TETRA

Key takeaways TETRA (Terrestrial Trunked Radio) is the ETSI EN 300 392 standard that dominates public safety and commercial radio outside North America — across Europe, Asia, the Middle East, and beyond. It packs four TDMA slots into a 25 kHz channel using π/4-DQPSK at 18000 symbols/s (36 kbps gross) with an ACELP voice codec. Radios work in TMO (through the network) or DMO (radio-to-radio, no infrastructure), and encryption is routine. On the waterfall it is notably wider — 25 kHz — than P25 or DMR, and you’ll rarely meet it in the US, where P25 won out.

You met TETRA briefly in the protocol landscape. Here is the full picture: a mature, feature-rich trunked standard that most of the world’s emergency services run on, even though it barely registers on North American airwaves.

Where TETRA came from

TETRA was designed by ETSI, the European Telecommunications Standards Institute, and published as the EN 300 392 family of specifications in the 1990s. The goal was a single open digital standard for professional mobile radio — the kind of mission-critical network police, fire, ambulance, railways, airports, and utilities depend on. It succeeded spectacularly outside the US: national public-safety networks across Europe, large parts of Asia, the Middle East, Africa, and Latin America are TETRA. Think of it as the rough-equivalent role that P25 plays in North America, filling the same public-safety niche with a different rulebook.

How the air interface works

TETRA’s physical layer is its most distinctive trait. A carrier is 25 kHz wide and carries four time slots in a TDMA frame, so up to four calls (or signaling plus three calls) share one frequency by taking turns — the same time-sharing idea you met in TDMA vs FDMA, but with four slots rather than DMR’s two.

Modulation: π/4-DQPSK — a differential four-phase scheme. Each symbol carries two bits, and the constellation rotates by a quarter-turn between symbols, which keeps the envelope from passing through zero and eases the transmitter’s amplifier. This is not the 4FSK of P25 and DMR; it’s phase modulation, and on the constellation it shows up as a ring of points rather than four frequency levels.
Symbol rate: 18000 symbols/s, giving a 36 kbps gross channel rate (2 bits per symbol). Split four ways, each slot gets a useful sub-rate for voice and signaling.
Voice codec: ACELP (Algebraic Code-Excited Linear Prediction), a different family from the AMBE/IMBE vocoders used by P25 and DMR. ACELP is the same general technique behind many cellphone codecs of the era.

A TETRA carrier is one fat 25 kHz block carrying four interleaved time slots — wider than the narrow channels P25 and DMR use, which is your first visual clue.

TMO and DMO

TETRA radios operate in two modes:

TMO — Trunked Mode Operation. The normal case: radios register with and talk through the infrastructure (base stations linked to a switching and management network), coordinated by a control channel. This gives you all the trunking benefits — group calls, individual calls, priority, area-wide coverage.
DMO — Direct Mode Operation. Radios talk directly to one another with no infrastructure at all, like a simplex walkie-talkie. Crews use DMO when they’re out of network coverage or want a local tactical channel that doesn’t load the network.

The distinction matters for monitoring: a DMO conversation never touches a control channel, so there’s no central signaling to follow — you’d watch the simplex frequency directly.

Why it’s wide, and why that helps you

The headline practical fact is bandwidth. A TETRA carrier is 25 kHz, double the 12.5 kHz of P25 Phase 1 or a DMR channel. On the waterfall that fat block stands out, and combined with the phase-modulated constellation (a ring, not four FSK levels) it’s a reasonable bet you’re looking at TETRA rather than a 4FSK system — especially if you’re outside North America.

Encryption is common

Unlike many systems where clear voice is the default, encryption is routine on TETRA. The standard defines both air-interface encryption (scrambling the whole radio link, including signaling) and end-to-end encryption for the most sensitive users. Where it’s enabled, you’ll see a healthy control channel and active traffic slots but get no intelligible audio — the same wall you met in encryption and authentication. That’s expected on a lot of TETRA networks.

Quick check: what makes a TETRA carrier stand out on the waterfall compared with P25 or DMR?

Recap

TETRA is the ETSI EN 300 392 trunked standard that dominates public safety and commercial radio outside North America.
It uses four-slot TDMA in a 25 kHz channel with π/4-DQPSK at 18000 symbols/s (36 kbps) and the ACELP voice codec.
Radios run in TMO (through the network) or DMO (direct, no infrastructure).
It is wider than P25/DMR — a 25 kHz block — and uses phase rather than frequency modulation, both handy identification clues.
Encryption is common, so don’t be surprised by active-but-silent TETRA traffic.

Next we cross back to a narrowband North-American favourite and its global cousins: NXDN.

Frequently asked questions

What is TETRA?

TETRA (Terrestrial Trunked Radio) is an ETSI digital trunking standard, defined in EN 300 392, used heavily by public safety, transport, and industry outside North America. It uses four-slot TDMA in a 25 kHz channel with π/4-DQPSK modulation and an ACELP voice codec. It is a complete trunked system with strong support for group calls, direct mode, and encryption.

Why do I rarely hear TETRA in the United States?

TETRA was developed in Europe and its standard 25 kHz channels and frequency plan did not fit cleanly into US allocations or the FCC narrowbanding push toward 12.5 kHz and below. North American public safety standardised on P25 instead. TETRA dominates Europe, much of Asia, the Middle East, Africa, and Latin America, but it is uncommon on US airwaves.

How wide is a TETRA channel on the waterfall?

A TETRA carrier occupies 25 kHz, noticeably wider than the 12.5 kHz of P25 Phase 1 or DMR. That extra width is a useful first clue when you spot one on the waterfall — a single fat 25 kHz block carrying four interleaved time slots rather than a narrow 12.5 kHz lane.

What is the difference between TMO and DMO?

TMO (Trunked Mode Operation) is the normal mode where radios talk through the infrastructure — base stations and the switching network — using the control channel. DMO (Direct Mode Operation) lets two radios talk directly to each other without any infrastructure, like a walkie-talkie, for when units are out of network range.