Also known as: group call, group voice call
A group call is a one-to-many voice transmission addressed to a talkgroup: when a user presses to talk, every affiliated member of that group hears it, no matter which physical frequency the system assigns.1 It is the everyday mode of trunked land-mobile radio — a dispatcher calling a fleet, a supervisor addressing a shift — and it is set up by a channel grant on the control channel.
How it works
A radio requests a group call by sending its talkgroup and its own radio ID to the control channel. The system finds a free voice channel, issues a grant naming the talkgroup and that channel, and every radio affiliated to the talkgroup retunes to it — including radios on other sites where members are present. The call is half-duplex and push-to-talk: only one person transmits at a time, and when the talker releases, the channel is freed. Successive transmissions in the same conversation may land on different voice channels, because the system re-grants each time.
A group call contrasts with a private call, which is a one-to-one transmission addressed to a single radio ID rather than a talkgroup. Emergency and priority calls are group calls flagged for preferential handling, so the system grants them a channel ahead of routine traffic.
Relevance to SDR
Group calls are the bulk of what a trunking monitor hears, and following them is the whole point of decoding the control channel. GopherTrunk reads each grant, tunes a receiver to the assigned voice channel, and presents the call tagged with its talkgroup and the transmitting radio ID — then follows the conversation as the system re-grants it across the channel pool. GopherTrunk decodes clear and de-scramblable group-call audio; it cannot recover calls protected by keyed encryption, though it still logs the call metadata (talkgroup, radio ID, channel) even when the voice is encrypted.
Sources
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Trunked radio system — Wikipedia, on group (one-to-many) calls in trunked systems. ↩