SLh (application id: 16777291)

The SLh interface is a key reference point for supporting Location Services (LCS) within GSM, UMTS, and EPS networks. The SLh interface facilitates communication between the Gateway Mobile Location Center (GMLC) and the Home Subscriber Server (HSS). It primarily functions to enable the GMLC to obtain routing information necessary for locating a mobile subscriber (User Equipment, or UE) within the network.

In the context of Location Services, the SLh interface connects the GMLC to the HSS. The GMLC is responsible for handling location requests from external clients, such as emergency services or lawful interception authorities, and coordinating the retrieval of location information for a target UE. The HSS, on the other hand, stores and manages subscriber data, including information on the current serving node (e.g., MME, MSC, or SGSN) for each UE.
The SLh interface enables the GMLC to query the HSS for this routing information. This data is essential for the GMLC to accurately direct location requests to the correct network entity that can provide the UE's real-time location.

The primary purpose of the SLh interface is to allow the GMLC to retrieve routing information from the HSS. This routing information typically includes details such as the address of the Home GMLC (H-GMLC), the address of the visited MSC/MSC server, SGSN, or MME that is currently serving the UE. This information is crucial for the GMLC to effectively process and fulfill location requests.

SLh interface workflow:

1. Location Request Initiation: An external Location Services client, such as emergency services, initiates a location request for a specific UE. This request is forwarded to the GMLC.
2. Routing Information Query: Upon receiving the location request, the GMLC uses the SLh interface to send a "Send Routing Information for LCS" request to the HSS. The purpose of this request is to obtain the routing information required to locate the UE.
3. Processing by HSS: The HSS processes the request and retrieves the relevant routing information, such as the address of the node (e.g., MME, MSC) currently serving the UE.
4. Response to GMLC: The HSS returns the requested routing information to the GMLC via the SLh interface.
5. Forwarding the Location Request: With the routing information now available, the GMLC forwards the location request to the appropriate serving node (e.g., MME, MSC) to retrieve the UE's location data.
6. Location Data Delivery: The serving node processes the request and sends the UE’s location information back to the GMLC.
7. Completion: The GMLC then forwards the obtained location data to the requesting LCS client, thereby completing the process.

For complete technical specification of SLg interface in Diameter protocol please refer to: [3GPP TS 29.173]