Trunked Radio Systems Explained: Digital Doesn't Mean Encrypted
When police departments announce radio encryption, they often claim it's a necessary part of "going digital." This is technically false. Understanding how trunked radio systems actually work helps you counter these misleading claims and advocate for transparency.
What Is Trunking?
Traditional (conventional) radio systems assign specific frequencies to specific users. Your local fire department might always use 154.280 MHz, while police use 155.475 MHz. This is simple but inefficient—those frequencies sit idle when not in use.
Trunked systems take a different approach. Instead of dedicated frequencies, multiple agencies share a pool of channels. A computer controller dynamically assigns frequencies as needed:
Trunking Analogy: Phone Lines
Think of conventional radio like having a dedicated phone line for each department—expensive and mostly unused. Trunking is like a modern phone system where everyone shares lines and the system routes calls automatically. Same capability, much more efficient.
Key Trunking Concepts
- Talk Group
- A virtual channel that groups related users. "Police Dispatch," "Fire Tactical," and "EMS" might be different talk groups on the same physical system.
- Control Channel
- A dedicated frequency that coordinates the system. Scanners monitor this to know which frequency each talk group is using at any moment.
- System ID
- A unique identifier for each trunked system. Your scanner needs this to properly track conversations.
- NAC (Network Access Code)
- A code that identifies specific talk groups. Not encryption—just organization.
Analog vs. Digital vs. Encrypted: Three Separate Things
This is the most important distinction for advocacy. These three terms describe completely different aspects of radio technology:
| Technology | What It Means | Can Public Listen? |
|---|---|---|
| Analog | Audio transmitted as continuous radio waves (like FM radio) | Yes, with basic scanner |
| Digital | Audio converted to data, transmitted as digital signal | Yes, with digital scanner |
| Encrypted | Audio scrambled using encryption keys | No, regardless of scanner |
The Critical Point
Digital and encrypted are not the same thing. A radio system can be:
- Analog and unencrypted (traditional, fully public)
- Digital and unencrypted (modern, still fully public)
- Analog and encrypted (rare, but exists)
- Digital and encrypted (common choice to block public)
When officials say encryption was "required" by their digital upgrade, they are conflating two separate decisions.
P25: The Public Safety Standard
Project 25 (P25) is the dominant digital radio standard for public safety in North America. Developed through collaboration between government and industry, P25 ensures different manufacturers' radios can communicate.
P25 Phases
P25 Phase I
Uses FDMA (Frequency Division Multiple Access). One conversation per 12.5 kHz channel. Compatible with existing analog infrastructure.
P25 Phase II
Uses TDMA (Time Division Multiple Access). Two conversations per 12.5 kHz channel—double the capacity. Requires newer infrastructure.
P25 Encryption Options
P25 supports several encryption modes, but none are mandatory:
- Clear Mode - No encryption, anyone can listen
- DES - Older 56-bit encryption (being phased out)
- AES 256 - Current standard encryption
The Transparency-Compatible Path
Many departments use P25 in clear mode, gaining digital audio quality and interoperability benefits while maintaining public access. The FCC and P25 standards do not require encryption—that's a policy choice by local leadership.
How Trunked Systems Actually Work
Step-by-Step: A Trunked Radio Call
- Officer keys microphone
Radio sends digital request to control channel: "Talk group 1001 wants to transmit"
- Controller assigns channel
System checks available frequencies, responds: "Use channel 3 (frequency 851.2125)"
- All radios in talk group tune
Every radio monitoring talk group 1001 automatically switches to assigned frequency
- Transmission occurs
Audio transmits on the assigned frequency
- Call ends, channel released
Frequency returns to the available pool for other talk groups
What Scanners Must Do
To follow trunked conversations, your scanner must:
- Continuously monitor the control channel
- Decode the digital control data
- Identify which talk groups you want to hear
- Rapidly tune to assigned frequencies when those talk groups become active
- Return to monitoring the control channel between transmissions
Modern trunking scanners handle all of this automatically—you just program which talk groups to monitor.
Debunking Common Claims
When advocating against encryption, you'll encounter these misleading statements. Here's how to counter them:
Claim: "We had to encrypt when we went digital"
Reality
Digital and encrypted are separate features. P25 digital systems work perfectly in "clear" (unencrypted) mode. Thousands of departments use digital radio without encryption. The decision to encrypt is a policy choice, not a technical requirement.
Claim: "Modern systems don't support unencrypted operation"
Reality
All P25 systems support clear mode. The P25 standard specifically includes unencrypted operation. Vendors like Motorola and Harris sell systems configured for transparent operation daily. Ask which specific system "requires" encryption—there isn't one.
