UE available

Description

The 'UE available' event is a specific trigger in the context of deferred location requests within 3GPP networks. A deferred location request is a request for location information that is not required immediately but rather when a specific event occurs. The 'UE available' event is one such trigger: it indicates that the Mobile Switching Center (MSC) or Serving GPRS Support Node (SGSN) has successfully established contact with the User Equipment (UE). This event is particularly relevant when the UE was previously unreachable due to reasons such as the user not interacting with the device, temporary loss of radio coverage, or the UE having performed an IMSI detach (i.e., detached from the network). The event is defined in 3GPP TS 22.071 (Location Services) and TS 22.935 (Location Service requirements).

In the architecture of Location Services (LCS), the Gateway Mobile Location Center (GMLC) may request location information for a target UE. If the UE is not immediately reachable, the request can be deferred until a specific event occurs. The 'UE available' event is one of the possible event types. When the MSC/SGSN detects that the UE has become reachable—for example, after a periodic location update, a routing area update, or a service request—it can trigger the deferred location request. The MSC/SGSN then initiates the location procedure, which may involve paging the UE, performing authentication, and obtaining the location estimate. The 'UE available' event is thus a mechanism to ensure that location services can be provided even when the UE is temporarily out of contact, without requiring the requesting application to continuously poll.

The event is applicable in both circuit-switched (CS) and packet-switched (PS) domains. In the CS domain, the MSC handles the event; in the PS domain, the SGSN is responsible. The event is considered to be applicable when the UE is temporarily unavailable due to inaction by the UE user, temporarily loss of radio connectivity, or IMSI detach. It is important to note that IMSI detach is only applicable if the UE has previously been registered and information is still kept in the node. This means that if the UE has explicitly detached and the MSC/SGSN has deleted its context, the event may not be triggered. The event is part of the broader set of deferred location event types, which also include 'UE reachable', 'UE enters/leaves area', and 'UE available'.

The implementation of the 'UE available' event involves interaction between the LCS entities and the mobility management functions. When a deferred location request is received by the GMLC, it forwards the request to the MSC/SGSN. The MSC/SGSN stores the request and monitors the UE's state. When the UE becomes available (e.g., after a location update), the MSC/SGSN sends a location request to the Radio Access Network (RAN) to obtain the location estimate. The result is then returned to the GMLC and the requesting LCS client. The event is defined in the specifications to ensure interoperability between different vendors' equipment and to provide a consistent behavior for location-based services.

The 'UE available' event is crucial for many location-based applications, such as emergency services, fleet management, and location-based advertising. It allows these applications to request location information without requiring the UE to be continuously reachable, thus saving network resources and improving scalability. It also enables the network to provide location information when the UE becomes available, even if the request was made earlier. The event is part of the 3GPP's efforts to standardize location services and ensure that they work across different access technologies and network domains.

Purpose & Motivation

The 'UE available' event exists to support deferred location requests in 3GPP networks. Before its introduction, location requests typically required the UE to be immediately reachable, which was inefficient for applications that did not need real-time location but rather location when the UE becomes available. The event solves the problem of unreachable UEs by allowing the network to defer the location procedure until the UE is contactable. It was introduced in Release 5 as part of the Location Services (LCS) enhancements. It addresses limitations of previous approaches where location requests would fail if the UE was temporarily unavailable due to user inactivity, loss of coverage, or IMSI detach. By defining a standardized event, it enables consistent behavior across different networks and vendors, facilitating the deployment of location-based services that can tolerate delays. It also optimizes network resource usage by avoiding unnecessary paging and signaling when the UE is not reachable.