S7a (application id: 16777308)

The S7a interface enables the transfer of subscriber-related Closed Subscriber Group (CSG) data between the Mobility Management Entity (MME) and the Closed Subscriber Group Subscription Server (CSS) within a Visited Public Land Mobile Network (VPLMN). Similarly, the S7d interface serves the same purpose but operates between the Serving GPRS Support Node (SGSN) and the CSS.

The S7a interface's primary role is to facilitate the management of CSG subscription data for users when they are connected to a CSG cell, especially while roaming. It ensures that the user's access to the CSG is correctly authenticated and authorized based on their subscription data stored in the CSS. The S7a interface is responsible for updating the MME with the most current CSG subscription data, thereby maintaining the continuity of service and ensuring that the user has appropriate access rights.
This interface is particularly important in scenarios where users are moving between different network areas, such as during handovers between cells in a VPLMN, to ensure that their subscription data is up-to-date and that they can access the CSGs to which they are entitled.

S7a interface workflow:
The workflow of the S7a interface involves several key procedures to manage CSG subscription data efficiently:

1. Update VCSG Location Procedure:
   • Initiation: When a user equipment (UE) attaches to a CSG cell or initiates a tracking area update or routing area update, the MME initiates the Update VCSG Location procedure by sending an Update VCSG Location Request to the CSS.
   • Data Retrieval: The CSS checks whether the user is known and retrieves the corresponding CSG subscription data. This data is then sent back to the MME in an Update VCSG Location Answer.
   • Data Storage: Upon receiving the CSG subscription data, the MME updates its records with the new information and stores the CSS identity received in the Origin-Host AVP.
2. Handling of CSG Subscription Data:
   • If the user’s CSG subscription data is already available from a previous location update, the MME may set the "Skip Subscriber Data" flag to avoid redundant data retrieval.
   • If new subscription data is retrieved, the MME will delete any previously stored CSG data and replace it with the new data received from the CSS.
3. Error Handling:
   • If the CSS cannot find the user (e.g., the user is not known in the CSS), it returns an error code (e.g., DIAMETER_ERROR_USER_UNKNOWN) to the MME, which then handles the situation according to predefined procedures, such as rejecting the UE's request to access the CSG.
   • If there is a mismatch between CSG IDs in different records, the MME prioritizes the data from the CSS over any other sources.
4. Insert VCSG Subscriber Data Procedure:
   • This procedure is used when there are administrative changes in the user’s data that require updating the CSG subscription data in the MME or SGSN.
   • The CSS initiates this procedure, ensuring that the MME or SGSN maintains up-to-date subscription data, particularly when the user’s location or CSG subscription changes.

For complete technical specification of S7a interface in Diameter protocol please refer to: [3GPP TS 29.272]