AX.25 (Amateur X.25) is a
data link layer
The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...
protocol
Protocol may refer to:
Sociology and politics
* Protocol (politics), a formal agreement between nation states
* Protocol (diplomacy), the etiquette of diplomacy and affairs of state
* Etiquette, a code of personal behavior
Science and technology
...
originally derived from layer 2 of the
X.25
X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts a ...
protocol suite and designed for use by
amateur radio
Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communic ...
operators.
It is used extensively on amateur
packet radio
In digital radio, packet radio is the application of packet switching techniques to digital radio communications. Packet radio uses a packet switching protocol as opposed to circuit switching or message switching protocols to transmit digital dat ...
networks
Network, networking and networked may refer to:
Science and technology
* Network theory, the study of graphs as a representation of relations between discrete objects
* Network science, an academic field that studies complex networks
Mathematics
...
.
AX.25 v2.0 and later occupies the
data link layer
The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...
, the second layer of the
OSI model
The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...
. It is responsible for establishing link-layer connections, transferring data encapsulated in
frames between
nodes
In general, a node is a localized swelling (a "knot") or a point of intersection (a Vertex (graph theory), vertex).
Node may refer to:
In mathematics
*Vertex (graph theory), a vertex in a mathematical graph
*Vertex (geometry), a point where two ...
, and detecting errors introduced by the
communications channel
A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for informat ...
. As AX.25 is a pre-OSI-model protocol, the original specification was not written to cleanly separate into OSI layers. This was rectified with version 2.0 (1984), which assumes compliance with OSI level 2.
AX.25 v2.2
(1998) added improvements to improve efficiency, especially at higher data rates. 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 th
Dire Wolfsoftware TNC.
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
In computer networking, source routing, also called path addressing, allows a sender of a packet to partially or completely specify the route the packet takes through the network. In contrast, in conventional routing, routers in the network determ ...
. Although it is possible to build AX.25 switches similar to the way Ethernet switches work, this has not yet been accomplished.
Specification
AX.25 does not define a physical layer implementation. In practice 1200
baud
In telecommunication and electronics, baud (; symbol: Bd) is a common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel.
It is the unit for symbol rate or modulatio ...
Bell 202 tones and 9600 baud G3RUH DFSK are almost exclusively used on
VHF
Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter.
Frequencies immediately below VHF ...
and
UHF
Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...
. On
HF the standard transmission mode is 300 baud
Bell 103 tones. At the physical layer, AX.25 defines only a "physical layer state machine" and some timers related to transmitter and receiver switching delays.
At the link layer, AX.25 uses
HDLC
High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002.
HDLC provides both c ...
frame syntax and procedures. (ISO 3309) frames are transmitted with
NRZI encoding. HDLC specifies the syntax, but not the semantics, of the variable-length address field of the frame. AX.25 specifies that this field is subdivided into multiple addresses: a source address, zero or more repeater addresses, and a destination address, with embedded control fields for use by the repeaters. To simplify compliance with amateur radio rules, these addresses derive from the station call signs of the source, destination and repeater stations.
Media access control follows the
Carrier sense multiple access
Carrier-sense multiple access (CSMA) is a medium access control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus or a band of the electromagnetic s ...
approach with collision recovery (CSMA/CR).
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
Automatic Packet Reporting System (APRS) is an amateur radio-based system for real time digital communications of information of immediate value in the local area. Data can include object Global Positioning System (GPS) coordinates, weather st ...
(APRS).
A simple
source routing
In computer networking, source routing, also called path addressing, allows a sender of a packet to partially or completely specify the route the packet takes through the network. In contrast, in conventional routing, routers in the network determ ...
mechanism using
digipeater
In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Some ...
s is available at the datalink level. Digipeaters act as
simplex
In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given dimension. ...
repeater
In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Some ...
s, receiving, decoding and retransmitting
packets from local stations. They allow multi-hop connections to be established between two stations unable to communicate directly. The digipeaters use and modify the information in the frame's address field to perform this function.
