CHF and PCF Integration for Xn Handover Call Flow
This section describes the call flow for the CHF and PCF Integration for the Xn handover.

Step | Description |
---|---|
1 |
The PDU session is established over MME, SGW, 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 SMF as the Charging Data Create Response. |
2 | The AMF sends SM Context Update Request to the SMF. The SM Context Update Request includes the information on UE location, UE time zone, and path switch request N2 message. |
2a | The SMF detects access-side changes that are received in the SM Context Update Request and the charging triggers with the information that is available in Step 2. |
2b | The SMF detects the PCF triggers with the information that is available in Step 2. |
3 |
The Xn Handover Preparation procedure starts from SMF toward UPF and the opposite way, as defined in 3GPP TS 23.502, section 4.9.1.2. SMF sends the N4 Modification Request to UPF and updates the received DL tunnel information of T-gNB. After the tunnel information is updated, UPF provides the usage report as part of N4 modification response. The SMF holds the final SM Context Update Response when the SMF detects the CHF or PCF triggers. |
4 | The SMF sends the Charging Data Update Request to the CHF. This request includes the information on session-level triggers, multi-unit-Information (with rating-group-level triggers and usage report), customer identification, and PDU session charging information. |
5 | The CHF sends the Charging Data Update Response with optional multi-unit-information. The CHF also sends the new session or rating-group-level triggers to the SMF. |
5a | The SMF processes the Charging Data Update Response and updates the PDU session. The SMF does not send the N4 modification request to the UPF for the newly received information from the CHF. |
6 | The SMF posts the internal transaction to send the SM policy update information for PCF triggers. |
7 | The SMF sends the SM Context Update Response to the AMF. This response includes the path switch request acknowledgment N2 message. |
8 | The SMF sends the SM Policy Control Update information to the PCF. The SM Policy Control Update includes details, such as the user location information and UE time zone. |
9 | The PCF sends the SM Policy Control Update Response, which is the SM policy decision, to the SMF. |
10 | The SMF processes the SM policy decision that is received as response and handles the response as PCF Initiation Modify procedure, as defined in 3GPP TS 23.502, section 4.3.3.2. |