11.5 Functional requirement of Relay MSC

3GPP43.068Release 17Stage 2TSVoice Group Call Service (VGCS)

The VGCS handling process in the relay MSC is shown in figure 10.

Successful call set-up initiated by a service subscriber

When the VGCS handling process in the relay MSC receives a VGCS 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 VGCS prefix plus group 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 group 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 group 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 group 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 group 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 group 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 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.

Successful call set-up initiated by the anchor MSC

When the VGCS 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 "talker channel parameter" is used, the relay MSC shall additionally send the parameter to the affected BSCs

If the network supports talker priorities and the group call was initiated by a service subscriber currently located in the relay MSC’s area, then the relay 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.

If the group call was initiated by a service subscriber currently located in the relay MSC’s area, then the relay MSC shall send a SEND_GROUP_CALL_END_SIGNAL message, including the IMSI and additional information of the service subscriber, to the anchor MSC when the downlink has been set up successfully to the originating cell. Additionally, if the network supports talker priorities, the relay MSC includes the talker priority of the service subscriber in the SEND_GROUP_CALL_END_SIGNAL message.

In a RANflex configuration, the IMSI, talker priority and additional information (initial talker information) shall be deleted by the relay MSC after sending SEND_GROUP_CALL_END_SIGNAL. If initial talker information is received in a subsequent FORWARD_GROUP_CALL_SIGNALLING message, the relay MSC shall use this information for the voice group call.

NOTE: The initial talker information sent within SEND_GROUP_CALL_END_SIGNAL can be invalid, e.g. due to a race condition, if the voice group call is simulateously set up by a dispatcher or another service subscriber.

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 a downlink has been set up successfully to any cell.

Then the relay MSC waits for uplink management messages.

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_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 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".

Uplink management

If a relay MSC not supporting talker priorities receives an Uplink Release Indication message from a BSC, the relay MSC marks the uplink as free, sends a Process Group Call Signalling message indicating "uplink release indication" to the anchor MSC, sends Uplink Release Command messages to all other BSCs, and waits for further uplink management messages.

NOTE: If there is a dedicated connection for the talking service subscriber between the relay MSC and the anchor MSC, the anchor MSC will release this connection with cause ‘normal, unspecified’.

If a relay MSC supporting talker priorities receives an Uplink Release Indication message from a BSC, the relay MSC compares the talker priority included in the Uplink Release Indication with the stored talker priority. If they are equal, the relay MSC proceeds as specified above for the relay MSC not supporting talker priorities, except that it includes the talker priority in the Process Group Call Signalling message to the anchor MSC; otherwise the relay MSC discards the Uplink Release Indication.

If the relay MSC receives an Uplink Request message without talker priority or with talker priority "normal subscriber" from a BSC, the relay MSC checks whether the uplink is marked as free. If so, a Process Group Call Signalling message indicating "uplink request" is sent to the anchor MSC, 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, the request is rejected.

If a relay 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 relay MSC checks whether the subscriber has the subscription for the requested talker priority for that group ID. In a RANflex configuration and in a RANflex configuration with 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 relay MSC:

– stores the received data;

– sends a Process Group Call Signalling message indicating "uplink request" and the requested talker priority to the anchor MSC;

– sends Uplink Seized Command messages with the requested talker priority to all other BSCs involved in the call and connected to this relay MSC;

– releases the current talker by sending a Clear Command message, if the talker is on a dedicated channel and is located in this relay MSC;

– marks the uplink as busy; and

– waits for further uplink management messages.

Additionally,

– if the requested talker priority is "emergency subscriber", the relay MSC sets the emergency mode and includes the "emergency mode indication" in the Uplink Seized Command messages;

– if the relay 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 and in a RANflex configuration with 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 relay MSC includes the "additional information" in the Process Group Call Signalling message indicating "uplink request" to the anchor MSC.

If a relay 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 relay MSC sends an Uplink Reject Command message with the current talker priority to the BSC.

If the relay MSC receives an Uplink Request Confirm message from a BSC, it stores the data and waits for further uplink management messages. Additionally, if the relay 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 and in a RANflex configuration with 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 a Forward Group Call Signalling message with the additional info to the anchor MSC.

If a relay MSC supporting talker priorities receives an Emergency Reset Indication from a BSC and the subscription check is successful, the relay MSC sends a Progress Group Call Signalling message with "emergency mode reset command" to the anchor MSC and Emergency Reset Command messages to all BSCs involved in the call and connected to this relay MSC, and waits for further uplink management messages. If the talker priority at receipt of the Emergency Reset Indication is "emergency subscriber", then it is changed in the relay MSC to "normal subscriber".

If the relay MSC receives a Forward Group Call Signalling message from a anchor MSC indicating "uplink release indication", it marks the uplink as free, sends Uplink Release command messages to all BSCs and waits for further uplink management messages.

