Called-party pre-emption

Description

Called-party pre-emption is a supplementary service feature defined in 3GPP TS 22.067 and TS 23.067 that enables the termination of an ongoing call to a particular user when a higher priority call is directed towards that same user. It is a key component of the enhanced Multi-Level Precedence and Pre-emption (eMLPP) service, which provides priority handling for calls in GSM and UMTS networks. The feature ensures that high-priority users, such as emergency responders or government officials, can reach a called party even if that party is already engaged in a lower-priority call. The decision to pre-empt is made by the Mobile Station (MS) of the called party, based on the priority level of the incoming call compared to the priority level of the ongoing call.

In the context of point-to-point calls, called-party pre-emption is implemented using the Call Waiting (CW) supplementary service. When a higher priority call arrives for a user who is already in a call, the network sends a Call Waiting indication to the MS. The MS, configured with the appropriate priority thresholds, automatically accepts the waiting call, which results in the termination of the existing call. This automatic acceptance is transparent to the user and ensures that the higher priority communication is established without manual intervention. The pre-emption decision is thus delegated to the MS, which must be provisioned with the necessary eMLPP subscription information and priority levels.

For group calls, such as Voice Group Call Service (VGCS), the pre-emption mechanism may operate differently, but the core principle remains: a higher priority call can pre-empt an ongoing lower priority call to the same user. The network elements involved include the Mobile Switching Center (MSC), the Visitor Location Register (VLR), and the Home Location Register (HLR), which store the subscriber's eMLPP priority levels and enforce the pre-emption policies. The MS plays a critical role by evaluating the priority of the incoming call and deciding whether to accept it, thereby terminating the existing call.

The feature is particularly important in Public Land Mobile Networks (PLMNs) that support eMLPP, as it allows for the prioritization of calls during emergencies or when network resources are scarce. It ensures that critical communications can take place even when the called party is busy. The pre-emption is typically applied only when the incoming call has a higher priority than the ongoing call, and the called party has subscribed to the eMLPP service with the appropriate priority levels.

Called-party pre-emption has been part of 3GPP specifications since Release 4 and has remained largely unchanged through Release 19. Its definition and mechanisms are stable, reflecting the maturity of the eMLPP service. The feature is applicable to both GSM and UMTS networks, and its principles can be extended to LTE and 5G networks through the concept of priority services (e.g., MCPTT in mission-critical systems). However, in 5G, the implementation may leverage different enablers such as the 5G QoS Identifier (5QI) and pre-emption capabilities at the network level.

Purpose & Motivation

Called-party pre-emption exists to ensure that high-priority calls can reach their intended recipient even when that recipient is already engaged in a lower-priority call. In traditional telephony, a busy line would result in a busy tone, preventing urgent calls from getting through. This is unacceptable in scenarios where priority communications are essential, such as emergency services, military operations, or government communications. The eMLPP service was developed to provide multiple levels of precedence and pre-emption, allowing authorized users to gain access to network resources and called parties based on their priority level.

The feature solves the problem of called party busyness by enabling the network and the mobile station to automatically terminate an ongoing lower-priority call when a higher-priority call arrives. This is particularly important in networks that support group calls and dispatcher communications, where a dispatcher may need to reach a field worker urgently. Without called-party pre-emption, the dispatcher would have to wait for the ongoing call to end, which could delay critical response.

Historically, pre-emption capabilities were introduced in GSM as part of the eMLPP service in Release 4. The need for such features arose from the requirements of professional mobile radio (PMR) users, who demanded functionality similar to that of TETRA or other dedicated systems. The 3GPP specifications 22.067 and 23.067 define the service aspects and technical realization, respectively. The feature has been carried forward into later releases, ensuring backward compatibility and continued support for priority communications in evolving networks.