Automatic Commencement Mode

Description

Automatic commencement mode is a feature defined in 3GPP specifications for Mission Critical Push To Talk (MCPTT) services, introduced in Release 13. It describes a mode of operation for private calls where the call is established automatically upon initiation by the calling party, without requiring the called party to perform any action (such as pressing a button or accepting the call). This mode is particularly useful in mission-critical scenarios where immediate communication is essential, and the callee may be unable to interact with the device (e.g., during an emergency or when hands are occupied).

In MCPTT, a private call is a point-to-point communication between two users. The automatic commencement mode is one of two possible modes for private call setup, the other being manual commencement mode, where the callee must explicitly accept the call. The mode is typically configured per user or per group and is negotiated during call setup. When automatic commencement mode is used, the MCPTT server (or the originating user's client) sends a call setup request to the terminating user's client, and the call is established immediately, with the terminating user's device automatically answering and alerting the user (e.g., via a tone or vibration) without requiring any action.

The automatic commencement mode is implemented using the MCPTT signalling protocols defined in 3GPP TS 23.179 and TS 23.379. The call setup procedure involves the MCPTT server, which acts as a central control point, and the participating MCPTT clients. The server checks the user's configuration and permissions to determine whether automatic commencement is allowed. If so, it proceeds to establish the call, allocating necessary resources and notifying both parties. The terminating client, upon receiving the call setup request, automatically accepts and transitions to the talking state, allowing the caller to speak immediately.

This mode is particularly beneficial for group calls as well, but the definition specifically addresses private calls. It enhances the user experience by reducing call setup time and eliminating the need for manual intervention, which is critical in emergency situations. However, it also raises privacy and security considerations, as the callee may not have the opportunity to reject the call. Therefore, network operators and administrators can control the availability of automatic commencement mode through policy settings.

The automatic commencement mode is a key differentiator of MCPTT compared to commercial push-to-talk services, which often require manual acceptance. It aligns with the mission-critical requirements for fast, reliable, and seamless communication. The feature has been maintained across subsequent releases, with no changes to its definition, indicating its stability and continued relevance in the MCPTT framework.

Purpose & Motivation

Automatic commencement mode was created to address the need for instantaneous communication in mission-critical situations. In emergency response, law enforcement, and military operations, delays caused by manual call acceptance can have severe consequences. Traditional voice call setup requires the callee to answer, which may be impractical when the user is engaged in physical activity or unable to access the device. Automatic commencement mode eliminates this step, ensuring that the call is connected as soon as it is initiated.

The feature also supports the push-to-talk paradigm, where communication is typically half-duplex and initiated by pressing a button. In such scenarios, requiring the callee to accept a call would be counterproductive and slow down the communication flow. By automatically commencing the call, MCPTT services can mimic the immediacy of traditional radio systems while leveraging the capabilities of LTE and 5G networks.

Furthermore, automatic commencement mode enables efficient group communication and prioritisation. It allows dispatchers and commanders to reach individuals or groups instantly, without waiting for them to accept. This is essential for coordinating responses and ensuring that critical information is disseminated quickly. The mode is configurable, so organisations can choose to enable or disable it based on their operational needs and privacy policies.