Section: 6.5 MBS Session associations in NG-RAN Node
Content: The following MBS Session associations are defined in the NG-RAN node to support NR MBS: NG-RAN MBS session resource context: Encompasses CP and UP, transport and radio resources to support an MBS Session. For multicast it also encompasses the MBS Session state (active, de-activated) and information about joined UEs. If an MBS session resource within a gNB serves multiple MBS service areas, as specified in TS 23.247[ Architectural enhancements for 5G multicast-broadcast services ]  [27] the same NG RAN MBS session resource context may be associated with multiple NG-U resources. For a multicast MBS session, NG-U resources are setup or released by the gNB upon UE mobility or UEs leaving or joining the multicast MBS session. MBS Session context in a gNB-DU: The definition of an MBS Session context in a gNB-DU applicable for broadcast and multicast. An MBS Session context in a gNB-DU - is a block of information associated to an MBS Session, which may consist of one or several MRB Contexts; - corresponds to either one or several F1-U tunnels. MRB Context in a gNB-DU: An MRB Context is a block of information in a gNB-DU associated to an MRB (MRB “instance”). The gNB-DU sets up resources for each MRB Context in a gNB-DU associated to an MBS Session context - based on information provided within MBS Session Context related information as received by the gNB-DU (e.g. MRB QoS, MBS service area information, etc.), and, - for multicast, based on the UE Contexts established for RRC_CONNECTED UEs within the gNB-DU containing joining information of the UE for the respective multicast session. - for broadcast, the gNB-DU determines whether F1-U tunnels are setup per gNB-DU or per MBS Area Session ID served by the gNB-DU. - for multicast, the gNB-DU determines whether F1-U tunnels are setup per gNB-DU or per cell served by the DU or per MBS Area Session ID served by the gNB-DU or for ptp restransmissions or for a ptp-only MRB leg. For multicast, for each MRB, the gNB-DU provides the MRB specific Uu configuration to the gNB-CU to configure the UE. Multicast F1-U Context: A Multicast F1-U Context is a block of information in a gNB-DU to control the F1-U tunnels associated to the MRB Contexts established for a multicast MBS session. A Multicast F1-U Context is either established per gNB-DU or per cell served by the gNB-DU or per MBS Area Session ID served by the gNB-DU or for ptp restransmissions or for ptp forwarding or for a ptp-only MRB leg. Several Multicast F1-U contexts may exist in parallel in a gNB-DU for the same multicast MBS session. Allocation and usage of MRB ID values on NG-RAN interfaces for multicast MBS sessions: - F1 interface: an MRB ID signalled on an F1 interface instance identifies uniquely an MRB among all MRB contexts in an gNB-DU, allocated for all active multicast MBS sessions served by that gNB-DU. The value of each MRB ID is the same value as communicated to UEs served by that gNB-DU. - E1 interface: an MRB ID signalled on an E1 interface instance identifies uniquely an MRB among all MRBs allocated for a multicast MBS session. - Xn interface, NG interface: MRB IDs are signalled on Xn/NG interfaces for providing MBS QoS flow to MRB mapping information and data forwarding information from the source gNB. The value of the MRB ID signalled on the Xn/NG interface is the same value as communicated to UEs at the source cell. Allocation and usage of MRB ID values on NG-RAN interfaces for broadcast MBS sessions: - An MRB ID signalled on NG-RAN interfaces identifies uniquely an MRB among all MRBs allocated for a broadcast MBS sesssion. MBS-associated logical F1/E1-connection: F1AP and E1AP provide means to exchange control plane messages associated with an MBS session over the respective F1/E1 interface. An MBS-associated logical connection is established during the first F1AP/E1AP message exchange between the F1/E1 peer nodes. The connection is maintained as long as MBS associated F1AP/E1AP messages need to be exchanged over the F1/E1 interface. The MBS-associated logical F1-connection uses the identities gNB-CU MBS F1AP ID and gNB-DU MBS F1AP ID. The MBS-associated logical E1-connection uses the identities gNB-CU-CP MBS E1AP ID and gNB-CU-UP MBS E1AP ID. When a node (DU or CU or CU-CP and CU-UP) receives an MBS associated F1AP/E1AP message the node retrieves the associated MBS session based on the F1AP/E1AP ID. MBS-associated signalling: MBS-associated signalling is an exchange of F1AP/E1AP messages associated with one MBS session over the MBS-associated logical F1/E1-connection.