Trunked-radio control channel
A nonstop data channel that runs a trunked radio system, telling radios which channel to use for each call.
What does it sound like?
A constant, never-ending digital buzz or warble.
Who uses it?
Public safety, government and business radio systems.
Where and when can I hear it?
VHF, UHF, 700, 800 and 900 MHz.
The science
In a trunked system, a controller shares a pool of voice channels among many user groups, and the control channel is where radios ask for and are granted a channel: when someone keys up, the controller assigns a traffic channel and every radio in that talkgroup moves to it, returning to the control channel afterward. In Project 25 Phase 1 the control channel uses C4FM, a four-level FM that sends 4800 symbols per second at two bits each for 9600 bits per second in a 12.5 kHz channel. Radios register on it when switched on, and send requests in time slots marked by status symbols on the outbound channel, a random-access scheme called slotted ALOHA. Motorola's older SmartNet and SmartZone systems run their control channels at 3600 bits per second using FSK, sending outbound signaling words that carry system identity, status and other control information.
Why use it?
Trunking lets many groups share a few channels automatically, with a controller handing out channels as calls start, instead of each group being tied to its own frequency. Registration also lets the network admit only authorized radios and keep track of where each one is.
Lore
The P25 standard was first established in October 1989. P25 radios can also use the busy and idle status symbols on the outbound channel to measure the base station's carrier and correct their own frequency reference, a technique called automatic frequency control.