SLg (application id: 16777255)

The SLg interface is a crucial component within the Evolved Packet Core (EPC) that facilitates location-based services (LBS). It operates between the Gateway Mobile Location Center (GMLC) and the Mobility Management Entity (MME) or Serving GPRS Support Node (SGSN). The primary function of the SLg interface is to enable the exchange of location information between these network elements to support various location services.

The SLg interface is used to:

• Request and provide subscriber location information: The GMLC requests the location of a target user equipment (UE) from the MME or SGSN using the Provide Subscriber Location message. The MME or SGSN responds with the location estimate, which may include latitude, longitude, and other positioning details.
• Handle unsolicited location reports: The MME or SGSN can send an unsolicited location report to the GMLC when it detects a location event, such as a change in the UE's location.
• Support emergency services: The SLg interface is also used in emergency scenarios to provide location information to the GMLC when a UE makes an emergency call.
• Positioning support: The SLg interface supports different positioning procedures, including Mobile Terminated Location Request (MT-LR) and Mobile Originated Location Request (MO-LR), which are essential for services like navigation, tracking, and emergency location reporting.

SLg interface workflow:

1. Initiation of Location Request:
   • The process begins when the GMLC needs to determine the location of a specific UE. It sends a Provide Subscriber Location Request to the MME or SGSN over the SLg interface.
   • The request specifies the type of location service needed (e.g., emergency, commercial) and includes the identifier of the target UE.
2. Processing by MME/SGSN:
   • Upon receiving the request, the MME or SGSN retrieves the necessary positioning information, which may involve interacting with other network elements like the eNodeB (in LTE) or Base Station Subsystem (BSS) (in 3G networks).
   • The MME or SGSN performs the necessary calculations or retrieves the latest location estimate available.
3. Location Response:
   • The MME or SGSN sends back a Provide Subscriber Location Answer message to the GMLC, containing the requested location information.
   • If the location request cannot be fulfilled (e.g., due to the UE being unreachable), the response includes an appropriate error or status code.
4. Unsolicited Location Reporting:
   • In cases where the MME or SGSN detects a significant location event (e.g., a sudden movement of the UE), it can send a Subscriber Location Report to the GMLC without an explicit request. This report ensures the GMLC always has the most accurate and up-to-date location data for the UE.

For complete technical specification of SLg interface in Diameter protocol please refer to: [3GPP TS 29.172]