11.4 Functional requirement of Anchor MSC
3GPP43.068Release 17Stage 2TSVoice Group Call Service (VGCS)
The VGCS handling process in the anchor MSC is shown in figure 8.
Successful call set-up
When the VGCS handling process in the anchor MSC receives a VGCS 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 group call attributes, and waits for a response.
If the GCR returns a positive response containing the group 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.; If the "talker channel parameter" is used, the anchor MSC shall additionally send the parameter to the affected BSCs;
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;
starts the No Activity Timer;
sends Forward Group Call Signalling messages to all selected relay MSCs if the call was not initiated by a dispatcher (however in a non-RANflex configuration not to the relay MSC from which the IMSI was received within the Send Group Call End Signal message if the call was initiated by a service subscriber located in the relay MSC area). The Forward Group Call Signalling messages shall contain the IMSI, talker priority and additional information of the service subscriber who has initiated the call. In a RANflex configuration and in a RANflex configuration with group call redundancy, simultaneous group call setup of two service subscribers may result in two Send Group Call End Signal messages being received by the anchor MSC from two different relay MSCs, with both messages containing an IMSI. In this situation the anchor MSC shall discard the IMSI received from the relay MSC whose MSC address does not match the generic number with the number qualifier indicator set to "additional calling party number" from the IAM received and accepted from the originating MSC.
NOTE: In a network configuration involving also pre-Rel-7 relay MSCs, the IAM sent by such a relay MSC as originating MSC will not include a generic number parameter. If the anchor MSC accepts the IAM from the pre-Rel-7 relay MSC and receives further IAM(s) including a generic number with number qualifier indicator set to "additional calling party number", the anchor MSC can store these additional calling party numbers in a list and discard the IMSI received with a Send Group Call End Signal message from a relay MSC whose MSC address matches any of the numbers in the list.
If the network supports talker priorities, the anchor MSC includes the talker priority of the service subscriber who has initiated the call in the Forward Group Call Signalling messages; and
waits for uplink management messages.
Procedure Set-up Connections to Relay MSCs and BSCs
The procedure is shown in figure 9.
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. If the network supports talker priorities, the anchor MSC additionally sends the talker priority of the service subscriber who has initiated the call and, if applicable, the "emergency mode indication" with Uplink Seized Command messages" to the affected BSCs;
– 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 the "talker channel parameter" is used, the anchor MSC shall include this parameter in the PREPARE_GROUP_CALL messages.
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 area.
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. If the negative response on the PREPARE_GROUP_CALL message is received from originating relay MSC, the call shall be released.
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 VGCS 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 VGCS 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 VGCS 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 anchor MSC sends an ABORT message to that 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 VGCS call.
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 sends a SEND_GROUP_CALL_END_SIGNAL_ACK message or ABORT (depending on the state of the MAP dialogue) to all relay MSCs, releases the connections to the 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 group 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 group 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 group 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 group 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 group 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 group 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 group call.
If the negative response from the GCR indicates any other reason than "on-going call" the VGCS call set-up request is rejected by sending a release message back to the initiator and the process returns to the idle state.
Uplink management
If an anchor MSC not supporting talker priorities receives an Uplink Release Indication message from a BSC, the anchor MSC marks the uplink as free, sends Forward Group Call Signalling messages indicating "uplink release indication" to all relay MSCs, sends Uplink Release command messages to all other BSCs, restarts the No Activity Timer and waits for further uplink management messages.
If an anchor MSC supporting talker priorities receives an Uplink Release Indication message from a BSC, the anchor MSC compares the talker priority included in the Uplink Release Indication with the stored talker priority. If they are equal, the anchor MSC proceeds as specified above for the anchor MSC not supporting talker priorities; otherwise the anchor MSC discards the Uplink Release Indication.
If the anchor MSC receives an Uplink Request message without talker priority or with talker priority "normal subscriber" from a BSC, it checks whether the uplink is marked as free. If so, an Uplink Request Acknowledge message is returned to the BSC, Forward Group Call Signalling messages indicating that the uplink is no longer free are sent to all relay MSCs, Uplink Seized Command messages are sent to all other BSCs, the uplink is marked busy and the process waits for further uplink management messages. If the uplink was not free when receiving the Uplink Request message, the request is rejected.
