5G: Do we need a BSF?

5G Core (5GC) is the heart of a 5G mobile network; it handles a wide variety of essential functions in the mobile network, such as connectivity and mobility management, authentication and authorization, and policy management, among others. Like 4G networks, the new 5G core domain supports the PCC (Policy and Charging Control) architecture where the PCF (Policy Control Function) takes the role of a central network component interacting with other network functions (SMF, AMF, NEF/AF…) and feeds them with policy rules needed for requesting and managing service quality on a per session basis.

With the advent of 5G, PCC networks are expected to support massive number of devices with various data, transmission, and bandwidth requirements. These requirements are driven mainly by use cases ranging from mobile broadband (eMBB), ultra-reliable low-latency communications (URLLC), and massive machine-type communications (mMTC). To cope with the growing number of connected devices and huge amount of policy traffic, Communication Service Providers (CSP) are opting for deploying multiple PCFs as a scalability solution and thus securing their policy networks. However, having numerous PCFs in a network comes at a cost, mainly increasing the complexity of selecting a PCF when many NFs are getting involved during a session.

In PCC architecture with multiple PCFs, policy rules shall be applied from the access network to the core with consistency and the PCF needs to be able to maintain the relations of different sessions over different interfaces. Therefore, all messages of the same subscriber from different interfaces and for a specific session must be routed to the same PCF. This routing mechanism is called “Session Binding” and it is handled by a new 5G network function called BSF (Binding Support Function).

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BSF in 5G - Binding Support Function (BSF)

  • 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

    • 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.
    • 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.

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