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.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (103 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-7, normative work from Rel-15.
In Release 15, the LRF's capabilities were expanded to support location retrieval for emergency services in the 5G System, including handling UE location for Non-3GPP access and ensuring support for Network Provided Location Information (NPLI). Specific procedures were introduced for the SMF to request and validate UE location from the AMF, particularly for LADN PDU Session establishment and when no QoS Flow is involved. Furthermore, enhancements were made for location reporting with the UE in RRC Inactive state and for EPS interworking contexts.
- Network Provided Location for non-3GPP access TS 23.501CR0078
- EPS Interworking: 5G-S-TMSI derivation and context retrieval TS 23.501CR0085
- Partitioning of Identifier space to ensure success of Context retrieval for EPS Interworking TS 23.501CR0090
- Clarification on location reporting for LADN in RRC Inactive clause 5.3.3.2.5 - TS 23.501 TS 23.501CR0136
- SMF getting UE location from the AMF for NPLI when no QoS Flow to create/Update/modify TS 23.501CR0266
- How the SMF validates UE location when requested for LADN PDU Session establishment TS 23.501CR0298
In Release 16, the LRF (Location Retrieval Function) saw enhancements including support for concurrent location requests and improved procedures for low power periodic and triggered location reporting. The release also completed the unified location service exposure procedure, clarifying interactions with the GMLC and NEF for authorization and privacy checks. Furthermore, it introduced location information handling for trusted non-3GPP access and added exception support for periodic or triggered location reporting.
- Support of Concurrent Location Request TS 23.273CR0006
- Location information TS 23.501CR0941
- Description regarding NEF support of data retrieval from external party TS 23.501CR1362
- Location information for trusted N3GPP TS 23.501CR1420
- Correction and Improvement of Low Power Periodic and Triggered Location TS 23.273CR0011
- Handling of NG-RAN Location Service Exposure Procedure TS 23.273CR0021
+ 26 more changes
In Release 17, key enhancements for the LRF included the introduction of a Scheduled Location Time for deferred location requests and support for requesting location in Local Coordinates. The release also added a new procedure for unified location service exposure for Network Functions within the serving PLMN and introduced UE location verification procedures specifically for NR satellite access. Furthermore, it provided clarifications on requesting current location from the AMF and on handling location reports for UEs in RRC Inactive state.
- Introduction of the Scheduled Location Time TS 23.273CR0187
- Addition of a Scheduled Location Time TS 23.273CR0151
- Location estimate in Local Coordinates TS 23.273CR0144
- Clarification on requesting current location from AMF. TS 23.273CR0146
- Support for Multiple QoS Class in deferred location requests TS 23.273CR0150
- Add a new procedure for unified location service exposure for NF in serving PLMN TS 23.273CR0163
+ 14 more changes
In Release 18, the LRF (Location Retrieval Function) was enhanced with new capabilities including support for Mobile Base Station Relays (MBSR) in location procedures like MT-LR, finer granularity for triggered UE location reporting, and the introduction of local LMF and GMLC selection. It also gained the ability to provide multiple location reports for a single MT-LR request and to verify location for Non-Terrestrial Network (NTN) access. Furthermore, the architecture was updated to support location service continuity between EPS and 5GS, both for immediate and deferred requests.
- Multiple location report for MT-LR Immediate Location Request for the regulatory service TS 23.273CR0237
- Support of Mobile Base Station Relay for Location Service TS 23.273CR0240
- UE location reporting only when the UE locates in the target area defined with finer granularity TS 23.273CR0242
- Privacy Check for NWDAF requesting UE location from GMLC TS 23.273CR0249
- Introduce new feature: local LMF and GMLC selection TS 23.273CR0254
- Verification of location for NTN access TS 23.273CR0258
+ 32 more changes
In Release 19, the LRF saw enhancements including support for the deferred 5GC-MT-LR procedure using NRPPa periodic measurement reports and the ability to provide multiple location reports for next-generation emergency routing. It also gained explicit support for retrieving location for UEs served by a Multi-Wireless Access Band (MWAB) gNB and introduced the GMLC into the charging architecture for 5GS. Furthermore, procedures were clarified for handling UEs in overlapping non-allowed and slice location availability areas.
- Adding Deferred 5GC-MT-LR Procedure for Periodic Location Events based NRPPa Periodic Measurement Reports TS 23.273CR0589
- Multiple Location Report for Next Generation Emergency Routing TS 23.273CR0608
- Support for Location Service Support of UEs served by MWAB TS 23.501CR5544
- Introduction of GMLC in charging architecture for 5GS TS 32.240CR0491
- Clarification of location services when a UE is served by a MWAB-gNB TS 23.273CR0670
- Inter PLMN GMLC communication via NEF removal TS 23.273CR0736
+ 1 more changes
Explore further
Broader topics and technologies where LRF plays a role.
Defining Specifications
3GPP specifications that define or reference LRF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.167 vj11 | IMS Emergency Sessions | Rel-19 |
| TS 23.271 vj00 | LCS Stage 2 Specification | Rel-19 |
| TS 23.273 vj50 | 5G Location Services Stage 2 Architecture | Rel-19 |
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 24.229 vj50 | IMS call control protocol based on SIP and SDP | Rel-19 |
| TR 29.949 vj00 | VoLTE IMS Roaming Architecture & Procedures | Rel-19 |
| TS 32.240 vj40 | Charging Management Architecture & Principles | Rel-19 |