Sy (application id: 16777302)

The Sy interface is a critical reference point within the Diameter protocol architecture, primarily designed to facilitate communication between the Policy and Charging Rules Function (PCRF) and the Online Charging System (OCS). This interface plays a vital role in managing policy control and charging for subscribers based on their spending limits.

The primary purpose of the Sy interface is to enable the PCRF to make informed policy decisions by retrieving subscriber spending limit information from the OCS. It supports functions such as requesting policy counter statuses, subscribing to or unsubscribing from spending limit reports, and receiving notifications of changes in spending limit statuses.
The Sy interface is essential in scenarios where the PCRF needs to adjust policy decisions dynamically based on a subscriber's spending behavior, ensuring that the network enforces appropriate charging rules and quality of service (QoS) settings.

In the overall PCC (Policy and Charging Control) architecture, the Sy interface connects the PCRF and the OCS. The PCRF acts as a client that requests and utilizes policy counter status information, while the OCS serves as the server that maintains these counters and provides the necessary data.
The Sy interface is implemented over the Diameter protocol, utilizing Diameter commands and Attribute-Value Pairs (AVPs) specific to the Sy application. 

Sy interface workflow:

1. Initial Spending Limit Report Request: 
   • When the PCRF determines that it needs information about a subscriber's spending limit to make a policy decision, it sends an Initial Spending Limit Report Request to the OCS over the Sy interface.
   • The request includes details about which policy counters are required. If necessary, the PCRF may also subscribe to ongoing updates regarding these counters.
2. Spending Limit Report Provisioning:
   • Upon receiving the request, the OCS processes it and retrieves the requested policy counter statuses.
   • The OCS then sends a Spending Limit Report Answer back to the PCRF, providing the current statuses of the requested counters.
   • If the PCRF has subscribed to updates, the OCS will notify the PCRF of any changes in these counters over time.
3. Intermediate and Final Spending Limit Reports:
   • As the subscriber's spending changes, the OCS generates intermediate reports to update the PCRF with the latest status of the counters.
   • Once the subscriber reaches a specific spending threshold or a billing cycle completes, the OCS sends a final report to the PCRF, summarizing the final status of the counters.
4. Policy Decision and Enforcement:
   • The PCRF uses the information provided by the OCS to make real-time policy decisions. These decisions could involve adjusting the subscriber's QoS settings, modifying access policies, or triggering specific charging rules.
   • The PCRF then communicates these decisions to other network elements, such as the Policy and Charging Enforcement Function (PCEF), to enforce the new policies.
5. Session Termination:
   • Once the PCRF determines that it no longer needs updates from the OCS (e.g., the subscriber session ends or the required information is no longer relevant), it sends a request to terminate the Sy session.
   • The OCS acknowledges this request and ceases to send further updates.

For complete technical specification of Sy interface in Diameter protocol please refer to: [3GPP TS 29.219]