Location IMS – Interworking Function

Description

The Location IMS – Interworking Function (LIMS-IWF) is a critical functional entity defined in 3GPP specifications for enabling Location Services (LCS) within the IP Multimedia Subsystem (IMS), particularly for emergency call scenarios. It resides within the IMS network architecture and serves as a protocol gateway and service enabler. When an IMS emergency call is initiated (e.g., via a SIP INVITE to the Emergency-Call Identification Service), the IMS network identifies the call as an emergency session. The LIMS-IWF is then invoked to obtain the location of the caller. Its primary role is to interwork between the IMS signaling plane, which uses Session Initiation Protocol (SIP) and its extensions, and the legacy circuit-switched or packet-switched mobile network's location infrastructure, which typically uses the Mobile Location Protocol (MLP) to communicate with a Gateway Mobile Location Center (GMLC). The LIMS-IWF receives a location request triggered by IMS signaling, often containing an IMS Public User Identity. It then maps this identity to a corresponding mobile network identifier (like MSISDN or IMSI), formulates a standardized MLP request (e.g., an Emergency Location Immediate Request - ELIR), and sends it to the appropriate GMLC. Upon receiving the location estimate (geodetic or civic) from the GMLC, the LIMS-IWF translates it into a format suitable for IMS, such as a PIDF-LO (Presence Information Data Format Location Object), and delivers it to the IMS core for routing the emergency call to the correct Public Safety Answering Point (PSAP). This seamless translation is vital for meeting regulatory mandates for emergency caller location in next-generation networks.

Purpose & Motivation

The LIMS-IWF was developed to solve a fundamental incompatibility between the emerging all-IP IMS network architecture and the existing location service infrastructure designed for circuit-switched and packet-switched GSM/UMTS networks. As operators began deploying IMS for voice and multimedia services, a critical challenge arose: how to provide accurate caller location for IMS-based emergency calls (like VoIP 911) when the traditional location mechanisms relied on protocols and interfaces not natively supported in the SIP-based IMS core. Without the LIMS-IWF, IMS networks would have been unable to leverage the mature, network-based positioning capabilities (e.g., Cell-ID, OTDOA, A-GPS) already deployed in the radio access and core network. Its creation was motivated by stringent regulatory requirements (e.g., FCC E911 rules) that mandate the provision of location information with emergency calls, regardless of the underlying technology (circuit-switched, packet-switched, or IMS). The LIMS-IWF bridges this technological gap, allowing operators to reuse their existing GMLC and positioning infrastructure while migrating to IMS, ensuring regulatory compliance, protecting subscriber safety, and avoiding the need for a costly, parallel location system for IMS.

Key Features

• Protocol interworking between IMS/SIP signaling and legacy Mobile Location Protocol (MLP)
• Identity mapping from IMS Public User Identity to cellular identifiers (MSISDN, IMSI)
• Supports Emergency Location Immediate Request (ELIR) for IMS emergency calls
• Converts location data from GMLC into IMS-compatible formats like PIDF-LO
• Integrates with IMS service triggering mechanisms for emergency session detection
• Enables reuse of existing network-based location infrastructure (GMLC, LMU) for IMS services

Evolution Across Releases

Defining Specifications