Interworking Mobile Terminated Location Request

Description

IW-MT-LR is a standardized procedure defined in 3GPP TS 23.271 for performing a Mobile Terminated Location Request for a User Equipment (UE) that is currently attached via an Interworking Wireless Local Area Network (I-WLAN). The I-WLAN architecture allows UEs to access 3GPP packet-switched services through a trusted WLAN access network, integrating it with the core network. The IW-MT-LR process is initiated by an external Location Service (LCS) Client, which sends a location request to the Gateway Mobile Location Centre (GMLC) in the home Public Land Mobile Network (HPLMN). The GMLC then routes the request through the core network to the appropriate entity serving the UE over the I-WLAN.

Architecturally, the request traverses from the GMLC to the Home Subscriber Server (HSS) to retrieve the UE's serving node information. For an I-WLAN connected UE, this serving node is the Packet Data Gateway (PDG) or, in later architectures, the evolved Packet Data Gateway (ePDG) or Trusted WLAN Access Gateway (TWAG). The location request is then forwarded to the UE's serving node. The actual positioning mechanism depends on the capabilities of the UE and the I-WLAN. It can utilize network-based methods (e.g., using WLAN access point information), UE-based methods (where the UE calculates its own position using, for example, GNSS), or UE-assisted methods. The resulting location estimate is then formatted and sent back through the chain to the originating LCS Client.

This procedure is a critical component of the Interworking WLAN Location Services (I-WLAN LCS) architecture. It ensures that regulatory, emergency, and commercial location services (like friend-finder or asset tracking) can be provided consistently, regardless of whether the UE is connected via a 3GPP radio access network or a trusted WLAN. The process involves standard interfaces like the Le interface between the LCS Client and GMLC, and the Lg interface between the GMLC and the HSS/HLR, with extensions defined for the I-WLAN specific nodes. Security and privacy are paramount, requiring authorization of the LCS Client and adherence to the user's privacy settings (LCS Client authorization, subscriber privacy profile check) before any location information is disclosed.

Purpose & Motivation

IW-MT-LR was introduced to extend the mature cellular Location Services (LCS) framework to users connected via trusted WLAN networks. As 3GPP networks evolved to integrate WLAN as a trusted access technology (I-WLAN), a gap existed for providing standardized, network-initiated location services for these users. Without IW-MT-LR, external applications could not request the location of a user on WLAN in a standardized, secure, and privacy-compliant manner, limiting service continuity.

The primary motivation was service parity and convergence. Operators wanted to offer the same portfolio of value-added LCS services—such as emergency location, lawful intercept, fleet management, and location-based charging—to subscribers regardless of their access network. Prior to standardization, ad-hoc solutions for WLAN location were proprietary and not integrated with the core network's subscriber and privacy management systems. IW-MT-LR solves this by leveraging the existing LCS architecture and interfaces, applying them to the I-WLAN context. It addresses the problem of how to route a location request to a UE that is not attached to a traditional MSC, SGSN, or MME, but to a WLAN gateway, and how to execute positioning in a WLAN environment.

Its creation was driven by the industry trend of network convergence and the increasing importance of WLAN offloading. It ensured that critical services like E911 (enhanced emergency services) could, in principle, be supported over WLAN if the regulatory framework and network implementation allowed it, future-proofing the architecture. It also enabled new commercial services that rely on knowing a subscriber's location even when they are on Wi-Fi, which is often used indoors where cellular signals may be weak.

Key Features

• Enables network-initiated (mobile terminated) location requests for UEs connected via I-WLAN
• Integrates with the core 3GPP LCS architecture (GMLC, HSS) for authorization and routing
• Supports multiple positioning methods applicable to WLAN (e.g., WLAN AP-based, UE-based GNSS)
• Ensures subscriber privacy through standard LCS privacy profile verification
• Uses standardized interfaces extended for I-WLAN nodes (e.g., towards PDG/ePDG)
• Provides a consistent location service experience across 3GPP and trusted non-3GPP access

Evolution Across Releases

Defining Specifications