S9a* (application id: 16777320)

The S9a* interface is a specialized reference point primarily utilized for Non-Seamless Wireless Offload (NSWO) scenarios in the context of interworking between a Policy and Charging Rules Function (PCRF) and a Broadband Policy Control Function (BPCF). The S9a* interface facilitates the transfer of Policy and Charging Control (PCC) rules and Quality of Service (QoS) rules between these entities, specifically for Fixed Broadband Access networks. The role of the S9a* interface is critical in enabling operators to manage policies and charging rules when a User Equipment (UE) is offloaded to a fixed broadband network while still maintaining consistency with the mobile network's policies.

The primary purpose of the S9a* interface is to ensure seamless policy enforcement and charging control across different access networks—specifically between the mobile network's PCRF and the fixed broadband network's BPCF. This interface is essential for maintaining QoS levels and applying consistent PCC rules even when a UE is offloaded from a mobile network to a fixed broadband network, a scenario common in NSWO cases.
In the architecture, the S9a* interface is located between the V-PCRF (Visited PCRF) or H-PCRF (Home PCRF) and the BPCF. The V-PCRF or H-PCRF acts as the central policy decision point, while the BPCF enforces these policies in the fixed broadband environment. The S9a* interface carries the necessary signaling messages to establish, modify, and terminate policy sessions in the fixed broadband network, ensuring that the UE experiences consistent service quality.

S9a* interface workflow:
The workflow of the S9a* interface involves several key steps:

1. Session Establishment:
   • When a UE connects to a fixed broadband network, and NSWO traffic is detected, the BPCF triggers the establishment of a session with the PCRF over the S9a* interface.
   • The BPCF sends a Credit Control Request (CCR) command to the PCRF with the CC-Request-Type AVP set to INITIAL_REQUEST.
   • The PCRF verifies the policy control for NSWO traffic based on operator policies. If the policy control is approved, the PCRF responds with a Credit Control Answer (CCA) containing the result in the Result-Code AVP.
   • If policy control cannot be performed, the PCRF sends a response with DIAMETER_AUTHORIZATION_REJECTED.
2. Session Modification:
   • Session modifications can be initiated by either the PCRF or the BPCF, depending on changes in policy information, such as subscription updates or requests from an Application Function (AF).
   • The initiating entity sends a Re-Authorization Request (RAR) command to modify the session, which may include changes to QoS rules or the addition/removal of charging rules.
   • The receiving entity validates the changes and responds with a Re-Authorization Answer (RAA) command, confirming or rejecting the modifications.
3. Session Termination:
   • When the UE is no longer attached to the fixed broadband network, the BPCF initiates the termination of the S9a* session by sending a CCR command with the CC-Request-Type AVP set to TERMINATION_REQUEST.
   • The PCRF acknowledges the termination with a CCA command, ensuring that the session is properly closed.

For complete technical specification of S9a* interface in Diameter protocol please refer to: [3GPP TS 29.215]