AX.25
AX.25 (Amateur X.25) is a data link layer protocol originally derived from layer 2 of the X.25 protocol suite and designed for use by amateur radio operators.[1] It is used extensively on amateur packet radio networks.
AX.25 v2.0 is responsible for establishing
AX.25 v2.2 [1] (1998) added improvements to improve efficiency, especially at higher data rates.[2] Stations can automatically negotiate payload sizes larger than the previous limitation of 256 bytes. Extended sequence numbers (7 vs. 3 bits) allow a larger window size, the number of frames that can be sent before waiting for acknowledgement. "Selective Reject" allows only the missing frames to be resent, rather than having to wastefully resend frames that have already been received successfully. Despite all these advantages, few implementations have been updated to include these improvements published more than 20 years ago. The only known complete implementation of v2.2, at this time (2020), is the Dire Wolf software TNC.[3]
AX.25 is commonly used as the data link layer for network layer such as IPv4, with TCP used on top of that. AX.25 supports a limited form of source routing. Although it is possible to build AX.25 switches similar to the way Ethernet switches work, this has not yet been accomplished.[citation needed]
Specification
AX.25 does not define a physical layer implementation. In practice 1200
At the link layer, AX.25 uses
Media access control follows the
AX.25 supports both virtual-circuit connected and datagram-style connectionless modes of operation. The latter is used to great effect by the Automatic Packet Reporting System (APRS).
A simple
The AX.25 specification defines a complete, albeit point to point only
Implementations
Traditionally, amateur radio operators have connected to AX.25 networks through the use of a
AX.25 has also been implemented on
Dire Wolf is a free open-source replacement for the 1980s-style TNC. It contains DSP software modems and a complete implementation of AX25 v2.2 plus FX.25 forward error correction. It can function as a digital repeater, GPS tracker, and APRS Internet Gateway (IGate) without any additional software.
KISS-mode framing
- See full article at KISS (TNC)
AX.25 is often used with a
The KISS framing is not part of the AX.25 protocol itself nor is it sent over the air. It merely serves to encapsulate the protocol frames in a way that can successfully pass over a serial link to the TNC. The KISS framing is derived from SLIP, and makes many of the same assumptions, such as there only being two "endpoints" involved in the conversation. With SLIP, these were the two SLIP-connected hosts; with KISS, it is assumed that the KISS framing link is over serial with only the host computer and the TNC involved. Among other things, this makes it awkward to address multiple TNCs without having multiple (serial) data channels.
Alternatives to KISS do exist that address these limitations, such as 6PACK.[8]
Applications
AX.25 has most frequently been used to establish direct, point-to-point links between packet radio stations, without any additional network layers. This is sufficient for keyboard-to-keyboard contacts between stations and for accessing local bulletin board systems and DX clusters.
In recent years, APRS has become a popular application.
For tunneling of AX.25 packets over IP, AXIP and AXUDP are used to encapsulate AX.25 into IP or UDP packets.
Limitations
At the speeds commonly used to transmit packet radio data (rarely higher than 9,600
HDLC protocols identify each frame by an address. The AX.25 implementation of HDLC includes sender and destination station call-sign plus four-bit Secondary Station Identifier (SSID) value in range 0 through 15 in the frame address. At
AX.25 lacks an explicit around with varying degrees of success.
Some amateurs, notably
- a large existing deployment of recycled narrowband FM radios and especially existing APRS applications,
- easy availability of cheap, low-power FM transmitters, especially for the 430 MHz UHFband, to match existing legacy radio gear,
- new radio level modulations would need different radio gear than what is currently in use and the resulting system would be incompatible with the existing one – thus requiring a large initial investment in new radio gear,
- adoption of newer 8-bit microprocessors with 128 bytes of RAM would not be enough, and new ones might cost US$30 instead of US$3. Phil Karn did demo decoding of this new modulation of his by running it on a Pentium IImachine – some 10 years later, mid-level embedded microprocessors are capable enough to do the same with under US$50 system cost.
Despite these limitations, an extension to the AX.25 protocol, supporting forward error correction, has been created by the TAPR. This extension is called FX.25.
Small gadget transmitters do not need to know what is being transmitted. There is only a need to monitor channel occupation by radio receiver
It seems, however, that any new system that is not compatible with the current Bell 202 modulation is unlikely to be widely adopted. The current modulation seems to fulfill sufficient need that little motivation exists to move to a superior design, especially if the new design requires significant hardware purchases.
Most recently, a wholly new protocol with forward error correction has been created by Nino Carillo, KK4HEJ, called Improved Layer 2 Protocol (IL2P).
See also
- Packet radio
- Automatic Packet Reporting System (APRS)
- FX.25 Forward Error Correction
- Improved Layer 2 Protocol (IL2P) [10]
References
- ^ a b "AX.25 Link Access Protocol for Amateur Packet Radio" (PDF). Tucson Amateur Packet Radio. 1997. Archived (PDF) from the original on 2014-06-11. Retrieved 2014-01-15.
- ^ "AX.25 Throughput: Why is 9600 bps Packet Radio only twice as fast as 1200?" (PDF). GitHub. 12 November 2021. Archived (PDF) from the original on 8 March 2021. Retrieved 7 May 2020.
- ^ Dire Wolf github.com
- ^ James Miller (1995). "9600 Baud Packet Radio Modem Design". AMSAT. Archived from the original on 2012-06-07. Retrieved 2012-02-03.
- ^ ISO/IEC 3309: "Information technology. Telecommunications and information exchange between systems. High-level Data Link Control (HDLC) procedures. Frame structure" (1984).
- ^ John Ackermann (2002). "Linux AX.25 Configuration". febo.com. Archived from the original on 11 March 2008. Retrieved 2008-03-05.
- ^ Mike Chepponis; Phil Karn. "The KISS TNC: A simple Host-to-TNC communications protocol". Archived from the original on 25 July 2008. Retrieved 2008-08-18.
- ^ "6PACK a "real time" PC to TNC protocol". Archived from the original on 2012-02-24. Retrieved 2009-05-28.
- ^ Jeff Tranter (1997). "Packet Radio Under Linux". Linux Journal. Archived from the original on 22 November 2008. Retrieved 2009-01-01.
- ^ "TARPN Software Products - Protocols and Modulation". Archived from the original on 2021-07-09. Retrieved 2021-07-08.
Further reading
- Terry L. Fox (1984). AX.25 Amateur Packet-Radio Link-Layer Protocol, Version 2.0 (PDF). Newington, CT: ISBN 0-87259-011-9. Archived(PDF) from the original on 2014-01-07. Retrieved 2014-01-07.
- Rich Roznoy, ed. (1997). Packet: Speed, More Speed and Applications (2nd ed.). Newington, CT: ISBN 0-87259-605-2.
- AMPRNet Archived 2001-03-31 at the TCP/IPover AX.25 links
- Linux-AX25.org Archived 2008-11-20 at the Wayback Machine – a site dedicated to packet radio on Linux
- AX.25 Layer 2 Archived 2006-07-07 at the Wayback Machine – a web site established to be a concise repository for AX.25 layer 2 design activities
- APRS Archived 2021-05-06 at the Wayback Machine – Bob Bruninga's official APRS website
- TARPN Archived 2021-06-21 at the Wayback Machine – Tadd Torborg KA2DEW - Terrestrial Amateur Radio Packet Network site
- AX.25 Specification ax25.net –