Voice Broadcast Call (VBS Call)

Description

The voice broadcast call is the fundamental service instance of the GSM Voice Broadcast Service (VBS), a point-to-multipoint circuit-switched teleservice standardized in 3GPP TS 22.069 (service requirements) and TS 42.069 (GERAN stage 1 requirements). A voice broadcast call is created when a VBS service subscriber — a user whose subscription includes the VBS service and who is authorized to act as the broadcaster — initiates a broadcast to a predefined group of listeners. The term is used interchangeably with VBS call throughout the specifications, and it describes a single, active instance of the service rather than the service capability itself.

Architecturally, VBS reuses the GSM circuit-switched core network and the GERAN radio access network. The MSC/VLR handles call control, the HLR holds the VBS subscription data, and the Group Call Register (GCR) stores the attributes of the broadcast call, most importantly the broadcast call area, which is expressed as a list of cells in which the broadcast must be distributed. When the broadcaster dials the broadcast call reference, the MSC retrieves the broadcast call attributes from the GCR, determines the current location of the broadcaster, and sets up the call. In the cell serving the broadcaster a downlink is allocated, and in every other cell of the broadcast call area a downlink-only radio resource — the voice broadcast call channel — is established. Because listeners never transmit, no uplink is required outside the broadcaster's cell, and no talker arbitration is needed.

Listeners are alerted through the Notification Channel (NCH), which is carried on the cell broadcast channel of each cell in the broadcast call area. The notification carries the broadcast call reference and the description of the channel to be used, so that a mobile station belonging to the group can join the call quickly without any signalling towards the network. Once a listener has tuned to the downlink, it simply receives the speech of the broadcaster; joining and leaving are effectively passive operations from the network point of view, although the network tracks listeners in order to decide when a cell no longer needs a channel and can release it.

The service supports the usual GSM supplementary mechanisms. Enhanced Multi-Level Precedence and Pre-emption (eMLPP) allows a broadcast call to be established with a priority level and to pre-empt lower-priority traffic; ciphering and authentication are inherited from the GSM circuit-switched security architecture; and dispatchers, typically fixed or operator-controlled terminals, can be attached to the group. Because the broadcast is one-way, VBS is the natural counterpart of the Voice Group Call Service (VGCS): VGCS provides a bidirectional group conversation with a shared uplink, while VBS provides a one-way distribution with a downlink-only channel.

VBS is a GERAN-only feature. It was never defined for UTRAN, E-UTRAN or NR, and its functionality in modern networks is largely superseded by IMS-based group communication such as Mission Critical Push To Talk (MCPTT) and by multicast/broadcast delivery over LTE and 5G. Nevertheless, the specifications continue to be maintained through Release 19 because VBS and VGCS remain deployed in legacy GSM networks used by railways, utilities, airports and public safety organizations, where wide-area, fast, resource-efficient group speech distribution is still required.

Purpose & Motivation

VBS and the voice broadcast call exist to solve a specific problem: delivering the same speech to a large, geographically dispersed group of users quickly and at low radio cost. Before group services, the only way to reach many users was to place many individual point-to-point calls, which consumes one traffic channel per listener in every cell and requires the network to set up each call separately. For command-and-control applications — a dispatcher addressing a fleet of vehicles, a controller instructing railway staff along a line, an operator issuing instructions to field crews — this is both wasteful and slow.

The broadcast model replaces N point-to-point calls with one downlink per cell. Because listeners do not transmit, no uplink is needed outside the broadcaster's cell, and no arbitration of the uplink is required. Notification through the NCH lets hundreds of mobile stations join the broadcast almost simultaneously, which matters when the message is time-critical. The service also inherits GSM mechanisms that professional users expect: subscription-based authorization, priority and pre-emption through eMLPP, ciphering, and dispatcher participation.

Historically, VBS was specified in the same Release 4 timeframe as VGCS and eMLPP, when GSM was being positioned for professional mobile radio markets. It addressed the limitation that GSM offered only point-to-point calls, leaving dedicated trunked radio systems as the only option for group communication. Although newer technologies have taken over the role, the VBS definition is still maintained because the service remains in operational use in legacy networks.