7.2.6 Multicast session update procedure

23.2473GPPArchitectural enhancements for 5G multicast-broadcast servicesRelease 18TS

Multicast MBS session update procedure is invoked by the AF to update the service requirement (result in multicast QoS parameters update and/or multicast QoS flow addition/removal) and/or MBS Service Area for an ongoing Multicast MBS session.

If the MBSF acts as the MBS security function for multicast as defined in TS 33.501 [20], it may use this procedure to provide an updated MSK and optional updated MTK in addition for the MBS session via the control plane.

NOTE: The procedure is not applicable if no MSK but only the MTK is to be updated.

For the interaction between AF or MBSF and MB-SMF, see clauses 7.1.1.6 and 7.1.1.7.

Figure 7.2.6-1: Multicast MBS Session update procedure

1. This procedure is triggered by the MB-SMF receiving the updated service requirement, an updated multicast session security context from the MBSF and/or MBS Service Area for a multicast MBS Session, see clauses 7.1.1.6 and 7.1.1.7.

2. The AF providing the updated service area may also inform UEs at application level about the new service area via a service announcement.

NOTE 1: If a UE is located in a cell which was previously outside the service area and is now inside the updated service area, the UE can join the multicast service as specified in clause 7.2.1.3.

For QoS updates steps 3 to 7 are performed.

For MBS Service Area update steps 3 to 7 may be performed to allow NG-RAN to terminate data transmission in the area which is no longer in the MBS Service Area.

3. The MB-SMF invokes Namf_MBSCommunication_N2MessageTransfer service operation (MBS Session ID, [Area Session ID], N2 SM message container (TMGI, [QoS profile(s) for multicast MBS session], [MBS Service Area], [Area Session Id])) to the AMF(s).

4. The involved AMF sends N2 MBS session request (N2 SM message container) to NG-RAN nodes handling the multicast MBS session and possible Area Session ID based on the RAN node IDs stored in the AMF for the MBS session.

5. The NG-RAN node updates the QoS profile and/or MBS Service Area for the multicast MBS session based on the N2 MBS session request. If only QoS parameters are updated without multicast QoS flows added/removed, the NG-RAN may also update the QoS parameters of the associating PDU Sessions.

For MBS Service Area update, the NG-RAN updates the MBS Session Context with the updated MBS Service Area. The NG-RAN stops transmission of the related multicast data in the cell(s) which is within the old MBS Service Area but now outside the updated MBS Service Area. The NG-RAN also configures the UE not to receive the MBS data over the radio interface if the NG-RAN detects the UE(s) was in the previous MBS Service Area but is outside the updated MBS Service Area. If the NG-RAN node no longer serves any cells within the updated MBS service area, it requests to release shared delivery resource as defined in clause 7.2.2.4.

6. The NG-RAN node(s) acknowledges N2 MBS session request by sending an N2 MBS session Response message to the AMF.

7. The AMF invokes the Nmbsmf_MBSSession_ContextUpdate () to the MB-SMF.

8. The MB-SMF sends Nmbsmf_MBSSession_ContextStatusNotify request (MBS Session ID, [QoS profiles for multicast for MBS session], [MBS Service Area], [Area Session ID], [updated multicast session security context]) to the SMFs. For MBS Service Area updates, if an Area Session ID exists, the MB-SMF provides the MBS Service Area corresponding to the Area Session ID to the SMFs involved in the multicast MBS session. For QoS updates, the MB-SMF notifies SMFs handling all MBS service areas.

9. The SMF determines the affected UEs it serves based on the multicast MBS Session ID and Area Session ID (if provided) received in the step 8.

The subsequent steps 10 to 12 are executed for each affected UE. For QoS updates, steps 10 and 11 are skipped.

10. [Conditional] For an MBS Service Area update, if the SMF previously subscribed at the AMF to notifications about the UE moving in or out of a subscribed "Area Of Interest", the SMF updates the subscription with the new MBS Service Area as area of interest.

11. [Optional] When the MBS Service Area is updated, if the SMF does not have the latest UE location, the SMF queries AMF which then query the NG-RAN for the current location of the UE to determine whether the UE is within the updated MBS Service Area.

12. [Conditional] For QoS Updates, if the 5GC Individual MBS traffic delivery is used, or if the associated QoS flows is to be added/removed, the SMF triggers PDU Session Modification procedure as defined in TS 23.502 [6]. If the 5GC Individual MBS traffic delivery is used, the SMF may also configure the UPF with the updated QoS and/or updated or removed QoS flows.

For MSK updates, the SMF also triggers PDU Session Modification procedure and provides the updated multicast session security context in the N1 SM container.

For MBS Service Area update, the SMF triggers the PDU Session Modification procedure as defined in TS 23.502 [6] with the following enhancement:

– The SMF also updates the PDU session resources associated to the multicast MBS session with the new MBS service area in an N2 container. The RAN node serving the PDU session starts or terminates transmission of multicast content in cells which are added or removed in the updated service area, respectively, and if necessary, interacts with the MB-SMF to start or terminate the distribution of multicast data to the RAN node.

– Towards the UE, the SMF provides the MBS service area in N1 SM container to the UE. For a UE previously inside the MBS service area but now outside the updated MBS service area of the multicast MBS session, the SMF may alternatively, based on operator policy, inform the UE in the N1 SM container that the UE has been removed from the multicast MBS session.

– Towards the NG-RAN, the SMF provides the updated MBS service area in N2 SM information. For a NG-RAN node supporting MBS, it starts transmission of multicast content in cells which are added in the updated MBS service area if UEs within the Multicast MBS session are within those cells, and if necessary, the NG-RAN interacts with the MB-SMF to start the distribution of multicast data to the RAN node. The RAN node stops transmission of multicast content in cells which are removed from the updated MBS service area, and if necessary, the NG-RAN interacts with the MB-SMF to terminate the distribution of multicast data to the RAN node

– For Individual delivery and a local Multicast MBS session the following applies: For a UE previously inside the service area but now outside the updated MBS service area, the SMF removes associated unicast QoS flows for the multicast MBS session. For a UE previously outside the service area but now inside the updated service area, the SMF adds associated unicast QoS flows for the multicast MBS session to the PDU session resources.