Before this:What is a radio wave?Digital modulation & constellations
What is trunked radio?
Key takeaways Trunked radio lets many user groups share a small pool of frequencies by assigning a free channel to each call on demand, then reclaiming it. A dedicated control channel carries data — not voice — and orchestrates the whole system, announcing every call and the voice channel it’s been assigned. Users belong to talkgroups (virtual channels), so you follow who’s talking, not a fixed frequency. This is why GopherTrunk decodes the control channel first: it’s the map that tells the software where each call goes so it can follow them across the system.
This is the idea GopherTrunk was built around. Once trunking clicks, the whole design of the software — control channels, talkgroups, the activity panels — turns from jargon into something obvious.
What problem does trunking solve?
Radio spectrum is scarce and expensive. Imagine a county with 40 agencies — police, fire, public works, transit, schools. Give each its own permanent frequency and you’ve burned 40 channels, most sitting silent most of the time while a few are congested. That’s conventional radio: simple, but wasteful.
Trunking fixes the waste by sharing. The agencies pool, say, 10 frequencies, and a central computer hands out a free one per call, just for that call’s duration, then takes it back. Because real conversations are short and bursty, 10 shared channels can serve all 40 groups with almost no waiting. It’s the same logic as a bank with one queue feeding several tellers instead of a separate line per teller.
How does a trunked call actually work?
The magic is coordination, and it happens on the control channel — one frequency that carries a constant stream of data (never voice). Here’s a single call, step by step:
- A user keys up. Their radio sends a request on the control channel.
- The system computer finds a free voice channel and broadcasts an assignment on the control channel: “Talkgroup 101, go to voice channel 3.”
- Every radio affiliated with talkgroup 101 hears that data message and instantly retunes to voice channel 3 to listen.
- The call happens on channel 3. When it ends, the channel is released back into the pool for the next call — maybe a completely different talkgroup.
The next call from the same talkgroup might land on channel 7, then channel 2. The frequency is never fixed — which is exactly why you can’t scan a trunked system the old-fashioned way.
Watching the pool churn
To see why this defeats an old scanner, follow ten seconds on a busy system — this is exactly the stream GopherTrunk reads on the control channel:
| Time | Control-channel message | What a follower does |
|---|---|---|
| 0.0 s | TG 101 (Dispatch) → channel 3 | Tune a receiver to ch 3, record |
| 1.2 s | TG 250 (Fire) → channel 7 | Tune a second receiver to ch 7 |
| 3.8 s | TG 101 ends; ch 3 released | Free ch 3 back to the pool |
| 4.1 s | TG 101 (Dispatch) → channel 2 | The same talkgroup, a new channel |
| 5.5 s | TG 412 (Transit) → channel 3 | Reuse ch 3, now a different group |
An old scanner parked on “channel 3” would hear Dispatch, then silence, then Transit — a meaningless jumble. A trunk-tracker following the control channel knows who’s on each channel at every instant, which is the whole point of decoding it first.
What is a talkgroup?
Because the physical frequency keeps changing, trunked systems give users a stable virtual identity: the talkgroup. A talkgroup is just a number with a label — “Police Dispatch,” “Fire Ground 2,” “City Transit.” Everyone in a talkgroup hears each other’s calls no matter which voice channel the system happens to assign.
So when you monitor a trunked system, you follow talkgroups, not frequencies. In GopherTrunk you’ll lock, prioritise, or mute talkgroups — and the software handles the frequency-hopping underneath. Each transmitting radio also has its own radio ID, so you can see individual units, not just groups.
What is affiliation?
Affiliation is how the system keeps track of who’s listening to what. When a radio is turned on or switches to a talkgroup, it registers (affiliates) with the system over the control channel. That lets the system route calls efficiently and, helpfully for monitoring, it means the control channel is a constant source of information about which radios and talkgroups are active — visible in GopherTrunk’s Radio IDs and CC Activity panels.
Why does GopherTrunk decode the control channel first?
Everything above runs through the control channel, so it is the single most important thing to decode. By locking onto it and reading its data stream in real time, GopherTrunk learns:
- the instant any talkgroup starts a call,
- which voice channel that call was assigned to,
- which radio ID is transmitting, and
- the system’s identity and parameters.
With that map, GopherTrunk can point a receiver at the right voice channel at the right moment, capture the audio, and immediately be ready for the next assignment — following dozens of conversations as they scatter across the channel pool. Lose the control channel and you’re blind; that’s why so much of operating GopherTrunk comes down to getting a clean, solid lock on it.
The control-channel data itself is sent using the digital modulation you met earlier — for P25 and DMR, that’s the 4FSK whose symbols you can watch in the constellation and eye-diagram panels. Different systems package this differently, which is the subject of the protocol landscape.
Quick check: on a trunked system, what do you follow to hear a particular group of users?
Recap
- Trunking shares a small pool of frequencies across many groups by assigning a channel per call.
- The control channel carries data and orchestrates everything — it’s the map.
- Talkgroups are virtual channels; you follow them, not frequencies.
- Affiliation registers radios so the system (and you) know who’s active.
- GopherTrunk decodes the control channel first so it can follow every call.
Next, we’ll survey the actual protocols — P25, DMR, NXDN and friends — and how to tell them apart. Or see the whole chain assembled in From antenna to audio.
Frequently asked questions
What is trunked radio?
Trunked radio is a system where many user groups share a small pool of radio channels automatically. Instead of each group having its own permanent frequency, a computer assigns a free channel for the duration of each call, then releases it. A dedicated control channel coordinates everything, telling radios where to go for each conversation. This packs far more users onto the same spectrum than giving everyone a fixed frequency.
What is a control channel?
A control channel is one frequency in a trunked system that continuously carries data rather than voice. It manages the system — handling radios registering, requesting calls, and being told which voice channel to switch to. To monitor a trunked system you decode the control channel first, because it’s the map that tells you where every call goes.
What is a talkgroup?
A talkgroup is a virtual channel — a label that identifies a group of users, like “Police Dispatch” or “Fire Ground 2.” Members of a talkgroup hear each other regardless of which physical frequency the system assigns to a given call. In a trunked system you follow talkgroups, not frequencies, because the frequency changes call to call.
What is the difference between conventional and trunked radio?
In conventional radio, each group has a fixed frequency it always uses. In trunked radio, frequencies are shared and assigned on demand by a control channel. Conventional is simple to scan — just listen to the frequency. Trunked needs you to decode the control channel to follow conversations as they hop between voice channels.
Why does GopherTrunk follow the control channel?
Because the control channel is where the system announces every call and the voice channel it’s assigned to. By decoding it in real time, GopherTrunk knows the instant a talkgroup starts a call and which frequency to tune a receiver to in order to capture the audio — then it follows the next call, and the next, across the whole system.