QoS Flow Failure Handling During Xn Handover
This section describes the QoS flow failure handling during the Xn 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 AMF sends SM Context Update Request to SMF. This request includes the information on UE location, UE time zone, and N2 message path switch request with the list of the accepted QoS Flow Identifier (QFI). |
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 SMF extracts the list of failed QFI, failed rule report, and failed EPS bearer ID (EBI) from the received list of the accepted QFIs. |
2c | The PCF triggers are identified through the information that is received in Step 2. |
3 | For Xn 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. |
4 | 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. |
5 | The CHF sends the Charging Data Update Response to the SMF. This response may include the multi-unit information with new session or rating-group-level triggers. |
5a | The SMF processes the Charging Data Update Response and updates the PDU session. SMF does not send the N4 Mode Request to the UPF for the information that is received from the CHF. |
6 | The SMF sends the SM Context Update Response by including the N2 message path switch request acknowledgment and the list of failed EBI list. |
7 | The SMF and T-AMF process the N1 N2 Transfer Request for the PDU Session modification for the QoS about failed QFIs. The SMF includes the PDU Session Modification command to communicate the information on the QoS flow failure list to the UE. |
8 | The AMF sends the SM Context Update Request N1 message to the SMF to communicate about the handover completion. |
9 | The SMF sends the SM Context Update Response as “200/204 OK” to T-AMF. The SMF does not process the received N1 message from UE. |
10 | The SMF posts the internal transaction to send the SM Policy Update for PCF triggers to send to the PCF. The SMF posts this information to communicate Rule Report for the failed QFIs or any identified armed access-side triggers. |
11 | The SMF sends the SM Policy Control Update to the PCF. This update includes details, such as user location information and UE time zone. |
12 | The PCF sends the SM Policy Control Update Response, which is the SM policy decision, to the SMF. |
13 | The SMF processes the received SM policy decision and initiates the PCF modify procedures. |