval of VGCS subscription data

The procedure in the group call serving MSC to retrieve subscription information from the VMSC is shown in figure 10A2.

In a RANflex configuration with or without group call redundancy, if the group call serving MSC receives an uplink request or a request to transmit application-specific data, it determines from the NRI in the requesting subscriber’s TMSI whether it is the requesting subscriber’s VMSC.

If it is not, then the group call serving MSC retrieves the IMSI and information about subscribed talker priorities from the VLR of the VMSC by means of the MAP service SEND_GROUP_CALL_INFO.

– If a response is received from the VMSC, the procedure ends.

As an implementation option the group call serving MSC may initiate a MAP Update_Location_Area procedure towards its VLR in order to retrieve the subscriber data from the HLR. During this procedure the VLR allocates a new TMSI including an NRI referring to the group call serving MSC.

As an alternative implementation option the group call serving MSC may store the TMSI temporarily together with the IMSI and the other information received from the VLR of the VMSC and use these data for subscription checks during subsequent talker requests. Use of the temporary subscriber data may be limited e.g. to an implementation dependent time or for an implementation dependent number of subsequent talker requests. The group call serving MSC shall delete the temporary subscriber data when the group call is released.

– If no response is received or if the VMSC is out of service, the group call serving MSC should initiate a MAP Update_Location_Area procedure towards its VLR. During this procedure the MSC requests the MS to provide its IMSI and the VLR allocates a new TMSI including an NRI referring to the group call serving MSC. The group call serving MSC checks whether the MS has the necessary subscription for the requested action when the Update_Location_Area procedure is completed.

NOTE: As an operator option, for this case of a network internal failure, the group call serving MSC can skip the MAP Update_Location_Area procedure and perform the subscription check using a backup subscription profile.

As an operator option, the group call serving MSC may proceed with the handling of the request received from the MS, assuming that the MS has the necessary subscription, and perform the Update_Location_Area procedure in parallel. If the result of the subscription check after completion of the Update_Location_Area procedure is negative and the MS still has the uplink seized, the group call serving MSC shall initiate an uplink release procedure.

Implicit location update procedure

The procedure for the implict location update in the group call serving MSC is shown in figure 10A3.

Sheet 1: The procedures Check_IMEI_MSC, Obtain_IMEI_MSC, Obtain_IMSI_MSC and Authenticate_MSC are specified in 3GPP TS 23.018 [18].

Figure 10A2: The VGCS subscription info retrieval procedure in the group call serving MSC (sheet 1 of 1)

Figure 10A3: The implicit location update procedure in the group call serving MSC (sheet 1 of 3)

Figure 10A3: The implicit location update procedure in the group call serving MSC (sheet 2 of 3)

Figure 10A3: The implicit location update procedure in the group call serving MSC (sheet 3 of 3)