If an anchor MSC supporting talker priorities receives an Uplink Request message with a talker priority higher than "normal subscriber" from a BSC, and the uplink is free or it was seized by the current talker with a lower talker priority, the anchor MSC checks whether the subscriber has the subscription for the requested talker priority for this group ID. In a RANflex configuration (with or without group call redundancy) this check may require retrieval of information from the visited MSC/VLR by means of the MAP service SEND_GROUP_CALL_INFO. If so, the anchor MSC:
– stores the received data;
– sends Forward Group Call Signalling messages indicating "uplink seized command" with the requested talker priority to the relay MSCs;
– returns an Uplink Request Acknowledge message with the requested talker priority to the BSC which has requested the uplink;
– sends Uplink Seized Command messages with the requested talker priority to all other BSCs;
– releases the current talker by sending a Clear Command message, if the talker is on a dedicated channel and is located in the anchor MSC;
– marks the uplink busy; and
– waits for further uplink management messages.
Additionally,
– if the requested talker priority is "emergency subscriber", the anchor MSC sets the emergency mode and includes the "emergency mode indication" in the Uplink Request Acknowledge message and the Uplink Seized Command messages; furthermore the anchor MSC alerts all active dispatchers with an appropriate emergency indication (e.g. in-band tone or announcement); furthermore the anchor MSC alerts all inactive dispatchers by setting up dedicated connections with an appropriate emergency indication (e.g. special CLI prefix or postfix, and in-band tone or announcement) and connects the dispatchers to the voice group call.
– if the anchor MSC supports the transmission of additional subscriber-related information, it interrogates the VLR to get the "additional information" assigned to the new talker. In a RANflex configuration (with or without group call redundancy) this may require retrieval of information from the visited MSC/VLR by means of the MAP service SEND_GROUP_CALL_INFO. If "additional information" is available, the anchor MSC includes the "additional information" in the Forward Group Call Signalling messages indicating "uplink seized command" to all relay MSCs and sends VGCS Additional Info messages to all BSCs involved in the call and connected to the anchor MSC. Furthermore, the anchor MSC sends a VGCS Additional Info message on the dedicated connection to the BSC serving the current talker, before it releases the current talker.
If an anchor MSC supporting talker priorities receives an Uplink Request message with a talker priority higher than "normal subscriber" from a BSC, and the uplink was seized by the current talker with the same or a higher talker priority, then the anchor MSC sends an Uplink Reject Command message with the current talker priority to the BSC.
If the anchor MSC receives an Uplink Request Confirm message from a BSC, it stores the received data and waits for further uplink management messages. Additionally, if the anchor MSC supports the transmission of additional subscriber-related information, it interrogates the VLR to get the "additional information" assigned to the subscriber. In a RANflex configuration (with or without group call redundancy) this interrogation may require retrieval of information from the visited MSC/VLR by means of the MAP service SEND_GROUP_CALL_INFO. If "additional information" is available, the relay MSC sends VGCS Additional Info messages to all BSCs involved in the call and connected to this relay MSC, and Forward Group Call Signalling messages with the additional info to all relay MSCs.
If an anchor MSC supporting talker priorities receives an Emergency Reset Indication from a BSC and the subscription check is successful, the anchor MSC resets the emergency mode, sends Emergency Reset Command messages to all BSCs involved in the call and connected to the anchor MSC, and Forward Group Call Signalling messages indicating "emergency reset command" to all relay MSCs. If the talker priority at receipt of the Emergency Reset Indication is "emergency subscriber", then it is changed in the anchor MSC to "normal subscriber"; furthermore the anchor MSC alerts all active dispatchers with an appropriate emergency indication (e.g. in-band tone or announcement); furthermore the anchor MSC alerts all inactive dispatchers by setting up dedicated connections with an appropriate emergency indication (e.g. special CLI prefix or postfix, and in-band tone or announcement) and connects the dispatchers to the voice group call.
