Description
The LCS Application Protocol (LCS-AP) is a critical control plane protocol within the 3GPP architecture, specifically designed to support Location Services (LCS). It operates over the SLg interface, connecting the Gateway Mobile Location Centre (GMLC) with the core network's mobility management entities—namely the Mobility Management Entity (MME) for LTE/EPC and the Serving GPRS Support Node (SGSN) for 2G/3G GPRS networks. The protocol defines a set of procedures and messages that allow the GMLC, which acts as the external interface for location requests, to communicate with the network nodes that can initiate and manage the location determination process for a User Equipment (UE).
Architecturally, LCS-AP is an application layer protocol that typically uses SCTP (Stream Control Transmission Protocol) as its transport layer for reliable message delivery. Its primary function is to convey location service requests from the GMLC to the MME/SGSN. Upon receiving a request, the MME/SGSN interacts with the Radio Access Network (RAN) and potentially the UE itself (e.g., for UE-based or UE-assisted positioning methods) to obtain location measurements or a calculated position. The protocol carries essential information such as the target UE's identifier (e.g., IMSI, MSISDN), the required Quality of Service (QoS) for the location estimate (like accuracy and response time), and the location event (e.g., immediate location, deferred location for periodic reporting).
The protocol supports various positioning methods, including network-based (e.g., Observed Time Difference of Arrival - OTDOA in LTE), UE-based (where the UE calculates its own position), and UE-assisted methods. LCS-AP messages are used to transfer the resulting location estimate (e.g., geographic coordinates) back to the GMLC, which then delivers it to the external LCS Client (e.g., an emergency services center or a commercial application server). The protocol also handles error conditions, subscriber privacy verification (checking if the LCS client is authorized to locate the subscriber), and supports deferred location reporting for tracking applications. Its role is strictly in the control plane, managing the signaling required to set up, execute, and tear down a location session without carrying the actual positioning measurement data, which is handled by other protocols in the user plane in later 3GPP releases.
Purpose & Motivation
LCS-AP was introduced to standardize the control signaling for Location Services across 3GPP networks, addressing the growing need for reliable and efficient mobile device positioning. Prior to its standardization, location services were often implemented using proprietary interfaces, which hindered interoperability between network equipment from different vendors and between different network operators. The creation of LCS-AP as part of the overall LCS architecture in 3GPP Release 9 provided a unified protocol for core network entities to communicate location requests and responses, which was essential for the commercial rollout of location-based services and for meeting regulatory mandates like Enhanced 911 (E911) in North America and its equivalents globally.
The protocol solves the problem of how an external entity (the LCS Client) can request the network to locate a subscriber's device in a standardized manner. It abstracts the complexities of the underlying radio access technology (e.g., UMTS, LTE) from the service layer. By defining clear primitives for location invocation, cancellation, and reporting, LCS-AP enables a wide range of services including emergency caller location, asset tracking, location-based charging, and friend-finder applications. Its development was motivated by the telecommunications industry's shift towards service-oriented architectures and the need to expose network capabilities like positioning through open, standardized interfaces to foster innovation in the application layer.
Key Features
- Standardized control plane signaling for location service requests and delivery
- Operates over the SLg interface between GMLC and MME/SGSN
- Supports multiple location request types: immediate, deferred, and periodic
- Carries location QoS parameters like accuracy, response time, and vertical accuracy
- Includes privacy verification procedures to authorize location requests
- Transport-independent design, typically implemented over SCTP for reliability
Evolution Across Releases
Initial introduction of LCS-AP. Defined the basic protocol procedures for LTE/EPC networks, establishing the SLg interface between the GMLC and the MME. Supported core location service procedures like Location Request, Location Response, and Location Report for immediate location retrieval.
Defining Specifications
| Specification | Title |
|---|---|
| TS 29.171 | 3GPP TS 29.171 |
| TS 36.305 | 3GPP TR 36.305 |
| TS 37.857 | 3GPP TR 37.857 |