BSF is used for binding an application-function request to a specific Policy Control Function (PCF) instance. It is comparable to Policy and Charging Rules Function (PCRF) binding function provided by a 4G Diameter Routing Agent (DRA), for VoLTE and VoWiFi.
If multiple PCFs are deployed on a 5G network, the BSF queries session binding data for the PCF serving a subscriber, such as the IP address or DNN. Afterwards, it sends the PCF information to the NEF or AF. The NEF or AF then can route messages received from the subscriber over different interfaces (N7, N5, and Rx) to the target PCF during data and voice services.
5G voice services are still provided by the IMS network. To support VoNR services for 5G subscribers, the BSF needs to be able to interwork with the DRA on the IMS network over the Rx interface. The BSF can act as a proxy agent or a redirect agent to help the DRA address a PCF, as defined in 3GPP TS 29.513:
- As a proxy agent: After receiving an AAR message from the DRA, the BSF obtains the host name of the target PCF, includes the PCF host name in the AAR message, and sends the message to the target PCF over the Diameter link.
- As a redirect agent: After receiving an AAR message from the DRA, the BSF obtains the host name of the target PCF and returns a response carrying the PCF host name to the DRA. The DRA then redirects the AAR message to the target PCF.
