Before this:Conventional vs. trunked radio
Talkgroups, radio IDs & affiliation
Key takeaways A talkgroup is a virtual channel — a number plus a label like “PD Dispatch” — that binds a group of users together regardless of which frequency a call lands on. A radio ID (unit ID) identifies the individual radio behind each transmission. Affiliation (registration) is the message a radio sends to tell the system which talkgroup it’s monitoring, which both routes calls and gives a monitor a live who’s-active picture. Calls can be group, individual/private, or flagged emergency. Because frequencies change per call, you follow talkgroups, not frequencies.
A trunked system constantly moves conversations across its channel pool, so it needs stable identities to make sense of it all. Three identities matter, and once you know them, the control-channel data stops looking like noise.
Talkgroups — the virtual channel
A talkgroup is the unit of organisation users actually think in. It’s a number (say, 101) paired with a label (“Police Dispatch”). Every member of a talkgroup hears every call to that talkgroup, no matter which physical voice channel the system assigns. The talkgroup is virtual precisely because the frequency underneath it is not fixed — the system may grant channel 3 for one call and channel 7 for the next.
This is why, on a trunked system, you follow talkgroups, not frequencies. You decide you want “Fire Dispatch” and let the software chase the frequency for you. In GopherTrunk you lock, prioritise, or mute talkgroups, and the control-channel follower handles the hopping underneath.
Radio IDs — the individual unit
Where a talkgroup names a group, a radio ID (also unit ID or source ID) names a single radio. Every transmission on a trunked system carries the ID of the radio that keyed up, so you can see not just “Dispatch is busy” but “unit 1147 is transmitting to Dispatch.” Systems and databases map these numbers to aliases — a badge number, a vehicle, a dispatcher console — so a stream of IDs becomes a roster of who’s on the air.
GopherTrunk surfaces these in the Radio IDs panel, letting you watch individual units appear, affiliate, and transmit across the system.
Affiliation — registering interest
How does the system know who’s listening to “Fire Dispatch” so it can route a call there? Through affiliation, also called registration. When a radio powers on, joins a site, or turns its channel selector to a new talkgroup, it sends an affiliation message on the control channel declaring which talkgroup it now monitors.
Affiliation serves the system: it can grant a voice channel only at sites where members are actually present, saving capacity. It also serves you: because radios announce themselves this way, the control channel is a steady stream of information about which units and talkgroups are active right now — visible in GopherTrunk’s CC Activity and Radio IDs panels even before any voice is keyed.
Group, private, and emergency calls
Not every call goes to a whole group. The control-channel signalling distinguishes a few kinds:
| Call type | Goes to | Notes |
|---|---|---|
| Group call | A whole talkgroup | The common case; all affiliated members hear it |
| Individual / private call | One specific radio ID | Unit-to-unit, like a phone call between two radios |
| Emergency | A talkgroup, flagged urgent | A radio raises an emergency; the system prioritises it |
A group call is the everyday traffic — dispatch to a unit, units to each other within a talkgroup. An individual (private) call is one radio addressing another by its radio ID, heard only by those two. An emergency is a group call carrying an urgent flag, which the system prioritises and which usually stands out clearly in the control-channel log. All three are announced in the same data stream you decode, so you can see them coming.
Quick check: to hear a particular group of users on a trunked system, what do you follow?
Recap
- A talkgroup is a virtual channel — a number and label that binds a group of users.
- A radio ID (unit ID) identifies the individual radio behind each transmission.
- Affiliation registers a radio’s chosen talkgroup so the system can route calls.
- Calls can be group, individual/private, or flagged emergency.
- You follow talkgroups, not frequencies, because the frequency changes per call.
Next, we trace a single call from start to finish — the anatomy of a trunked call: request, grant, and release.
Frequently asked questions
What is a talkgroup?
A talkgroup is a virtual channel — a number with a label like “PD Dispatch” or “Fire Ground 2” — that identifies a group of users. Everyone in a talkgroup hears each other’s calls no matter which voice channel the system assigns. On a trunked system you follow talkgroups rather than frequencies, because the frequency changes from call to call.
What is a radio ID or unit ID?
A radio ID, also called a unit ID or source ID, is a number that identifies an individual radio on the system. Every transmission carries the ID of the radio that sent it, so you can see which specific unit is talking, not just which group. IDs are usually mapped to aliases like a badge number or vehicle in databases.
What is affiliation?
Affiliation, or registration, is how a radio tells the system which talkgroup it is monitoring. When a radio powers on or changes talkgroups, it sends an affiliation message on the control channel. This lets the system route calls only to sites where members are listening, and it gives a monitor a live picture of who is active.
What is the difference between a group call and a private call?
A group call goes to a whole talkgroup — everyone affiliated to it hears the audio. A private or individual call is one radio calling another by its radio ID, heard only by those two units. Most traffic on public-safety systems is group calls; private calls and emergency declarations also appear in the control-channel data.