CHF and PCF Integration for N26 5G to 4G Handover Call Flow
This section describes the call flow for the CHF and PCF Integration for N26 5G to 4G handovers.

Step | Description |
---|---|
1 |
The PDU session is established over S-AMF or 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 5G to 4G Handover procedure starts from AMF toward the SMF or PGW-C and vice versa. AMF initiates the SM Context Retrieve Request to establish the UL PDRs and send SM Context Update Response to start the IDFT tunnel, if necessary. |
3 | In the N26 5G to 4G Handover Execution procedure, the SGW sends the GTPv2 Modify Bearer Request to PGW-C. This request includes the information on UE location, UE time zone, RAT type, and Bearer Context List. |
3a | 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 3. |
3b | The SMF detects the PCF triggers with the information that is available in Step 3. |
4 | In the N26 Handover 5G to 4G Execution procedure, the PGW-C requests UPF to create a GTP-U tunnel for each flow. This tunnel is for the EBIs received in the Bearer Context List of GTPv2 Modify Bearer Request. After the DL PDRs are established, UPF sends the usage report as part of N4 modification response to SMF. The SMF holds the final GTPv2 Modify Bearer Response when the SMF detects the CHF or PCF triggers. |
5 | 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. |
6 | 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. |
6a | The SMF or PGW-C 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. |
7 | The SMF or PGW-C posts the internal transaction to send the SM policy update information for PCF triggers. |
8 | The SMF or PGW-C sends the SM Context Update Response, for which the handover state is complete, to the AMF. |
9 | The SMF or PGW-C sends the SM Policy Control Update information to the PCF. The SM Policy Control Update includes details, such as the user location information, UE time zone, and serving network. |
10 | The PCF sends the SM Policy Control Update Response, which is the SM policy decision, to the SMF. |
11 | 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. |