Response Method

Description

The Response Method is a concept in 3GPP location services (LCS) that defines how a Gateway Mobile Location Centre (GMLC) responds to a location request received from another entity, such as an LCS client or another GMLC. Introduced in Release 6 and specified in TS 23.271 and TS 23.273, this parameter determines whether the location procedure is synchronous (SYNC) or asynchronous (ASYNC). In SYNC mode, the requesting entity expects a single response to its location request. In ASYNC mode, the requesting entity expects multiple responses, which may include periodic or event-triggered location reports for one or more target UEs.

The Response Method is negotiated between the requesting entity and the receiving GMLC. The source entity of the location request can indicate a preferred method, but the final selection is made by the receiving GMLC. When the ASYNC method is selected, the GMLC notifies the source entity of a Request ID, which is used to correlate subsequent responses with the original request. This allows the source entity to manage multiple ongoing location procedures and match responses to the correct request.

A key feature of the Response Method is the ability of the receiving GMLC to convert a SYNC request into an ASYNC procedure. This can happen, for example, in an overload situation where the GMLC cannot provide an immediate response. In such cases, the GMLC may decide to handle the request asynchronously and send multiple responses over time. The source entity must be prepared to receive multiple responses even if it originally requested a SYNC procedure. This flexibility ensures that location services can adapt to changing network conditions and resource availability.

The Response Method is particularly important for location-based applications that require continuous or periodic tracking of a target UE. For example, a fleet management application may request periodic location updates for a vehicle. By using the ASYNC method, the application can receive multiple location reports from a single request, reducing signaling overhead and simplifying application logic. Conversely, a one-time location query, such as finding the nearest restaurant, would use the SYNC method to receive a single response.

The definition of the Response Method has remained remarkably stable since its introduction in Release 6. The core concepts of SYNC and ASYNC procedures, the negotiation between requesting and receiving entities, and the ability to convert SYNC to ASYNC have been consistently maintained through Release 19. This stability reflects the fundamental nature of the concept and its independence from specific location technologies or network architectures. Whether the location service is provided over GSM, UMTS, LTE, or 5G, the Response Method provides a consistent framework for managing location requests and responses.

Purpose & Motivation

The Response Method was created to address the diverse needs of location-based applications and to provide flexibility in how location requests are handled. Before its introduction, location services typically followed a simple request-response model where each request resulted in exactly one response. This model worked well for one-time location queries but was inefficient for applications that needed continuous or periodic location updates. Such applications had to send multiple requests, each generating its own signaling and processing overhead.

The Response Method solves this problem by introducing the ASYNC procedure, which allows a single request to generate multiple responses. This is particularly valuable for tracking applications, emergency services, and other use cases where ongoing location monitoring is required. By reducing the number of requests, the ASYNC method reduces signaling load on the network and simplifies the logic in client applications. It also enables more efficient use of network resources, as the GMLC can schedule and optimize location reports based on network conditions.

Another important motivation for the Response Method was the need to handle overload situations gracefully. In a purely synchronous model, an overloaded GMLC might have to reject requests or delay responses, leading to poor user experience. With the ability to convert SYNC requests to ASYNC procedures, the GMLC can accept the request and provide responses as resources become available. This improves resilience and ensures that location services remain available even under heavy load. The Response Method thus provides a robust and flexible foundation for location services in 3GPP networks.