S9a (application id: 16777319)

The S9a interface is a crucial component in the Diameter protocol, designed specifically for Fixed Broadband Access Network (FBAN) interworking within the Policy and Charging Control (PCC) architecture. It enables the transfer of Quality of Service (QoS) and Policy and Charging Control (PCC) rules between the Broadband Policy Control Function (BPCF) in the Fixed Broadband Access Network and the Policy and Charging Rules Function (PCRF) in the Packet Data Network (PDN). The S9a interface is primarily used in scenarios where there is a need to manage and enforce policy control and QoS for users accessing services over a fixed broadband network.

The S9a interface connects the BPCF, which is located in the Fixed Broadband Access Network, with the PCRF, which resides in the Packet Data Network. This setup is essential for ensuring that the policies governing QoS and charging in the broadband network are aligned with those in the PDN.

The S9a interface operates in two key scenarios:

• Non-Roaming Scenario: In this scenario, the BPCF communicates directly with the PCRF in the user's home network via the S9a interface. The PCRF in the home network (H-PCRF) manages the QoS and PCC rules, which are then transferred to the BPCF to ensure consistent policy enforcement across the broadband network.
• Roaming Scenario: When a user accesses services while roaming, the S9a interface facilitates communication between the BPCF and the V-PCRF (Visited PCRF) in the visited network. The V-PCRF interacts with the H-PCRF to retrieve the necessary policies, which are then enforced by the BPCF in the visited network.

S9a interface workflow:

1. Session Establishment:
   • When a user initiates a session in the Fixed Broadband Access Network, the BPCF sends a session establishment request to the PCRF over the S9a interface. This request includes information such as the user's IP address, access network information, and requested QoS parameters.
2. Policy Decision and Rule Transfer:
   • The PCRF evaluates the session request based on the user's subscription profile and the network conditions. It then generates the appropriate PCC and QoS rules. These rules are transferred to the BPCF over the S9a interface, ensuring that the fixed broadband network enforces the same policies as those defined by the PCRF.
3. Rule Enforcement:
   • Upon receiving the PCC and QoS rules from the PCRF, the BPCF enforces these rules within the Fixed Broadband Access Network. This ensures that the user's service quality is maintained according to the policies defined in the PDN.
4. Session Modification and Termination:
   • During the session, if there is a need to modify the QoS or PCC rules (for example, due to changes in user behavior or network conditions), the PCRF communicates these modifications to the BPCF via the S9a interface. When the session ends, the BPCF sends a termination message to the PCRF.

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