Also known as: channel grant
A channel grant is the control-channel message that assigns a talkgroup’s call to a specific voice channel (and timeslot on TDMA systems).1 It is the pivotal event in trunking: the instant a grant is broadcast, a call is starting and the message says exactly where.
How it works
When the controller accepts a call request, it packs a grant into a TSBK (P25) or CSBK (DMR) and broadcasts it on the control channel. The grant carries the addressing needed to join the call: the target identity (a talkgroup for a group call, or a source/destination pair for a private call), the physical voice channel — usually as a channel number the radio maps to a frequency through the system’s channel plan — and, on TDMA systems, the timeslot. Affiliated radios and monitors read it and retune together.
Because a radio might miss the initial grant — it was out of range, or just powered on — systems repeat a group update or grant update message while a call is in progress. This periodic re-announcement of active calls is what lets a late arrival find and join a call already underway, the mechanism behind late entry.
Variants
- Group voice grant — the common case, directing a talkgroup to a voice channel.
- Unit-to-unit (private) grant — assigns a channel for a one-to-one private call between two radio IDs.
- Grant / group update — an in-progress re-announcement of active calls for late entry and for radios that missed the original.
- Data / packet grant — assigns a channel for a short data transfer rather than voice.
In practice
Grants are the backbone of any trunk-following scanner. During a busy incident the control channel may issue several grants per second across many voice channels, and a monitor’s job is to task receivers to the ones a listener cares about. If all voice channels are busy, a call request is queued rather than granted, and the eventual grant may arrive seconds later.
Relevance to SDR
GopherTrunk reads grants in real time to task a receiver to the right channel and slot, which is how it follows conversations as they scatter across the channel pool. It maps the grant’s channel number to an actual frequency via the decoded channel plan, then down-converts and demodulates that voice channel — repeating the process for every grant so a single wideband capture reconstructs the whole system’s traffic.
Sources
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Trunked radio system — Wikipedia, on the grant message that assigns calls to voice channels. ↩