M17 is the newcomer and the outsider — a digital voice mode designed by hams, from the ground up, with nothing proprietary in it. An M17 repeater is an ordinary FDMA relay at heart, but what flows through it is unlike the others: an open-source voice codec, a callsign carried right in the signal with nothing to register anywhere, and a stream that announces its own destination so the machine can route without asking a server for permission. It’s also the mode where actual repeaters are still rare — and understanding why is half the story.
Like every repeater here, an M17 machine listens on an input frequency and retransmits on an output frequency, with a familiar offset. The modulation is 4FSK — four-level frequency-shift keying at 4800 symbols per second, 9600 bit/s gross, occupying about 9 kHz inside a 12.5 kHz channel. Access is FDMA: one conversation per channel at a time, no DMR-style timeslots. On the surface, very much like C4FM. Underneath, nothing alike.
The reason M17 exists is the vocoder. DMR, D-STAR, and Fusion all lean on the proprietary AMBE family, which carries licensing baggage. M17 instead uses Codec 2, a fully open-source voice codec — you can read it, modify it, and build a compatible radio without asking anyone. In stream mode the payload is one 3200 bit/s channel, either all voice or split into 1600 bit/s of voice alongside 1600 bit/s of data.
The other open touch is how the signal describes itself. Every M17 stream carries link information — who it’s from, where it’s going, and what type of traffic it is — and that information is repeated continuously across the frames, so a repeater (or a radio joining late) can lock on partway through and immediately know how to handle the stream.
Every M17 frame repeats the link info beside the Codec 2 voice, so a repeater can route straight from the stream.
Here is where M17 quietly departs from everything else. Your callsign travels inside the transmission — packed into a 48-bit field that encodes up to nine characters. There is no DMR-style numeric ID to apply for, and no D-STAR-style gateway registration to clear. The network doesn’t need a central database of who’s who, because every stream already carries its own identity.
No toll booth
Recall that a D-STAR gateway checks your registration before it will route you, and DMR expects a registered numeric ID. M17 has neither. Key up and your callsign is simply in the signal — the repeater and reflector read it directly. It’s the most decentralized of the common digital voice modes, by design.
Because the link information rides in the stream, the repeater’s job is refreshingly direct. It reads the destination field and acts on it: a reflector (and module) means “send this to the network,” while a callsign means “route this to that station.” To reach the internet, the repeater hands the stream to a piece of software called m17-gateway, which connects it to the M17 reflector network — over 130 reflectors worldwide as of 2026, each with lettered modules (A–Z) for regional and topical rooms, and interlinked by bridges to the other digital modes.
m17-gateway carries the repeater’s traffic to a reflector module; the destination came from the stream, not a lookup.
If you’ve never worked an M17 repeater, you’re in good company — there are only a couple of dozen on the air across the whole United States, and a given region may have none. The mode lives overwhelmingly on hotspots and reflectors, not hilltop machines. The reason is hardware.
A note on encryption
M17 is unusual in building optional AES encryption and ECDSA stream signatures right into the protocol. The signatures — which authenticate who sent a stream without hiding its content — are fine on the air; encryption that obscures the meaning of a message is another matter, and is generally not permitted on the U.S. amateur bands. See Encryption and Part 97 for where the line sits.
Say you key up on an M17 repeater whose gateway is linked to a reflector module:
The one-sentence version
An M17 repeater is a full-duplex 4FSK relay running an open Codec 2 stream that carries your callsign and destination inside itself, so the machine routes you to a reflector with nothing to register and nothing to look up — the catch being that full-duplex hardware keeps real repeaters rare.
That covers the machine and its linking. For the mode itself — how M17 encodes and where it fits — see How M17 Works; for the reflector side, Understanding M17 Reflectors; and for the backstory of this open-source upstart, History of M17.
A noncommercial hobby reference compiled by N6JET, gathered from public sources and shared freely for anyone interested in amateur digital voice.