Section: 8.15.1.2 Multicast MBS Session Context Establishment
Content: Figure 8.15.1.2-1 illustrates an exemplified interaction of NGAP, E1AP, F1AP and RRC protocol functions for Multicast MBS Session Context Establishment with a UE joining an active multicast session as the first UE joining in its serving gNB. Figure 8.15.1.2-1: Multicast MBS Session Context establishment 0. For a certain multicast MBS session (TMGI), which is currently active, no multicast MBS session context is yet established in the gNB. 1. The UE joins the multicast session. NOTE 1: NAS related details for PDU Session signalling are not shown in Figure 8.15.1.2-1. 2/3. The gNB-CU-CP establishes the multicast bearer context at the gNB-CU-UP. For unicast NG-U transport the GTP DL TEID is retrieved. 4/5. The gNB-CU CP triggers the NGAP Distribution Setup procedure. For unicast NG-U transport, DL GTP TEID is provided to the 5GC. For multicast NG-U transport, multicast address information is retrieved from the 5GC. Multicast session QoS parameters are provided by the 5GC. 5a/5b. The gNB-CU-CP triggers the setup of MRB resources via the E1AP MC Bearer Context Modification procedure. For multicast NG-U transport the 5GC shared NG-U multicast address information is provided to the gNB-CU-UP. Multicast session QoS parameters are provided to the gNB-CU-UP. 6/7. The gNB-CU-CP establishes the Multicast Context at the gNB-DU, providing MRB configuration. It may contain MBS Area Session ID information. 8/9. The gNB-CU-CP retrieves the MRB configuration for the joined UE from the gNB-DU via F1 UE Context Management procedures. NOTE 2: The interactions between the gNB-CU-CP and gNB-CU-UP for UE specific Berarer Context management before and after step 8/9 are not illustrated in this Figure. 10. The gNB-DU triggers the establishment of F1-U resources, which are established either per gNB-DU or per cell or per MBS Area Session ID. 11/12. The gNB-CU-UP side of the F1-U is established by means of the E1AP MC Bearer Context Modification procedure, providing the gNB-DU side F1-U TNL to the gNB-CU-UP, which provides the gNB-CU-UP side F1-U TNL address in return. 13. The gNB-CU-UP side F1-U TNL address is provided to the gNB-DU. 14. In case of NG-U multicast transport, the gNB-CU-UP joins the NG-U multicast group. 15. The gNB successfully terminates the NGAP procedure for establishing the multicast session context. 16. The gNB-CU-CP RRC-configures each UE which has joined the multicast group. 17. The multicast MBS media stream is provided to the UEs. On NG-U, in case of location dependent multicast MBS Sessions, multiple shared NG-U transport tunnels may need to be setup, one per Area Session ID served by the gNB. In case of shared NG-U termination, - the gNB-CU-UP may provide the gNB-CU-CP at E1 setup or configuration update about established shared NG-U terminations, indicated by one or several MBS Session IDs. - at establishment of the MC bearer context in the gNB-CU-UP, the gNB-CU- CP may request the gNB-CU-UP to either apply the available MRB configuration of the shared NG-U termination, or to apply the MRB configuration requested by the gNB-CU-CP. The gNB-CU-UP provides the MRB configuration to the gNB-CU-CP if the MRB configuration requested by the gNB-CU-CP and the available MRB configuration of the shared NG-U termination are different.