Claim: "Scanner users can't follow trunked systems anyway"
Reality
Trunking scanners have existed for decades. Modern digital scanners track trunked systems effortlessly. Services like Broadcastify stream thousands of trunked systems to millions of listeners. The technology to follow trunked radio is mature and widely available.
Claim: "Digital audio can't be broadcast online"
Reality
Broadcastify and similar services stream P25 digital audio from hundreds of departments. Digital scanners output standard audio that can be streamed like any other source. Digital format has zero impact on the ability to share publicly.
Claim: "The FCC requires encryption for public safety"
Reality
Completely false. The FCC has no encryption mandate. In fact, the Communications Act protects the public's right to receive radio transmissions. Encryption is entirely a local policy decision—the federal government doesn't require it.
Listening to Trunked Systems
Scanner Requirements
To monitor unencrypted trunked systems, you need a scanner that supports:
- Trunking - Ability to track control channels and follow talk groups
- Digital decode - P25 Phase I at minimum; Phase II for newer systems
- Your local system type - Motorola, P25, EDACS, or LTR depending on your area
Recommended Scanners for Trunked Systems
Uniden SDS100/SDS200
Top-tier digital trunking scanners. Support P25 Phase I & II, DMR, NXDN. SDS100 is portable; SDS200 is base/mobile.
Whistler TRX-1/TRX-2
Strong P25 performance at lower price point. Good choice for trunked system monitoring.
Uniden BCD436HP/536HP
Previous generation but still capable. More affordable entry into digital trunking.
Programming Resources
- RadioReference.com - Database of trunked systems with talk group IDs
- Sentinel/FreeSCAN - Software for programming Uniden scanners
- EZ-Scan - Software for Whistler scanners
Encryption Stops Everything
If your local system uses encryption, no consumer scanner can decode the audio. You'll hear the scanner lock onto transmissions but only receive garbled noise or silence. This is why fighting encryption policy matters—once implemented, technical solutions don't exist.
Advocacy Talking Points
When speaking at city council meetings or to local media, use these technically accurate points:
"Digital Does Not Mean Encrypted"
"The department's claim that encryption was required by their digital upgrade is technically incorrect. P25 digital systems fully support unencrypted operation. The choice to encrypt was a policy decision, not a technical necessity."
"Transparency-Compatible Options Exist"
"Hundreds of police departments across America use modern P25 digital radio in clear mode. They gain the benefits of digital audio quality and interoperability while maintaining public transparency. Our department can do the same."
"Ask the Right Questions"
"I'd like to see documentation from the radio vendor stating that encryption is technically required. If this is a policy choice rather than a technical requirement, that decision should involve public input."
"Selective Encryption Is Possible"
"P25 systems support talk-group-level encryption. Tactical and undercover operations can be encrypted while routine dispatch remains public. Full encryption is a choice, not the only option."
Technical Resources
- CISA P25 Compliance Assessment Program - Official P25 standards information
- RadioReference P25 Wiki - Community technical documentation
- How Police Scanners Work - Broader scanning fundamentals
- SDR for Beginners - Software-defined radio alternative
Frequently Asked Questions
What is a trunked radio system?
A trunked radio system is a shared communication network where multiple users and talk groups dynamically share a pool of radio frequencies. Instead of each agency having dedicated frequencies, a computer controller assigns available channels as needed, making more efficient use of limited radio spectrum.
Does digital radio automatically mean encrypted?
No. Digital and encrypted are completely separate technologies. Digital radio simply means audio is transmitted as data rather than analog signals. P25 Phase I and II digital systems can operate without any encryption, allowing anyone with a compatible scanner to listen. Encryption is an additional layer that departments must specifically enable and pay for.
What is P25 radio?
P25 (Project 25) is a set of digital radio standards developed for public safety. P25 radios allow interoperability between different agencies and manufacturers. P25 supports encryption but doesn't require it—many departments use P25 in unencrypted mode for transparency while gaining digital audio quality benefits.
Why do police departments claim they 'had to' encrypt when going digital?
This is a common misleading claim. Digital migration and encryption are separate decisions. Many departments upgrade to P25 digital for better audio quality and interoperability while keeping transmissions unencrypted. When officials claim encryption was 'required' by the digital upgrade, they are either misinformed or deliberately conflating the two technologies.
Can I listen to trunked radio with a regular scanner?
You need a scanner capable of tracking trunked systems. Many modern digital scanners (like Uniden SDS100/200 or Whistler TRX series) can follow trunked conversations by monitoring the control channel and hopping to active frequencies. However, if the system uses encryption, you'll only hear garbled audio regardless of your scanner's capabilities.
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