Location Retrieval Function

Description

The Location Retrieval Function (LRF) is a critical functional entity within the 3GPP IP Multimedia Subsystem (IMS) architecture, defined across multiple specifications including TS 23.167 (IMS emergency sessions) and TS 23.271 (Location Services). Architecturally, the LRF resides in the home IMS network or a visited network and interfaces with several other network nodes. Its primary role is to act as a location server and a gateway for retrieving and providing the geographic position of a User Equipment (UE) to authorized Application Functions (AFs), most notably the Emergency Call Session Control Function (E-CSCF) during an IMS Emergency Session.

The LRF operates through a series of standardized interfaces and procedures. When an emergency call is initiated via IMS, the E-CSCF queries the LRF for the caller's location. The LRF then orchestrates the retrieval of this location information. It may obtain the location directly from the UE if it supports location reporting, or more commonly, it interacts with the underlying access network's location infrastructure. For 3GPP access (like LTE or 5G NR), this involves communicating with the Gateway Mobile Location Centre (GMLC) via the Le interface. The LRF may also query the Home Subscriber Server (HSS) or Subscription Locator Function (SLF) to determine which GMLC serves the user.

Once the location is acquired (either as civic address or geodetic coordinates), the LRF formats it according to the required protocol (e.g., using the Presence Information Data Format - Location Object (PIDF-LO)) and delivers it to the requesting entity. Beyond emergency services, the LRF can also support other location-based services (LBS) as defined by the operator. Key components of the LRF include its service logic for handling location requests, its interfaces (Le towards GMLC, Ml towards E-CSCF, etc.), and its integration with IMS routing and subscription databases. Its function is indispensable for regulatory compliance with emergency caller location mandates (like E911 in the USA) in all-IP IMS networks.

Purpose & Motivation

The LRF was created to solve the problem of providing accurate caller location in IP-based telephony networks, specifically IMS, which lacked the inherent location capabilities of traditional circuit-switched mobile networks. In legacy GSM/UMTS circuits-switched calls, the cell ID could be easily passed to the emergency services network. With the migration to all-IP core networks and the decoupling of service layers (IMS) from access networks, a new, standardized mechanism was needed to query and retrieve a UE's location regardless of its access technology (3GPP, non-3GPP like WiFi).

The driving motivation was regulatory: governments worldwide mandate that emergency services must receive the caller's location. The LRF provides a standardized, access-agnostic function within the IMS architecture to meet this requirement. It addresses the limitations of earlier, proprietary, or access-specific solutions by defining a uniform interface (Ml) from the IMS core to a dedicated location retrieval system. This allows emergency services to work seamlessly across heterogeneous access networks and different device capabilities, future-proofing the IMS emergency call framework as new radio technologies emerge.