If an anchor MSC not supporting talker priorities receives a Process Group Call Signalling message indicating "uplink release indication" from a relay MSC , the anchor MSC marks the uplink as free, sends Forward Group Call Signalling messages indicating "uplink release indication" to all other relay MSCs, sends Uplink Release command messages to all BSCs, restarts the No Activity Timer and waits for further uplink management messages. If there is a dedicated connection for the talking service subscriber between the relay MSC and the anchor MSC, the anchor MSC shall release this connection with cause ‘normal, unspecified’.
If an anchor MSC supporting talker priorities receives a Process Group Call Signalling message indicating "uplink release indication" from a relay MSC, the anchor MSC compares the talker priority included in the Process Group Call Signalling message with the stored talker priority. If they are equal, the anchor MSC proceeds as specified above for the anchor MSC not supporting talker priorities; otherwise the anchor MSC discards the Process Group Call Signalling message.
If the anchor MSC receives a Process Group Call Signalling message from a relay MSC indicating "uplink request" without talker priority or with talker priority "normal subscriber", it checks whether the uplink is marked as free. If so, a Forward Group Call Signalling message indicating "uplink request acknowledgement" is returned to the relay MSC, Forward Group Call Signalling messages indicating that the uplink is no longer free are sent to all other relay MSCs, Uplink Seized Command messages are sent to all BSCs, the uplink is marked busy and the process waits for further uplink management messages. If the uplink was not free when receiving the Process Group Call Signalling message (Uplink Request), the request is rejected.
If an anchor MSC supporting talker priorities receives a Process Group Call Signalling message from a relay MSC indicating "uplink request" with a talker priority higher than "normal subscriber", and the uplink is free or it was seized by the current talker with a lower talker priority, then the anchor MSC:
– returns a Forward Group Call Signalling message indicating "uplink request acknowledgement" with the requested talker priority to the relay MSC;
– sends Forward Group Call Signalling messages indicating "uplink seized command" and the requested talker priority to all other relay MSCs;
– sends Uplink Seized Command messages with the requested talker priority to all BSSs involved in the call;
– releases the current talker by sending a Clear Command message, if the talker is on a dedicated channel and is located in the anchor MSC;
– marks the uplink as busy; and
– waits for further uplink management messages.
Additionally,
– if the requested talker priority in the Process Group Call Signalling message from the relay MSC is "emergency subscriber", the anchor MSC sets the emergency mode and includes the "emergency mode indication" in the Uplink Seized Command messages; furthermore the anchor MSC alerts all active dispatchers with an appropriate emergency indication (e.g. in-band tone or announcement); furthermore the anchor MSC alerts all inactive dispatchers by setting up dedicated connections with an appropriate emergency indication (e.g. special CLI prefix or postfix, and in-band tone or announcement) and connects the dispatchers to the voice group call.
– if "additional information" about the new talker was included in the Process Group Call Signalling message from the relay MSC, the anchor MSC includes the "additional information" in the Forward Group Call Signalling messages indicating "uplink seized command" to all other relay MSCs and sends VGCS Additional Info messages to all BSCs involved in the call and connected to the anchor MSC. Furthermore, the anchor MSC sends a VGCS Additional Info message on the dedicated connection to the BSC serving the current talker, before it releases the current talker.
If an anchor MSC supporting talker priorities receives a Process Group Call Signalling message from a relay MSC indicating "uplink request" with a talker priority higher than "normal subscriber", and the uplink was seized by the current talker with the same or a higher talker priority, then the anchor MSC returns a Forward Group Call Signalling message indicating "uplink reject command" with the current talker priority to the relay MSC.
If the anchor MSC receives a Process Group Call Signalling message with "additional info" from a relay MSC, the anchor MSC sends VGCS Additional Info messages to all BSCs involved in the group call and connected to the anchor MSC, and Forward Group Call Signalling messages with "additional info" to all other relay MSCs.
