Trunked Radio Systems

What is Trunking?

Trunked radio solves the problem of limited radio frequencies. Instead of each agency having dedicated channels (which sit idle most of the time), trunking systems pool frequencies and automatically assign them as needed. A computer controller — the trunking system controller— manages talk groups, channel assignment, and priority levels.

The term "trunking" comes from the telephone industry, where trunk lines carry multiple conversations over shared circuits. In radio, a trunked system might have 10–30 frequency pairs in a pool, serving 50+ talk groups with hundreds of users. The controller assigns an available frequency pair when a user keys the microphone and releases it when they release — most channels sit idle until needed.

Without trunking, a police department with 10 channels and 500 officers would need to manually assign channels — most officers would share channels, causing congestion. With trunking, the same 10 channels serve all 500 officers automatically, because not everyone talks simultaneously.

How Trunking Works

The trunking process follows these steps:

• 1. User keys PTT (Push-to-Talk): The radio sends a request on the control channel — a dedicated frequency that carries system commands, not voice.
• 2. Controller assigns channel: The controller finds an available frequency pair in the pool and sends a channel grant message to all radios in the user's talk group.
• 3. All radios retune: Every radio in the talk group automatically switches to the assigned frequency. Only the originating user transmits; the others receive.
• 4. User releases PTT: The radio sends a release message on the control channel. The controller marks the frequency as available for other talk groups.

The entire process — from PTT press to channel grant — takes 200–500 milliseconds. Users perceive this as near-instantaneous. The control channel operates continuously, even when no voice traffic is active, polling radios and delivering system information.

Talk Groups

Talk groups are virtual channels — users only hear transmissions from their own group, even though all groups share the same frequency pool. A police department might define talk groups such as:

• Dispatch (Primary): Routine patrol communications — the busiest talk group
• Tactical (Operations): Active incident coordination — separated from dispatch to avoid congestion
• Detective Bureau: Investigations — lower priority, less frequent
• SWAT/Tactical Response: High-priority, encrypted
• Interoperability: Multi-agency channel for joint operations (police + fire + EMS)
• Emergency (All Call): Priority override — all radios break through to receive emergency broadcasts

Interoperability talk groups are critical during major incidents (natural disasters, terrorist attacks, large-scale emergencies) where police, fire, EMS, and federal agencies must communicate. The lack of interoperability was identified as a major failure during the September 11, 2001 attacks, leading to the creation of nationwide interoperability channels.

Priority and Preemption

Trunked systems support priority levels that determine who gets a channel when the system is congested:

• Normal priority: Standard users — get a channel if one is available
• High priority: supervisors, command staff — jump ahead in the queue
• Emergency priority: overrides all other traffic — the system preempts a lower-priority call and reassigns the channel
• System-wide emergency: All Call — every radio in the system switches to the emergency channel regardless of current activity

Preemption means the controller can forcibly end a lower-priority call to make room for a higher-priority one. The preempted user hears a tone and their channel is reassigned. This is critical for public safety — an officer in distress must be able to communicate even if all channels are busy.

Trunking Standards

Motorola Type I / Type II / SmartZone

Motorola dominated the trunked radio market with proprietary standards:

• Type I (1970s): First commercial trunked system — 5-tone signaling, 5-bit user IDs, limited talk groups. Voice channels used analog FM.
• Type II (1980s): Expanded to 16-bit user IDs (65,536 possible), more talk groups, improved signaling. Type II SmartZone added multi-site roaming — a user could roam between trunked sites and automatically register.
• SmartZone Omnilink: Combined multiple trunked sites into a wide-area network with automatic site selection based on GPS or signal strength.
• ASTRO (25): Motorola's P25-compliant digital platform — migrated analog Type II systems to digital P25 Phase I/II.

MPT-1327 (UK Standard)

The British standard MPT-1327 (1984) used 10 kHz channels with 1,200 bps FSK (Frequency Shift Keying) signaling on the control channel. Voice was analog FM. MPT-1327 was widely deployed across Europe, Asia, and the Middle East. The MPT-1343 addendum defined Call Priority and other features. It supported up to 280,000 users per system with 32,768 talk groups.

EDACS (Enhanced Digital Access Communications System)

Developed by AT&T/Lucent (1985), EDACS was a digital trunked system using 25 kHz channels with 4,800 bps FSK control channel signaling. EDACS supported both analog and digital voice (CELP codec at 4.8 kbps). It competed with Motorola Type II in the US market but was eventually overtaken by P25.

