7.4 MBS procedures for inter System Mobility

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

7.4.1 Inter-system mobility with interworking at service layer

For inter-system mobility with interworking at service layer, i.e. the same multicast service is provided via eMBMS in E-UTRAN and MBS, the UE is instructed to switch between MBS and eMBMS:

NOTE 1: The source network can be configured with the frequencies where the same service is provided in the target network, i.e. the EPC network neighbouring the 5GC network support the same eMBMS service.

NOTE 2: The UE can be configured, per TMGI, to know that the same service is provided over MBS and eMBMS.

– Mobility from MBS to eMBMS.

When moving to E-UTRAN/EPC, the UE initiates procedures as defined in TS 23.246 [8] to receive MBMS service for the TMGI(s).

If the UE has one or more unicast PDU Sessions (including, but not limited to, the PDU Session used for MBS and for another service (e.g. Public Safety service) with the QoS Flow(s) for the other service) moving to EPS, and if the handover procedure from 5GS to EPS using N26 interface described in clause 4.11.1.2.1 of TS 23.502 [6] is used:

– For the PDU Session used also for MBS, the SMF+PGW-C removes the UE from the Multicast MBS session context(s), if it exists, upon receiving a Modify Bearer Request of the PDU Session from the SGW (i.e. step 14a of clause 4.11.1.2.1 of TS 23.502 [6]).

– The NG-RAN removes the UE from the Multicast MBS session context(s) if it exists, or removes the whole multicast session context if the UE is the last one for the Multicast MBS session (e.g. after receiving the UE Context Release Command message sent by the AMF).

For 5GS to EPS Idle mode mobility with no N26, when the UE moves to the EPS and performs E-UTRAN/EPS attach according to step 8 of clause 4.11.2.4.1 of TS 23.502 [6], if the UE does not maintain registration in 5GC, upon reachability time-out, the AMF may implicitly detach the UE and release the possible remaining PDU Session(s) in 5GC. The SMF/PGW-C removes the UE from the Multicast MBS session context(s), if it exists, upon receiving a tracking area update from the UE.

– Mobility from eMBMS to MBS.

When the UE has moved to NR/5GC, it triggers the multicast context and multicast flow setup/modification via PDU Session Modification procedures as defined in clause 6.8 to receive MBS transport for the TMGI(s).