Content for TS 38.401 Word version: 18.0.0

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8.15 Overall procedures for NR MBS 8.15.1 General 8.15.1.1 Broadcast MBS Session Setup 8.15.1.2 Multicast MBS Session Context Establishment 8.15.1.3 Multicast MRB type re-configuration with F1-U ptp-retransmission tunnel establishment
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8.15 Overall procedures for NR MBS |R17| p. 99

8.15.1 General p. 99

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

8.15.1.1 Broadcast MBS Session Setup p. 99

Figure 8.15.1.1-1 illustrates an exemplified 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

Figure 8.15.1.1-1: Broadcast MBS Session Setup

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).

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 gNB-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 gNB-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.

8.15.1.2 Multicast MBS Session Context Establishment p. 100

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.

Reproduction of 3GPP TS 38.401, Fig. 8.15.1.2-1: Multicast MBS Session Context establishment

Figure 8.15.1.2-1: Multicast MBS Session Context establishment

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. For unicast NG-U transport the GTP DL TEID is retrieved.

Step 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.

Step 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.

Step 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.

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 gNB-DU or per cell or per MBS Area Session ID.

Step 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.

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.

8.15.1.3 Multicast MRB type re-configuration with F1-U ptp-retransmission tunnel establishment p. 102

Reproduction of 3GPP TS 38.401, Fig. 8.15.1.3-1: Multicast MRB type re-configuration with F1-U ptp retransmission tunnel establishment

Figure 8.15.1.3-1: Multicast MRB type re-configuration with F1-U ptp retransmission tunnel establishment

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.