Before this:Talkgroups, radio IDs & affiliation
Anatomy of a trunked call: request, grant, release
Key takeaways Every trunked call follows the same arc. A radio keys up and sends a channel request on the control channel. The system finds a free voice channel and broadcasts a grant — “talkgroup 101, go to channel 3” — which affiliated radios hear and retune to. The conversation happens on that voice channel, with periodic grant updates so latecomers can join (late entry). When traffic stops, the channel is held briefly for hang-time, then released back to the pool. Decoding the grants is exactly how a follower knows where every call goes.
You now know the identities a trunked system tracks. This lesson stitches them together into a single call, from the moment a user presses the push-to-talk to the moment the channel returns to the pool.
The four phases
A trunked call moves through four phases, all coordinated on the control channel:
- Request. A user keys up. Their radio sends a channel request on the control channel, naming its talkgroup (and carrying its radio ID).
- Grant. The system computer finds a free voice channel and broadcasts a grant on the control channel: “talkgroup 101 → channel 3.” Every radio affiliated to talkgroup 101 hears that data message.
- Conversation. Those radios retune to channel 3 and the call happens there. While it’s active, the system periodically re-sends a grant update so radios that arrive late can still find the channel.
- Release. When traffic stops, the system holds the channel for a short hang-time in case of a reply. If none comes, the channel is released back to the pool for the next call — possibly a completely different talkgroup.
The voice channel is never owned by a talkgroup; it’s borrowed for the call and handed back. The next call from the same group may land somewhere else entirely.
Watching it on the control channel
Because every phase is announced as data, a follower sees the whole call as a sequence of control-channel messages — this is the stream GopherTrunk reads:
| Time | Control-channel message | What a follower does |
|---|---|---|
| 0.0 s | Radio 1147 requests TG 101 | Note the request |
| 0.1 s | Grant: TG 101 → channel 3 | Tune a receiver to ch 3, record |
| 0.1–6.0 s | Grant update: TG 101 still on ch 3 | Keep following; late joiners can enter |
| 6.0 s | (traffic stops) | Hold ch 3 through hang-time |
| 7.5 s | TG 101 released; ch 3 freed | Return ch 3 to the pool, ready for next |
The first grant is the cue to point a receiver at the voice channel. The grant updates are insurance: if a radio (or the follower) was busy at 0.1 s, it can still latch onto the call at 3 s and not miss the rest. When the release arrives, the follower frees the channel and is immediately ready for the next grant.
Hang-time and late entry
Two details make trunking feel seamless.
Hang-time is the short pause the system keeps a channel open after a transmission ends. Conversations are a back-and-forth, and re-requesting a channel for every single over would add latency and churn. Holding the channel briefly lets the reply land on the same channel, so a normal exchange stays put until the talking actually stops.
Late entry is joining a call already underway. Thanks to the repeated grant updates, a radio that powers on or switches talkgroups mid-call — or a monitor that tunes in late — can discover the active voice channel and pick up the conversation in progress. Without grant updates, you’d only ever hear calls you happened to catch at the instant of the first grant.
Quick check: what lets a radio join a trunked call that's already in progress?
Recap
- A call has four phases: request, grant, conversation, release.
- The request rides the control channel; the grant names the talkgroup and voice channel.
- Affiliated radios retune to the granted channel; grant updates repeat during the call.
- Late entry lets radios join a call in progress thanks to those updates.
- After traffic stops, hang-time holds the channel briefly, then it’s released.
Next, we open up the control channel itself and read what the data says — the full vocabulary of trunking messages.
Frequently asked questions
What happens when someone keys up on a trunked system?
Their radio sends a channel request on the control channel. The system finds a free voice channel and broadcasts a grant naming the talkgroup and the channel. Every radio affiliated to that talkgroup retunes to the voice channel, the conversation happens there, and when it ends the channel is released back to the pool after a short hang-time.
What is hang-time?
Hang-time is a short window the system holds the voice channel open after a transmission ends, in case someone replies. It keeps a back-and-forth conversation on the same channel instead of re-requesting one for every over. When hang-time expires with no further traffic, the channel is released to the pool.
What is late entry?
Late entry is joining a call already in progress. If a radio — or a monitor — affiliates or tunes in after the initial grant, it can still pick up the call because the control channel periodically repeats the grant as a grant update. That repeated announcement lets latecomers find the active voice channel.
What is a grant update?
A grant update is a control-channel message that re-announces an in-progress call’s talkgroup and voice channel. The system sends it periodically while a call is active so radios that missed or arrived after the first grant can still find and follow the call. It is how late entry and channel-following stay reliable.