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Content for  TS 38.401  Word version:  17.3.0

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8.15  Overall procedures for NR MBS |R17|p. 87

8.15.1  Generalp. 87

The following clauses describe the overall procedures for NR MBS involving E1 and F1.

8.15.1.1  Broadcast MBS Session Setupp. 87

Figure 8.15.1.1-1 illustrates an examplified interaction of NGAP, E1AP, F1AP and RRC protocol functions at Broadcast MBS Session Setup.
Reproduction of 3GPP TS 38.401, Fig. 8.15.1.1-1: Broadcast MBS Session Setup
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Step 1.
The 5GC starts the broadcast session by sending the NGAP Broadcast Session Setup Request message to the gNB containing the TMGI, S-NSSAI, 5G QoS Profile, area information and transport information (for NG-U multicast transport it provides the IP multicast address and the IP source specific multicast address, for NG-U unicast transport it provides an GTP UL TEID) and optionally an alternative set of transport information.
Step 2/3.
The gNB-CU-CP sets up the broadcast bearer context, providing NG-U transport information from the 5GC to the gNB-CU-UP and receiving from the gNB-CU-UP the NG-U GTP DL TEID in case NG-U unicast transport was selected and an F1-U GTP UL TEID per MRB.
Step 4.
In case of NG-U multicast transport, the gNB-CU-UP joins the NG-U multicast group.
Step 5/6.
The gNB-CU-CP establishes the Broadcast MBS Session Context at the DU, providing MRB configuration, other relevant session parameters and F1-U GTP UL TEID information, and receiving F1-U GTP DL TEID information.
Step 7/8.
The gNB-CU-CP triggers BC Bearer Context Modification Request towards the gNB-CU-UP to provide the F1-U GTP DL TEID information.
Step 9.
The DU configures broadcast resources and provides broadcast configuration information to the UEs by means of MCCH.
Step 10.
The gNB-CU CP successfully terminates the NGAP broadcast Session Setup procedure. In case the gNB has chosen NG-U unicast transport, NG-U GTP DL TEID information is provided to the 5GC.
Step 11.
The broadcast MBS media stream is provided to the UEs.
On NG-U, in case of location dependent broadcast 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 BC 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.
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8.15.1.2  Multicast MBS Session Context Establishmentp. 88

Figure 8.15.1.2-1 illustrates an examplified 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.
Reproduction of 3GPP TS 38.401, Fig. 8.15.1.2-1: Multicast MBS Session Context establishment
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Step 0.
For a certain multicast MBS session (TMGI), which is currently active, no multicast MBS session context is yet established in the gNB.
Step 1.
The UE joins the multicast session.
Step 2/3.
The gNB-CU-CP establishes the multicast bearer context at the gNB-CU-UP, in order to retrieve for unicast NG-U transport the GTP DL TEID, a shared resource address (GTP DL TEID).
Step 4/5.
The gNB-CU CP triggers the NGAP Distribution Setup procedure. For unicast transport, DL/UL GTP TEIDs are exchanged, for multicast transport, multicast address information is fetched from the 5GC.
Step 6/7.
The gNB-CU-CP establishes the Multicast Context at the DU, providing MRB configuration. It may contain MBS Area Session ID information.
Step 8/9.
The gNB-CU-CP retrieves the MRB configuration for the joined UE from the gNB-DU via F1 UE Context Management procedures.
Step 10.
The gNB-DU triggers the establishment of F1-U resources, which are established either per DU or per cell or per MBS Area Session ID. The receiving gNB-CU-CP fetches a gNB side F1-U TNL address information from the gNB-CU-UP by means of a E1AP MC Bearer Context Modification procedure.
Step 11/12.
The gNB-CU-UP side of the F1-U and NG-U UP entity is established by means of the E1AP MC Bearer Context Modification procedure, providing the DU side F1-U TNL address and the 5GC NG-U TNL address to the gNB-CU-UP, which provides the gNB-CU-UP side F1-U TNL address in return.
Step 13.
The gNB-CU-UP side F1-U TNL address is provided to the gNB-DU.
Step 14.
In case of NG-U multicast transport, the gNB-CU-UP joins the NG-U multicast group.
Step 15.
The gNB successfully terminates the NGAP procedure for establishing the multicast session context.
Step 16.
The gNB-CU-CP RRC-configures each UE which has joined the multicast group.
Step 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.
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8.15.1.3  Multicast MRB type re-configuration with F1-U ptp-retransmission tunnel establishmentp. 90

Reproduction of 3GPP TS 38.401, Fig. 8.15.1.3-1: Multicast MRB type re-configuration with F1-U ptp retransmission tunnel establishment
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Step 1.
The gNB-DU decides to change the MRB type configuration for a UE and provides the new configuration to the gNB-CU-CP along with a per-MRB MRB type reconfiguration indication and the MRB Reconfigured RLC mode.
Step 2.
The gNB-CU-CP confirms the modification request and indicates this to the gNB-DU. The gNB-CU-CP may request the gNB-DU to establish F1-U ptp retransmission resources in order for the UE to perform PDCP Status Report and receive retransmitted PDCP PDUs for the reconfigured MRBs.
Step 4-7.
If necessary, the gNB-DU requests the establishment of F1-U resources for ptp retransmission by means of triggering the F1AP Multicast Distribution Setup procedure, which in turn triggers the E1AP MC Bearer Context Modification procedure to exchange F1-U tunnel end-point addresses.
Step 8-9.
The RRC Reconfiguration procedure is performed
Step 10.
PDCP Status Report and PDCP PDU retransmission can take place via the established F1-U ptp retransmission resources.
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