LTR (Logic Trunked Radio)

LTR (Ef Johnson, 1989) used a distributed architecture — no central controller. Each repeater had its own logic, and radios scanned for the home channel. LTR was popular for commercial fleets (trucking, construction, taxi) due to lower infrastructure cost. LTR-Net added wide-area networking. End-of-line markers on each channel identified the system to scanning radios.

Project 25 (P25): The Digital Standard

Project 25 (P25) is the digital standard for public safety radio in North America, developed by APCO International with TIA (Telecommunications Industry Association) standardization. P25 ensures interoperability between different manufacturers — a critical requirement for multi-agency public safety operations.

P25 Phase I (FDMA)

• Channel width: 12.5 kHz (equivalent to analog narrowband)
• Modulation: C4FM (Continuous 4-level FM) — ±600 Hz deviation per symbol
• Data rate: 9,600 bps (raw), 6,000 bps (voice payload)
• Voice codec: IMBE (Improved Multi-Band Excitation) at 4,400 bps — 12 dB SINAD equivalent to analog
• Encryption: DES (56-bit, legacy), AES-256 (256-bit, current standard), ARC4 (proprietary, deprecated)
• Control channel: 9,600 bps FSK — carries talk group assignments, registration, alerts
• Coding: Golay (24,12) error correction for control channel, Trellis Coded Modulation for voice

P25 Phase II (TDMA)

• Channel width: 12.5 kHz (same as Phase I)
• Multiple access: TDMA — 2 time slots per 12.5 kHz channel (effectively 6.25 kHz per slot)
• Voice codec: AMBE+2 (Advanced Multi-Band Excitation) at 2,450 bps per slot
• Capacity: Doubled — 2 simultaneous voice calls per 12.5 kHz channel
• Data rate: 9,600 bps per slot, with 2 slots available
• Interference: Less susceptible to adjacent-channel interference than FDMA

P25 Phase II requires TDMA synchronization — all radios must be time-synchronized to the control channel. This is achieved through periodic synchronization broadcasts. The 2-slot TDMA frame is 40 ms long (20 ms per slot).

P25 Talk Group ID Structure

P25 uses a hierarchical talk group ID system:

• Radio ID: 24-bit unique identifier per radio (up to 16 million radios)
• Talk Group ID: 16-bit identifier per talk group (up to 65,535 groups)
• WACN (Wide Area Communication Network): 12-bit — identifies the system (up to 4,096 systems)
• BID (Block ID): 8-bit — identifies the site within a system

Standard talk group ranges: 1–15 for system management, 16–2,000 for local agencies, 2,001–10,000 for regional, 10,001–65,535 for nationwide. The FCC maintains a public database of P25 talk group assignments.

Encryption in Public Safety Radio

Public safety radio encryption protects sensitive communications (SWAT operations, undercover investigations, medical information) from scanning. P25 supports multiple encryption algorithms:

• AES-256: Advanced Encryption Standard — 256-bit key, currently recommended. Virtually unbreakable with current computing.
• DES (Data Encryption Standard): 56-bit key — deprecated, still found in legacy systems. Breakable with modern hardware.
• RC4 (ARC4): Variable key length — proprietary Motorola implementation. Deprecated due to known vulnerabilities.
• Multi-Key REK (Radio Emergency Key): Allows multiple encryption keys to be loaded, with automatic key management via the control channel.

The debate over public safety encryption is ongoing: agencies argue that encryption protects officer safety and operational security, while the public and media argue that encrypted radio prevents accountability and public oversight of police operations.

Comparison: P25 vs DMR vs TETRA

From Trunking to LTE: FirstNet and Beyond

The future of public safety radio is converging with cellular broadband.FirstNet (First Responder Network Authority, US) is a dedicated LTE/5G network for first responders, providing:

• Broadband data: Video streaming, image sharing, database access, mapping
• Priority and preemption: First responders get priority over commercial traffic during emergencies
• Push-to-talk over LTE: Voice communication via VoLTE with PTT functionality
• Nationwide coverage: Leveraging AT'T's commercial LTE/5G infrastructure with dedicated core network

FirstNet does not replace P25 trunking — it supplements it. P25 provides reliable voice and low-bandwidth data; FirstNet provides broadband capabilities. The two networks coexist, with bridges allowing interconnection. P25 Phase III (planned) will add OFDM broadband capabilities to the P25 standard.