5G System Learning---MBS Session Update

In a 5G broadcast system, session modification will update the PDU (Packet Data Unit) session; the update can be triggered by events such as the terminal device (UE), the network, or a radio link failure. The MBS session update process is specifically handled by the SMF, involving the UPF updating the user plane connection; then the UPF notifies the access network and AMF to modify session rules, QoS (Quality of Service), or other parameters.

I. Session Modification Initiation in 5G Systems can be triggered by multiple network elements, namely:

• UE-Initiated:The UE requests changes to its PDU session, such as modifying packet filters or QoS for a specific service.
• Network-Initiated:The network (typically a Policy Control Function (PCF)) initiates modifications, such as applying new policy rules or QoS changes.
• Access Network-Initiated: Events such as radio link failures, user inactivity, or mobility restrictions may trigger modifications, causing the AN to release the session or modify its configuration.
• AMF-Initiated:The AMF may also trigger modifications, such as due to unspecified network failures.

II. The MBS successful modification broadcast session modification procedure aims to request the NG-RAN node to update MBS session resources or areas related to previously established broadcast MBS sessions; this procedure uses non-UE associated signaling. A successful modification is shown in Figure 8.17.2.2-1, where:

MF initiates this process by sending a "BROADCAST SESSION MODIFICATION REQUEST" message to the NG-RAN node, in which:

• If the "Broadcast Session Modification Request" message contains an "MBS Service Area" IE, the NG-RAN node should update the MBS service area and send a "Broadcast Session Modification Response" message.
• If the "Broadcast Session Modification Request" message contains an "MBS Session Modification Request Transmission" IE, the NG-RAN node should replace the previously provided information with the newly received information and update the MBS session resources and area according to the request, and then send a "Broadcast Session Modification Response" message.
• If the "Broadcast Session Modification Request" message includes a "List of Supported User Equipment Types" IE (if supported), the NG-RAN node should consider this in the MBS session resource configuration.
• If the MBS NG-U fault indication IE is included in the broadcast session modification request message within the MBS session setup or modification request transmission IE and is set to "N3mb path failure," the NG-RAN node can provide new NG-U transport layer information to replace the failed transport layer information, or switch data transmission to another 5GC according to the N3mb path failure broadcast MBS session recovery procedure specified in TS 23.527.

III. MBS Modification Failure In the live network, NG-RAN nodes may experience broadcast session modification failures for various reasons; the modification failure is shown in Figure 8.17.2.3-1, where:

If an NG-RAN node fails to update any requested modifications, the NG-RAN node should send a "Broadcast Session Modification Failure" message.