Also known as: voice channel, traffic channel
A voice channel (or traffic channel) is a frequency that a trunked radio system temporarily assigns to carry one call.1 When the call ends, the channel returns to the pool for reuse. It is the counterpart of the control channel: control carries data and coordinates, while a voice channel carries the actual conversation for the brief life of a single transmission or call.
How it works
The control channel issues a grant directing a talkgroup to a specific voice channel. On a TDMA system the grant also names a timeslot, because two calls share one RF channel; on an FDMA system the channel is the whole frequency. Radios retune, the talker’s audio flows for the duration of the transmission, and a short hang time keeps the channel reserved so replies in the same conversation stay on it rather than being reassigned. When the hang timer expires with no more traffic, the controller marks the channel idle and it rejoins the pool. The next call from the same group may land on a completely different voice channel.
Beyond audio, a voice channel carries embedded low-speed signalling — the talkgroup and source radio ID, encryption status, and periodic frame synchronisation — so a receiver that tunes in mid-call can identify the participants and achieve late entry without having heard the original grant.
Variants
- Analog voice channel — FM audio (older systems, or analog talkgroups on hybrid systems), with unit IDs carried by in-band signalling.
- Digital voice channel — a vocoder bitstream (AMBE+2 on P25/DMR, ACELP on TETRA) wrapped in error-correction and framing.
- Data / packet channel — some systems grant channels for short data bursts rather than voice, using the same assignment machinery.
In practice
Voice channels are only busy in short bursts, which is exactly why trunking works: a pool far smaller than the number of talkgroups suffices. During a busy incident, though, all voice channels can be in use at once, at which point new call requests are queued on the control channel until one frees up. Encrypted calls still occupy a granted voice channel; a monitor sees the grant and the encryption flag but cannot recover the audio.
Relevance to SDR
GopherTrunk tunes a receiver to the granted voice channel to capture the audio, then returns to await the next assignment — following many calls from one wideband capture. On TDMA systems it decodes the specific granted slot of the assigned channel. Because the voice channel carries the source ID inline, each captured call is tagged with who was talking, not just which group — the basis of GopherTrunk’s per-call logging.
Sources
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Trunked radio system — Wikipedia, on per-call voice (traffic) channel assignment. ↩