If the relay MSC receives a Forward Group Call Signalling message from a anchor MSC indicating "uplink seized command" with or without talker priority, then the relay MSC

marks the uplink as busy,

sends Uplink Seized Command messages with the requested talker priority to all BSCs involved in the call and connected to this relay MSC;

– releases the current talker by sending a Clear Command message, if the talker is on a dedicated channel and is located in this relay MSC; and

– waits for further uplink management messages.

Additionally,

– if the Forward Group Call Signalling message from the anchor MSC contained the talker priority "emergency subscriber", the relay MSC sets the emergency mode and includes the "emergency mode indication" in the Uplink Seized Command messages;

– if "additional information" about the new talker was included in the Forward Group Call Signalling message from the anchor MSC, the relay MSC sends VGCS Additional Info messages to all BSCs involved in the call and connected to the relay MSC. Furthermore, the relay 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 the relay MSC receives a Forward Group Call Signalling message with the "additional info", the relay MSC sends VGCS Additional Info message to all BSCs involved in the group call and connected to this relay MSC, and waits for further uplink management messages.

If the relay MSC receives a Forward Group Call Signalling message from an anchor MSC indicating "uplink reject command" with or without talker priority, it returns an Uplink Reject message to the BSC which has requested the uplink and waits for further uplink management messages. If the "uplink reject command" included a talker priority, the relay MSC includes the talker priority in the Uplink Reject Command message

If the relay MSC receives a Forward Group Call Signalling message from an anchor MSC indicating "uplink request acknowledgement" with or without talker priority, then the relay MSC

– returns an Uplink Request Acknowledge message with the requested talker priority to the BSC which has requested the uplink;

– sets up a dedicated connection for the new talker to the anchor MSC (implementation option);

– releases the current talker by sending a Clear Command message, if the talker is on a dedicated channel and is located in this relay MSC; and

– waits for further uplink management messages.

Additionally,

– if the requested talker priority is "emergency subscriber", the relay MSC includes the "emergency mode indication" in the Uplink Request Acknowledge messages;

– if "additional information" about the new talker is available, the relay MSC sends VGCS Additional Info messages to all BSCs involved in the call and connected to the relay MSC. Furthermore, the relay 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 a relay MSC supporting talker priorities receives a Forward Group Call Signalling message with "emergency mode reset command" from the anchor MSC, the relay MSC resets the emergency mode, sends Emergency Reset Command messages to all BSCs involved in the call and connected to this relay MSC, and waits for further uplink management messages. If the talker priority at receipt of the "emergency mode reset command" is "emergency subscriber", then it is changed in the relay MSC to "normal subscriber".

If the relay MSC receives a Forward Group Call Signalling message from an anchor MSC indicating "uplink release command", it sends an Uplink Release Command message to the BSC which currently has access to the uplink and waits for further uplink management messages.

If the relay MSC receives an ABORT message from a anchor MSC, it sends release messages to all BSCs, informs the GCR that the call is no longer on-going and the process returns to the idle state.

Call release

When receiving a release message, with cause ‘normal call clearing’ from the anchor MSC for the dedicated connection which was set-up 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 releases the call while a dedicated connection to the anchor MSC is established, the relay MSC sends a release message with cause ‘normal call clearing’ for the dedicated connection to the anchor MSC and the process returns to the idle state.

When the initiating service subscriber releases the call, while on a group call channel or a dedicated connection to the relay MSC, the relay MSC sends a Process Group Call Signalling message to the anchor MSC indicating "release group call" and waits for the Release message and the Send Group Call End Signal Acknowledgement from the anchor MSC.

When receiving a Send Group Call End Signal Acknowledgement or ABORT from the anchor MSC, or a release message for the connection that was set up using the Group Call number, 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.

SM MT delivery

When the VGCS handling process in the relay MSC receives a FORWARD_GROUP CALL_SIGNALLING message containing a short message TPDU from the anchor MSC, the R-MSC shall deliver the short message to the relevant cells using the already established downlink channel for the voice group call.

Figure 10: The VGCS handling process in the relay MSC (sheet 1 of 8)

Figure 10: The VGCS handling process in the relay MSC (sheet 2 of 8)

Figure 10: The VGCS handling process in the relay MSC (sheet 3 of 8)

Figure 10: The VGCS handling process in the relay MSC (sheet 4 of 8)

Figure 10: The VGCS handling process in the relay MSC (sheet 5 of 8)

Figure 10: The VGCS handling process in the relay MSC (sheet 6 of 8)

Figure 10: The VGCS handling process in the relay MSC (sheet 7 of 8)

Figure 10: The VGCS handling process in the relay MSC (sheet 8 of 8)