If an anchor MSC supporting talker priorities receives a Process Group Call Signalling message with "emergency mode reset command" from a relay MSC, the anchor MSC resets the emergency mode, sends Forward Group Call Signalling messages with "emergency mode reset command" to all other relay MSCs, and sends Emergency Reset Command messages to all BSCs involved in the call and connected to the anchor MSC. If the talker priority at receipt of the "emergency mode reset command" is "emergency subscriber", then it is changed in the anchor MSC to "normal subscriber"; furthermore the anchor MSC alerts all active dispatchers with an appropriate emergency indication (e.g. in-band tone or announcement); furthermore the anchor MSC alerts all inactive dispatchers by setting up dedicated connections with an appropriate emergency indication (e.g. special CLI prefix or postfix, and in-band tone or announcement) and connects the dispatchers to the voice group call.
If the anchor MSC receives an ABORT message from a relay MSC, the connection to the relay MSC is released and the relay MSC is no longer considered to be part of the call.
Call release
If the anchor MSC receives a termination request from an entitled dispatcher (see figures 7b to 7d) or a Termination Request message from the initiating service subscriber who currently has access to the uplink, it sends Send Group Call End Signal ACK messages to all relay MSCs, sends Release messages to all relay MSCs, sends Release messages to all dispatchers and BSCs, informs the GCR that the call is no longer on-going and the process returns to the idle state. If the group call termination is initiated by an entitled dispatcher, and there is a dedicated connection for the talking service subscriber between the anchor MSC and the relay MSC, the anchor MSC shall release this connection with cause ‘normal call clearing’.
If the anchor MSC receives a Process Group Call Signalling message from a relay MSC indicating "release group call" or an ISUP Release message from a relay MSC indicating "Normal call clearing" for the calling service subscriber’s dedicated connection to the anchor MSC, then the anchor MSC sends Send Group Call End Signal ACK messages to all relay MSCs, sends Release messages to all relay MSCs, sends Release messages to all dispatchers and BSCs, informs the GCR that the call is no longer on-going and the process returns to the idle state.
If the anchor MSC receives an ISUP Release message from a relay MSC with cause value other than "Normal call clearing" for the calling service subscribers dedicated connection to the anchor MSC, then the anchor MSC shall send Uplink Release Command messages to all BSCs and Forward Group Call Signalling messages with Uplink Release Indication parameter to all relay MSCs.
If the no activity time in the anchor MSC expires, i.e. "no activity" (as specified in subclause 8.1.2.3) has been detected for the time specified in the GCR, the anchor MSC sends Send Group Call End Signal ACK messages to all relay MSCs, sends Release messages to all relay MSCs, sends Release messages to all dispatchers and BSCs, informs the GCR that the call is no longer on-going and the process returns to the idle state.
SM MT delivery
When the VGCS handling process in the anchor MSC receives an MT_ForwardSM_for_VGCS_REQ message from a SMS-GMSC, the anchor MSC shall interrogate the GCR to retrieve the group call attributes for short message transfer. The anchor MSC shall deliver to the SMS-GMSC the status of the voice group call and optionally a list of identities of dispatchers within the MT_ForwardSM_for_VGCS_RES message.
If the voice group call is established, the anchor MSC shall distribute the short message TPDU to all connected relay MSCs using the FORWARD_GROUP CALL_SIGNALLING message and shall deliver the short message to the relevant cells using the already established downlink channel for the voice group call.
If the voice group call is not established, the anchor MSC shall discard the short message TPDU.
Figure 8: The VGCS handling process in the anchor MSC (sheet 1 of 7)
Figure 8: The VGCS handling process in the anchor MSC (sheet 2 of 7)
Figure 8: The VGCS handling process in the anchor MSC (sheet 3 of 7)
Figure 8: The VGCS handling process in the anchor MSC (sheet 4 of 7)
Figure 8: The VGCS handling process in the anchor MSC (sheet 5 of 7)
Figure 8: The VGCS handling process in the anchor MSC (sheet 6 of 7)
Figure 8: The VGCS handling process in the anchor MSC (sheet 7 of 7)
Figure 9: Procedure Set-up Connections to Relay MSCs and BSCs (sheet 1 of 1)