QoS Flow Failure Handling During N2 Handover
This section describes the flow failure handling procedure during the N2 handover.

Step | Description |
---|---|
1 |
The PDU session is established over S-AMF and SMF by communicating with UPF, PCF, or CHF for IPv4, IPv6, or dual-stack. The PCF provides Policy Control Request trigger for SM policy decision as response to the request for creation of SM policy control. The CHF provides session-level and rating-group-level triggers to the SMF as the Charging Data Create Response. |
2 | The T-AMF sends SM Context Update Request to the SMF. This request includes the information on handover state as preparing, UE location, UE time zone, target serving NF ID, and serving network. In case of inter-AMF handoff, the AMF includes the target serving NF ID. |
2a | The SMF identifies the access-side modifications that are received in SM Context Update Request. The charging triggers are identified through the information that is received in Step 2. |
2b | The PCF triggers are identified through the information that is received in Step 2. |
3 | The T-AMF sends the SM Context Update Request to the SMF. This request includes the information on handover state as prepared along with N2 message on Handover Required Transfer Request. The transfer request includes the list of failed QFIs. |
3a | The SMF validates the list of failed QFIs to extract the list of failed rule report and failed EBIs. |
4 | For N2 handover preparation procedures, the SMF sends the N4 Session Modification Request to the UPF to update the received DL tunnel information of T-gNB. After the tunnel information is updated, the UPF sends the usage report to the SMF as N4 modification response. The SMF retains the final SM Context Update Response as charging or PCF triggers are identified. |
5 | The SMF sends the Charging Data Update Request to the CHF. This request includes the details, such as session-level triggers, multi-unit information with rating-group-level triggers and usage report, customer identification information, and the PDU session charging information. |
6 | The CHF sends the Charging Data Update Response to the SMF. This response contains the multi-unit information along with new session-level or rating-group level triggers. |
6a | The SMF processes the Charging Data Update Response and updates the PDU session. The SMF does not send the N4 Mode Request to the UPF for the information that is received from the CHF. |
7 | The SMF sends the SM Context Update Response to the T-AMF with handover state as completed. This response also includes list of the released EBIs and the failed EBIs. |
8 | The SMF and T-AMF process the N1 N2 Transfer Request. This request includes the N1 message as PDU Session Modification Command to communicate the information on the QoS flow failure list to the UE. |
9 | The CHF sends the SM Context Update Request with an N1 message for the completion of the PDU session modification. |
10 | The SMF sends the SM Context Update Response as “200/204 OK” to the T-AMF. The SMF does not process the received N1 message from the UE. |
11 | The SMF posts the internal transaction to send the SM Policy Update for PCF triggers. The SMF sends this update to communicate the rule report about the failed QFIs or any armed access-side triggers. |
12 | The SMF sends the SM Policy Control Update to the PCF. This update includes the details on the user location and UE time zone. |
13 | The PCF sends the SM Policy Control Update response, which is the SM policy decision, to the SMF. |
14 | The SMF processes the SM policy decision and treats it as the PCF-initiated PDU Session Modification procedure. |