Broadcast call

Description

In 3GPP terminology, a 'Broadcast call' is defined as being used in the same sense as 'voice broadcast call'. This term appears in specifications such as TS 24.069 and TS 44.069, which cover voice group call service (VGCS) and voice broadcast service (VBS). A broadcast call is a unidirectional voice communication where a single speaker transmits to a group of listeners, who cannot talk back. This is distinct from a group call, where multiple participants can speak. Broadcast calls are used in professional mobile radio (PMR) systems, public safety, and mission-critical communications, allowing dispatchers to address many field units simultaneously.

The technical realization of broadcast calls involves the establishment of a downlink-only channel to multiple recipients. In GSM and UMTS, Voice Broadcast Service (VBS) enables a calling subscriber to broadcast speech to a predefined group of listeners in a specific area. The network sets up a broadcast call using group call registers and group call areas, and listeners are paged and join the downlink channel. In LTE and 5G, similar functionality is provided by Mission-Critical Push-to-Talk (MCPTT) and related services, though the term 'broadcast call' is less commonly used. The key characteristic is that the communication is one-way, from the broadcaster to the listeners, with no reverse channel for voice.

Broadcast calls are managed by core network entities such as the Group Call Service Center (GCSC) and the Group Call Register (GCR) in GSM/UMTS. These entities maintain group definitions, user authorizations, and location information. When a broadcast call is initiated, the network allocates radio resources in the cells where listeners are located, often using a common downlink channel. Listeners are notified of the call and can choose to listen. The broadcast call can be terminated by the initiator or by the network.

In terms of service attributes, a broadcast call is a type of communication configuration that is unidirectional and group-oriented. It is used in scenarios where a central authority needs to disseminate voice information to many recipients efficiently, such as emergency announcements, dispatcher instructions, and public address. The term 'broadcast call' emphasizes the call-like nature of the service, distinguishing it from broadcast data services like cell broadcast.

3GPP specifications define the procedures for broadcast calls in TS 24.069 (Voice Group Call Service) and TS 44.069 (Voice Broadcast Service). These specifications cover call setup, group management, security, and interworking with other networks. As networks evolve, broadcast call functionality is being integrated into 5G mission-critical services, ensuring continuity for public safety and professional users.

Purpose & Motivation

The concept of a broadcast call exists to provide efficient one-to-many voice communication for professional and mission-critical users. In traditional two-way radio systems, dispatchers needed to communicate instructions to many field units simultaneously, but establishing individual calls was inefficient and slow. Broadcast calls solve this by allowing a single voice transmission to be received by an entire group, ensuring timely and coordinated communication.

Historically, broadcast calls were a feature of analog PMR systems like MPT 1327 and later digital systems like TETRA and APCO 25. When 3GPP standardized voice group call services in GSM, it introduced Voice Broadcast Service (VBS) to bring these capabilities to cellular networks. This allowed public safety organizations to use commercial cellular infrastructure for their communications, leveraging wider coverage and lower costs. The term 'broadcast call' was adopted to describe the unidirectional variant of group calls.

The motivation for broadcast calls is to support scenarios where a central dispatcher or commander needs to address many responders at once, such as during emergencies, large events, or military operations. Unlike multicast, which may allow multiple speakers, broadcast calls are strictly one-way, simplifying resource allocation and ensuring that the channel is not congested by multiple talkers. This is particularly important in critical situations where clear, uninterrupted communication from the command center is essential.