The Problem Trunking Solved
Traditional analog radio systems assigned each agency or unit a fixed frequency. The county sheriff always transmitted on 155.340 MHz, for example, and every radio tuned to that frequency could hear every transmission on it. This worked, but it was wasteful: during most hours, a given frequency sits idle while others are busy.
As public safety agencies grew and radio usage increased, spectrum became a genuine constraint. There were not enough available frequencies to give every department, every division, and every working group its own dedicated channel. Agencies needed a way to serve many users efficiently across a limited set of frequencies.
Trunking is the solution. A trunked radio system uses a pool of frequencies and a control channel to dynamically assign radios to whichever frequency is free at the moment a transmission begins. The radios do not sit on a fixed frequency. They are told in real time, by the control channel, which frequency to use for each call.
How a Trunked System Works
A trunked system has two main components: a control channel and a pool of voice channels. The control channel is a data channel that runs continuously. Every radio in the system listens to the control channel at all times, even when not in use.
When a dispatcher needs to talk to a group of units, or when a unit keys up to transmit, the control channel assigns that transmission to whichever voice frequency is currently free. All the radios belonging to that talk group instantly switch to the assigned frequency and hear the transmission. When the transmission ends, those radios return to monitoring the control channel, ready to be assigned to whichever frequency is free for the next call.
From the user's perspective inside the system, it feels like a conventional radio. You push a button, you talk, your colleagues hear you. The dynamic frequency assignment is invisible. But from a listener's perspective outside the system, without compatible equipment, it becomes nearly impossible to follow because the audio jumps between frequencies constantly.
This is why a basic analog scanner tuned to a single frequency will not work for following a trunked agency. The conversation fragments across multiple frequencies, and you catch only random pieces of it.
Talk Groups
Within a trunked system, users are organized into talk groups. A talk group is roughly analogous to a channel in a conventional system: it is a logical group of radios that can communicate with each other. The patrol division for a county sheriff might be one talk group. The detective bureau is another. Fire dispatch is a third.
A large county trunked system might have dozens or even hundreds of active talk groups covering every agency and division that shares the system. Statewide systems may have thousands of talk groups covering law enforcement, fire, EMS, public works, transportation, and other government functions across every county.
When a trunked system scanner is properly programmed, you tell it which talk groups you want to follow. It monitors the control channel, recognizes when one of your programmed talk groups is assigned a voice frequency, and switches to that frequency to capture the audio. This is how a trunked scanner reproduces the experience of tuning to a conventional channel even though the underlying frequencies change constantly.
P25 and Other Trunking Standards
Several different trunking protocols have been used in public safety over the years. The oldest and most widely known is Motorola Type II, a proprietary system deployed extensively in the 1980s and 1990s. Many agencies that built Motorola systems in that era are still running them or have upgraded to newer Motorola standards.
P25 is the current open standard for digital trunked public safety radio in North America. P25 was designed to ensure interoperability across different manufacturers and is now required for federally funded public safety radio systems. Most agencies building new systems today are building P25 networks.
P25 comes in two phases. Phase 1 uses conventional FM modulation on each voice channel and sounds similar to analog radio when received. Phase 2 uses time-division multiplexing to fit two voice calls on each channel simultaneously, doubling the system's capacity. Phase 2 requires digital-capable scanning equipment to decode. For a full explanation of P25 specifically, see our guide on what P25 radio is.
What Equipment You Need to Follow a Trunked System
To monitor a trunked system with a physical scanner, you need a scanner that supports the specific trunking protocol your local agency uses. Basic analog scanners cannot follow trunked systems at all. A scanner that supports analog trunking (Motorola Type II, for example) cannot follow P25 systems without a firmware or hardware upgrade.
Modern digital trunking scanners, such as those from Uniden and Whistler, support most common protocols including P25 Phase 1 and Phase 2, Motorola Type II, and others. These scanners are more expensive than basic analog scanners, typically ranging from $150 to over $500 depending on features.
Programming a trunking scanner is more involved than programming a conventional scanner. You need to know the control channel frequencies for the system, the talk group IDs for the agencies you want to follow, and in some cases additional system metadata. RadioReference.com's database is the primary resource for this information.
Online scanner feeds on Broadcastify largely sidestep this complexity. Volunteers who host feeds have already done the technical work of configuring equipment to follow the trunked system, and the feed streams the decoded audio directly. If there is a Broadcastify feed for your local agency, you can hear the audio without any special equipment.
Statewide and Regional Trunked Systems
Many states have built shared trunked radio infrastructure that multiple agencies share. These statewide interoperability networks allow a state police trooper, a county sheriff deputy, a fire department, and a city EMS unit to communicate on the same system using a standardized set of interoperability channels.
Examples include systems like the Illinois STARCOM21, the Texas CAPRAD network, and various statewide P25 networks deployed with First Responder Network Authority (FirstNet) funding. These systems have hundreds or thousands of talk groups and cover entire states from a shared radio infrastructure.
Statewide systems mean that a single scanner programmed correctly can potentially follow dozens of different agencies. They also mean that major incidents generate talk group activity across the system in ways that a single-agency scanner would not capture.
How to Tell If Your Local Agency Uses a Trunked System
The easiest way is to look up your county on RadioReference.com. The site maintains a comprehensive database of radio systems, and each county entry shows what systems are in use. If your local sheriff or police department is on a trunked system, it will be listed there along with the control channel frequencies and a catalog of talk groups.
A practical indicator that trunking is in use: if you tune a basic analog scanner to what you believe is your local police frequency and hear occasional data bursts or control-channel chatter but no voice traffic, the agency has likely migrated to a trunked system. The frequency you are on may be the control channel or a voice channel that the system is using.
Feed descriptions on Broadcastify also often note whether the feed is a P25 trunked system, which confirms what technology is in use locally.
Written by
PoliceScannerFinder Research Team
The PoliceScannerFinder Research Team studies publicly available scanner feeds, emergency communication systems, and public safety radio technology. Our mission is to make scanner listening approachable for beginners while providing accurate, responsible information about how emergency radio works across the United States.