Gy (application id: 14)

The Gy interface is a component in the 3GPP-defined online charging system (OCS), connecting the Policy and Charging Enforcement Function (PCEF) to the Online Charging System (OCS). This interface is responsible for enabling real-time, session-based charging in mobile networks. Diameter protocol supports various charging models, such as session-based, event-based, or volume-based charging.

OCS is responsible for managing the real-time credit control and charging of user sessions. It monitors the user’s account balance, processes credit requests from the PCEF, and sends back charging responses that determine whether a service can continue based on the user’s available credit.

Gy interface workflow

1. Session Initialization:
   • When a user initiates a session, such as starting a data connection, the PCEF generates a Credit Control Request (CCR) and sends it to the OCS via the Gy interface. 
2. Credit Authorization by OCS:
   • Upon receiving the CCR, the OCS checks the user’s account balance to determine if there is sufficient credit to authorize the session. The OCS then responds with a Credit Control Answer (CCA), which includes the granted credit units (e.g., data volume or time) and instructions on how the PCEF should proceed with the session.
3. Ongoing Monitoring and Mid-Session Requests:
   • As the session continues, the PCEF monitors the user’s data usage or session duration. If the user approaches the allocated credit limit, the PCEF sends another CCR to the OCS to request additional credit.
4. Service Continuation or Termination:
   • If the OCS grants additional credit, the session continues seamlessly. If the OCS denies the request due to insufficient credit, the PCEF may terminate the session or reduce the service level, depending on the instructions received.
5. Session Termination:
   • At the end of the session, the PCEF sends a final CCR to the OCS to report the total usage and finalize the charging process. The OCS deducts the appropriate amount from the user’s balance, closes the session, and updates the billing system accordingly.

For functionality and use cases of the Gy interface please refer to: [3GPP TS 32.251]
For the Diameter charging applications details please refer to: [3GPP TS 32.299]