Before this:What is trunked radio?
The digital protocol landscape
Key takeaways The digital airwaves are a patchwork of standards. P25 dominates North-American public safety (Phase 1 = FDMA 12.5 kHz; Phase 2 = TDMA, double capacity). DMR rules business and amateur digital (two-slot TDMA, AMBE+2). NXDN/dPMR are narrowband alternatives; TETRA is common in Europe. Older Motorola Type II, EDACS, LTR, MPT 1327 still linger. Most use 4FSK; they differ in how they pack calls (FDMA vs. TDMA) and signal their control channel. GopherTrunk identifies the protocol once it locks on.
You understand trunking in the abstract; this is the field guide to the actual standards you’ll meet, what’s behind each name, and how they relate. The Status reference tracks GopherTrunk’s coverage of them.
FDMA vs. TDMA — the key axis
One distinction explains a lot of the family tree. FDMA (frequency-division) gives each call its own frequency. TDMA (time-division) splits one frequency into rapid time slots, so two or more calls share it by taking turns. TDMA doubles capacity in the same spectrum, which is why newer standards adopt it. Keep this in mind as we go.
P25 (Phase 1 and Phase 2)
P25 is the open standard for North-American public safety:
- Phase 1 — FDMA, 12.5 kHz channels, C4FM 4FSK, IMBE vocoder. The workhorse for police/fire.
- Phase 2 — TDMA, two voice slots per channel for double capacity, a PSK-family modulation, AMBE+2 vocoder.
If you’re in the US tracking emergency services, P25 is what you’ll see most.
DMR (Tier II and Tier III)
DMR (Digital Mobile Radio) is an ETSI standard huge in business and amateur radio. It’s two-slot TDMA in a 12.5 kHz channel using AMBE+2:
- Tier II — conventional (non-trunked) two-slot DMR; very common commercially and on ham repeaters.
- Tier III — trunked DMR with a control channel.
DMR’s low cost made it ubiquitous outside public safety.
NXDN and dPMR
NXDN (used by Kenwood/Icom systems) and dPMR are narrowband standards (often 6.25 kHz) using 4FSK, found in business, transport, and utilities. They trade some audio quality for very efficient spectrum use, and both come in conventional and trunked flavours.
TETRA
TETRA (Terrestrial Trunked Radio) is a European-rooted standard widespread in public safety and transport outside North America. It uses a four-slot TDMA scheme with π/4-DQPSK modulation and is a complete trunked system with strong feature support.
Legacy systems
Plenty of older systems still run, frequently with analog voice but a data control channel for trunking:
| System | Notes |
|---|---|
| Motorola Type II | Classic analog trunking, widespread legacy |
| EDACS | GE/Ericsson trunking |
| LTR | Logic Trunked Radio, simple business trunking |
| MPT 1327 | Analog trunking common outside the US |
They’re fading as agencies migrate to P25/DMR, but a scanner enthusiast still meets them.
How to tell them apart
- Database first. RadioReference lists the system type for known systems in your area — by far the easiest route.
- On the air. Channel width and modulation footprint on the waterfall narrow it down; TDMA vs. FDMA behaviour (one slot vs. shared) is a clue.
- Let the decoder say. Once GopherTrunk locks a control channel, the signalling identifies the protocol and system parameters for you.
| Standard | Access | Typical use | Vocoder |
|---|---|---|---|
| P25 Phase 1 | FDMA | US public safety | IMBE |
| P25 Phase 2 | TDMA | US public safety (high capacity) | AMBE+2 |
| DMR | TDMA (2-slot) | Business, amateur | AMBE+2 |
| NXDN / dPMR | FDMA (narrow) | Business, utilities | AMBE+ variants |
| TETRA | TDMA (4-slot) | Public safety (EU and beyond) | ACELP |
Quick check: P25 Phase 2 and DMR both use TDMA. What does TDMA let them do?
Recap
- The big axis is FDMA (a frequency per call) vs. TDMA (time slots share a frequency).
- P25 = US public safety (Phase 1 FDMA, Phase 2 TDMA); DMR = business/amateur (2-slot TDMA).
- NXDN/dPMR are narrowband; TETRA is common in Europe; legacy Motorola/EDACS/LTR/MPT linger.
- Identify systems via a database, their on-air footprint, or by letting the decoder read the control channel.
Next: the reason some talkgroups stay silent no matter what — encryption.
Frequently asked questions
What is the difference between P25 and DMR?
Both are digital land-mobile-radio standards using 4-level FSK, but P25 is a North-American public-safety standard (Phase 1 is FDMA at 12.5 kHz; Phase 2 is TDMA for double capacity) using IMBE/AMBE+2 vocoders. DMR is an ETSI standard widely used in business and amateur radio, using two-slot TDMA in a 12.5 kHz channel with the AMBE+2 vocoder. P25 dominates US public safety; DMR dominates commercial and ham digital.
How can I tell which protocol a system uses?
Start with a database like RadioReference, which lists the system type for known systems. On the air, the modulation and channel width give clues, and decoder software identifies the protocol once it locks onto the control channel. GopherTrunk recognises the protocol from the control-channel signalling once it’s decoding.
What is the difference between FDMA and TDMA?
FDMA (frequency-division multiple access) gives each call its own frequency channel. TDMA (time-division multiple access) splits one frequency into time slots so several calls share it, alternating in rapid turns. P25 Phase 1 is FDMA; P25 Phase 2 and DMR are TDMA, which is how they fit more conversations into the same spectrum.
Are old analog trunking systems still around?
Yes. Legacy systems like Motorola Type II, EDACS, LTR, and MPT 1327 still operate in places, often with analog voice but a data control channel for trunking. Newer deployments are overwhelmingly digital (P25, DMR), but a scanner enthusiast still encounters legacy systems, and GopherTrunk supports a range of them.