# M17 (amateur radio)

M17 is a digital radio modulation mode developed by Wojciech Kaczmarski (amateur radio call sign SP5WWP) et al. M17 is primarily designed for voice communications on the VHF amateur radio bands, and above. The project received a grant from the Amateur Radio Digital Communications in 2021 and 2022. The protocol has been integrated into several hardware and software projects. In 2021, Kaczmarski received the ARRL Technical Innovation Award for developing an open-source digital radio communication protocol, leading to further advancements in amateur radio.

## Technical characteristics

M17 uses Frequency-Division Multiple Access (FDMA) technology in which different communication streams are separated by frequency and run concurrently. It utilizes 4,800 symbols per second, 4-level frequency-shift keying (4FSK) with a root Nyquist filter applied to the bitstream. Radio channels are 9 kHz wide, with channel spacing of 12.5 kHz. The gross data rate is 9,600 bits per second, with the actual data transfer at 3,200. The transmission, called stream, is divided into 40-millisecond long frames, each prepended with a 16-bit long synchronization word. A group of 6 frames form a superframe and is needed to decode the link information data. Protocol allows for low-speed data transfer (along with voice), e.g. GNSS position data. The mode has been successfully transmitted through EchoStar XXI and QO-100 geostationary satellites. The protocol's specification is released under GNU General Public License.

## Voice encoding

M17 uses Codec 2, a low bitrate voice codec developed by David Rowe VK5DGR et al. Codec 2 was designed to be used for amateur radio and other high compression voice applications. It is based on linear predictive coding with mixed-harmonic sinusoidal excitation. The protocol supports both 3200 (full-rate) and 1600 bits per second (half-rate) modes.

## Error control

Three methods are used for error control: binary Golay code, punctured convolutional code and bit interleaving. Additionally, exclusive OR operation is performed between data bits and a predefined decorrelating pseudorandom stream before transmission. This ensures that there are as many symbol transitions in the baseband as possible. A 16-bit cyclic redundancy check (CRC) code is used for data integrity assurance.

## Application functions

The M17 protocol was primarily designed for amateur radio use.
* Callsign encoding: 48-bit field holding up to 9 alphanumeric characters eliminates the need of a centralized user-ID database.
* Stream and packet modes of operation.
** Stream mode offers one 3200bps net bitrate channel (encoded speech or data) or two 1600bps channels (encoded speech alongside data).
** Packet mode supports text messaging, APRS and AX.25.
* Slow-speed side channel for short and repeated data transfers, e.g. GNSS position data or telemetry.
* Encryption:
** Bit scrambler encryption: a pseudorandom binary sequence created by combining an exclusive-or bitwise operation on the audio or data stream and a linear-feedback shift register using one of 3 feedback polynomials with 255, 65,535 and 16,777,215-bit repeat periods.
** AES encryption: 128-bit block encryption cipher operating in CTR mode with user-selectable 128, 192 or 256-bit key.

## Hardware support

With a small hardware modification, TYT MD-380, MD-390 and MD-UV380 handheld transceivers can be flashed with a custom, free, open source firmware to enable M17 support.
In July 2024, a US-based company Connect Systems, Inc. released the CS7000-M17, being the first commercial off-the-shelf handheld transceiver with native M17 support.

## Bridging with other modes

Links between M17 and other digital voice modes and Internet linked networks exist, with several networks providing M17 access. Modes bridged include DMR, P25, System Fusion, D-STAR, NXDN, AllStarLink, EchoLink and IRLP.

## Comparison with other standards

| | M17 | Digital Mobile Radio (DMR) | D-STAR | System Fusion (YSF) |
| Type | Open | Open | Partially open | Closed (proprietary) |
| Developer | Amateur radio community (decentralized) | European Telecommunications Standards Institute (ETSI) | Japan Amateur Radio League (JARL), ICOM Inc., Kenwood Corporation, and others | Yaesu |
| Modulation | 4FSK | 4FSK | GMSK | 4FSK |
| Channel (medium) access | FDMA | FDMA (Tier I), 2-slot TDMA (Tier II/III) | FDMA | FDMA |
| Voice codec | Codec 2 (open-source) | AMBE+2 (proprietary) | AMBE (proprietary) | AMBE+2 (proprietary) |
| Arbitrary data transfer support | Full | Limited (text messaging, geolocation) | Limited (text messaging, geolocation, images) | Limited (text messaging, geolocation, images) |
| Extensibility | Full | Fixed | Fixed | Fixed |

## M17 over IP

Access nodes and repeaters can be linked using reflectors. Over 200 M17 reflectors exist worldwide (November 2024).

## History

The project was started in 2019 by Wojciech Kaczmarski in Warsaw, Poland. A local amateur radio club he was a member of, was involved in digital voice communications. Kaczmarski, having experimented with TETRA and DMR, decided to create a completely non-proprietary protocol and named it after the club's street address - Mokotowska 17. As every part of the protocol was intended to be open source, Codec 2, released under the GNU LGPL 2.1 license, was chosen as the speech encoder.

## Applications and projects with M17 support

* OpenRTX - free and open-source firmware for ham radios
* WPSD - digital voice software suite for amateur radio personal hotspots and repeaters
* DroidStar - digital voice client for Android
* Multi-mode Digital Voice Modem (MMDVM)
* SDR++ - multiplatform, open-source software defined radio receiver
* SDRangel - multiplatform, open-source software defined radio receiver/transmitter
* OpenWebRX - web-based software defined radio receiver
* mrefd - M17 reflector
* rpitx - general radio frequency transmitter for Raspberry Pi
* dsd-fme - digital speech decoder
* mvoice - voice client and graphical repeater application (Raspberry and Linux)