The control channel is the signaling channel that every trunked radio on an SCNP25 system talks to. It does not carry voice itself — instead it manages who is on the system, which talkgroup each radio is affiliated to, which voice frequency a call should use, and which radios are allowed to do what.
There is one control channel per system. Voice traffic is carried separately on whichever frequency the control channel grants for a given call.
| Function | Trigger | Outcome |
|---|---|---|
| Registration | A radio registers with its Subscriber ID. | The radio is added to the system's subscriber list. Duplicate Subscriber IDs are rejected. |
| Deregistration | A radio cleanly disconnects, or has been silent for five minutes. | The Subscriber ID is freed, and any active emergency alarm for it is cleared. |
| Affiliation | A radio asks to join a talkgroup. | The talkgroup is recorded against that subscriber. Call requests for any other talkgroup will be denied. |
| Channel grant | A radio presses PTT on its affiliated talkgroup. | The control channel picks a free voice frequency, announces it to the system, and tells the talker to transmit there. |
| Channel release | The talker releases PTT. | The voice channel is marked released and enters hangtime — radios on that talkgroup can keep using it briefly. |
| Inhibit / Uninhibit | A dispatcher targets a specific Subscriber ID. | The control channel forwards the command; the targeted radio powers itself down or back up. |
| Emergency | A radio raises an emergency on its talkgroup. | The control channel rebroadcasts to the talkgroup and remembers the alarm so it can be cleared later. |
| Location query | A dispatcher queries a Subscriber ID. | The control channel forwards the query; the targeted radio replies with GPS coordinates if it allows it. |
When the control channel grants a voice channel for a PTT, it picks one in this order:
If none of those are available, the call is denied with NO_CHANNEL_FREE.
Hangtime is configured per system. After a transmission ends, the voice frequency stays bound to that talkgroup for the configured hangtime so the next reply lands on the same frequency.