S15 (application id: 16777318)

The S15 interface is a component in the management of Quality of Service (QoS) within the Home Node B (HNB) and Fixed Broadband Access Network environments. This interface enables communication between the HNB Gateway (HNB GW) and the Policy and Charging Rules Function (PCRF) or Virtualized PCRF (V-PCRF). The primary function of the S15 interface is to facilitate the dynamic provisioning, modification, and removal of QoS rules, ensuring that network resources are optimally allocated for HNB circuit-switched (CS) calls.

Architecture of the S15 Interface

• PCRF/V-PCRF acts as the Diameter server, responsible for making policy decisions and managing QoS rules.
• HNB GW serves as the Diameter client, responsible for enforcing the QoS rules provided by the PCRF.
• This relationship allows the PCRF to control and monitor the QoS resources in real-time, ensuring that the network can dynamically adapt to changing conditions and service demands.

S15 interface workflow: 

1. Session Initialization and Connection Establishment:
   • The HNB GW initiates a Diameter connection with the PCRF. Once the transport connection is established, both entities advertise their support for the S15 application by including the Auth-Application-Id (16777318) and the Vendor-Id (10415) in the Capabilities-Exchange-Request (CER) and Capabilities-Exchange-Answer (CEA) commands.
2. QoS Rule Provisioning:
   • When a service request that requires QoS management is initiated, the HNB GW sends a Diameter request message to the PCRF. This message includes relevant AVPs (Attribute-Value Pairs) that describe the service's QoS requirements and the current network conditions.
3. Policy Decision and Resource Allocation:
   • The PCRF evaluates the service request against the network's current state and policy rules. Based on this evaluation, the PCRF determines the appropriate QoS rules to apply and responds to the HNB GW with the necessary instructions to enforce these rules.
4. Resource Management:
   • The HNB GW applies the QoS rules as instructed by the PCRF. The PCRF may monitor the application of these rules and adjust them as necessary based on ongoing network conditions or changes in the service requirements.
5. Session Termination:
   • Upon completion of the service or session, the HNB GW sends a message to the PCRF to terminate the session. The PCRF then triggers the removal of the QoS rules, freeing up network resources for other services.
6. Failure Handling:
   • The S15 interface includes mechanisms to handle failures in the QoS provisioning process. If an error occurs during the resource allocation or enforcement, the PCRF and HNB GW communicate to resolve the issue, ensuring that service quality is maintained or appropriately adjusted.

For complete technical specification of S15 interface in Diameter protocol please refer to: [3GPP TS 29.212]