S13 (application id: 16777252)

The S13 interface is an essential component within the Diameter protocol framework, facilitating secure communication between the Mobility Management Entity (MME) or Serving GPRS Support Node (SGSN) and the Equipment Identity Register (EIR). This interface is primarily used for the Mobile Equipment (ME) Identity Check Procedure, ensuring that mobile devices attempting to access the network are legitimate, not stolen, and comply with network policies.

The main purpose of the S13 interface is to verify the identity of mobile devices through their International Mobile Equipment Identity (IMEI). The verification process ensures that only authorized devices are allowed to connect to the network, thereby enhancing overall network security and integrity. This check helps to prevent stolen, counterfeit, or unauthorized devices from gaining network access.

Architecture of the S13 Interface

• MME/SGSN: These entities manage the mobility and session control for mobile devices within the network. When a device attempts to connect, the MME or SGSN triggers the ME Identity Check via the S13 interface to verify the device's IMEI.
• EIR: The Equipment Identity Register is a database that stores and categorizes IMEI numbers into three lists:
• White List: Devices allowed to access the network.
• Black List: Devices that are barred from accessing the network.
• Grey List: Devices that may have restricted access or are flagged for monitoring.
• The S13 interface enables the exchange of messages between the MME/SGSN and the EIR using the Diameter protocol, specifically tailored for secure and reliable transmission of IMEI verification data.

S13 interface workflow: 
The S13 interface follows a specific workflow to perform the ME Identity Check:

1. ME Identity Check Request:
   • When a mobile device attempts to connect to the network, the MME or SGSN sends a ME-Identity-Check-Request (ECR) message to the EIR via the S13 interface.
   • This request includes the device's IMEI, and optionally, the IMSI (International Mobile Subscriber Identity) and software version, if available.
2. Processing by the EIR:
   • The EIR receives the ECR message and checks the provided IMEI against its internal database.
   • The EIR determines whether the IMEI is on the White, Black, or Grey list. If necessary, it may also consider the software version to verify the device further.
3. ME Identity Check Answer:
   • The EIR responds to the MME/SGSN with a ME-Identity-Check-Answer (ECA) message, containing the verification result.
   • If the IMEI is valid and allowed (on the White list), the EIR sends a DIAMETER_SUCCESS result code.
   • If the IMEI is unknown or on the Black list, the EIR returns an error code, such as DIAMETER_ERROR_EQUIPMENT_UNKNOWN.
4. Action by MME/SGSN:
   • Based on the ECA response, the MME or SGSN will determine whether to allow or deny the device's network access.
   • If the device is authorized, the network connection proceeds. If not, the MME/SGSN may reject the connection attempt, thus enforcing network security policies.

For complete technical specification of S13 interface in Diameter protocol please refer to: [3GPP TS 29.272]