Sd (application id: 16777303)

The Sd interface is a component of the Policy and Charging Control (PCC) architecture in mobile networks. This interface is established between the Policy and Charging Rules Function (PCRF) and the Traffic Detection Function (TDF). It plays a crucial role in the detection, reporting, and control of application-specific traffic within the network. The Sd interface leverages the Diameter protocol for communication, facilitating real-time exchange of Application Detection and Control (ADC) rules and associated information between the PCRF and the TDF.

The primary purpose of the Sd interface is to enable the PCRF to control and manage the behavior of application traffic as detected by the TDF. The PCRF provides ADC rules to the TDF, which in turn monitors network traffic to detect applications that match these rules. Once an application is detected, the TDF reports this event to the PCRF, which then enforces the relevant policies, such as Quality of Service (QoS) adjustments or charging mechanisms.
Architecture and Key Components

• PCRF (Policy and Charging Rules Function): The PCRF is responsible for making policy decisions in real-time and enforcing these policies across the network. It determines how resources are allocated and manages charging based on the detected application traffic reported by the TDF.
• TDF (Traffic Detection Function): The TDF monitors network traffic to detect specific applications based on the ADC rules provided by the PCRF. It is responsible for reporting these detections back to the PCRF, enabling the network to apply the necessary controls and policies.
• Sd Reference Model: The Sd interface connects the PCRF and the TDF, forming a logical link that enables the exchange of ADC rules and application traffic information. This interface is depicted in the PCC architecture diagrams provided by 3GPP, illustrating how it fits within the broader network ecosystem.

Sd interface workflow:

1. Provisioning of ADC Rules:
   • The PCRF provides ADC rules to the TDF over the Sd interface. These rules define which applications the TDF should monitor and the specific conditions under which the traffic should be reported. ADC rules can be either dynamic, responding to real-time network conditions, or predefined, set up in advance based on expected traffic patterns.
2. Application Traffic Detection:
   • As the TDF monitors network traffic, it uses the ADC rules to identify specific application flows. When an application that matches one of these rules is detected, the TDF prepares a report containing details such as the application identifier, the associated IP addresses, and any relevant event triggers (e.g., application start or stop).
3. Reporting to PCRF:
   • The TDF sends this report to the PCRF via the Sd interface. The report includes critical information that the PCRF requires to enforce the appropriate policies. This could involve adjusting QoS parameters, modifying charging rules, or implementing other control measures depending on the nature of the detected traffic.
4. Policy Enforcement:
   • Upon receiving the application detection report, the PCRF evaluates the information and enforces the relevant policies. For example, if the detected application requires high bandwidth, the PCRF might increase the QoS for that traffic. Alternatively, if the application is identified as chargeable, the PCRF might update the charging mechanisms accordingly.
5. Solicited vs. Unsolicited Reporting:
   • The Sd interface supports both solicited and unsolicited reporting. In solicited reporting, the PCRF explicitly requests the TDF to monitor certain applications and report on them. In unsolicited reporting, the TDF autonomously monitors traffic and reports to the PCRF based on pre-configured ADC rules without needing explicit requests.

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