1. Service continuity :The mobility of the terminal (UE) in 5G-supported multicast service (MBS), in principle, is the same as for other services in 5G (NR) systems.
2.Multicast switching :The mobility procedure for multicast reception allows the UE to continue to receive multicast services via PTM or PTP in the new cell after the switchover; where:
2.1.Switchover preparation phase :The source gNB transmits to the target gNB the UE context information of the MBS multicast sessions to which the UE has joined. in order to support the provision of local multicast services with location-dependent content (as described in TS 23.247 [45]) for each active multicast session, the target gNB may be provided with service area information for each regional session ID. The source gNB may propose data forwarding for certain MRBs to minimize data loss and may exchange the corresponding MRB PDCP sequence numbers with the target gNB during switchover preparation:
If the UE configures a PTP RLC AM entity in the target cell MRB, the MBS supports inter-cell switching and lossless switching of multicast services regardless of whether the UE configures a PTP RLC AM entity in the source cell.
To support lossless switching of multicast services, the network shall ensure synchronization and continuity of the DL PDCP COUNT values between the source and target cells. Additionally PDCP status reports from source gNB to target gNB data forwarding and/or UE for multicast session MRBs may be used during lossless handover.
2.2 Multicast Session Processing : For each multicast session that is performing user data transmission:
If MBS session resources do not exist on the target gNB. the target gNB triggers the setting of MBS user-plane resources to the 5GC using the NGAP distribution setup procedure.
If unicast transmission is used, the target gNB provides the DL tunnel endpoint to be used for MB-SMF.
If multicast transmission is used, the target gNB receives the IP multicast address from the MB-SMF.
2.3 Switchover execution :The MBS configuration decided by the target gNB during the period is sent to the UE via the source gNB within the RRC container (as described in TS38.331 [12]). the PDCP entity of the multicast MRB in the UE may be re-established or may remain as it is. When the UE connects to the target gNB. the target gNB sends an indication to the SMF that it is an MBS Supporting Node in a Path Switching Request message (Xn Switching) or Switching Request Acknowledgement message (NG Switching).
2.4 After Successful Switchover Completion : For any multicast session with no remaining joining UEs in the gNB, the source gNB may trigger the release of MBS user plane resources to the 5GC using the NGAP distribution release procedure.