The AX.25 specification defines a complete, albeit ''point to point only''
network layer
In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers.
Functions
The network layer provides the means of transfe ...
protocol, but this has seen little use outside of keyboard-to-keyboard or keyboard-to-BBS connections.
NET/ROM,
ROSE
A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...
, and
TexNet exist to provide routing between nodes. In principle, a variety of
layer 3 protocols can be used with AX.25, including the ubiquitous
Internet protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
IP h ...
. This approach is used by
AMPRNet
The AMPRNet (AMateur Packet Radio Network) or Network 44 is used in amateur radio for packet radio and digital communications between computer networks managed by amateur radio operators. Like other amateur radio frequency allocations, an IP ra ...
, which is an amateur radio TCP/IP network using AX.25 UI-frames at the datalink layer.
Implementations
Traditionally, amateur radio operators have connected to AX.25 networks through the use of a
terminal node controller
A terminal node controller (TNC) is a device used by amateur radio operators to participate in AX.25 packet radio networks. It is similar in function to the Packet Assembler/Disassemblers used on X.25 networks, with the addition of a modem to ...
, which contains a
microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
and an implementation of the protocol in
firmware
In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide h ...
. These devices allow network resources to be accessed using only a
dumb terminal
A computer terminal is an electronic or electromechanical hardware device that can be used for entering data into, and transcribing data from, a computer or a computing system. The teletype was an example of an early-day hard-copy terminal and ...
and a
transceiver
In radio communication, a transceiver is an electronic device which is a combination of a radio ''trans''mitter and a re''ceiver'', hence the name. It can both transmit and receive radio waves using an antenna, for communication purposes. The ...
.
AX.25 has also been implemented on
personal computers
A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or techn ...
. For example, the
Linux kernel
The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...
includes native support for AX.25. The computer connects to a transceiver via its audio interface or via a simple modem. The computers can also interconnect to other computers or be bridged or routed to TNCs and transceivers located elsewhere using BPQ over Ethernet framing, which is also natively supported by the Linux kernel to facilitate more modern setups with the actual transceivers directly placed under or in the antenna mast, creating a 'low loss', shorter RF wiring need, and replacing expensive and long and thick coax cables and amplifiers with cheap fiber (RFI (both ways)/EMP/lightning resistant) or copper Ethernet wiring. BPQ Ethernet framing allows to connect entire stacks of TNC+transceiver pairs to any existing network of computers which then can all access all radio links offered simultaneously (transparently bridged), communicate with each other internally over AX.25, or with filtered routing select specific TNCs/radio frequencies.
Note that the Linux kernel AX.25 implementation does contain some bugs, mainly in disconnecting sessions (this bug appears to have slipped in over time due to maintenance by some people using it exclusively for APRS and not in virtual channel connected mode), and for example TAP interfaces not supporting the full 7 bytes of call sign string length as some interface drivers expect a 6 byte HW address (MAC), not an up-to-7 byte one like AX.25 use requires.
Dire Wolfis 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) KISS (Keep It Simple, Stupid) is a protocol for communicating with a serial terminal node controller (TNC) device used for amateur radio. This allows the TNC to combine more features into a single device and standardizes communications. KISS was dev ...
''
AX.25 is often used with a
TNC that implements the KISS framing as a low-cost alternative to using expensive and uncommon
HDLC
High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002.
HDLC provides both c ...
controller cards.
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.
Applications
AX.25 has most frequently been used to establish direct,
point-to-point links between
packet radio
In digital radio, packet radio is the application of packet switching techniques to digital radio communications. Packet radio uses a packet switching protocol as opposed to circuit switching or message switching protocols to transmit digital dat ...
stations, without any additional network layers. This is sufficient for keyboard-to-keyboard contacts between stations and for accessing local
bulletin board system
A bulletin board system (BBS), also called computer bulletin board service (CBBS), is a computer server running software that allows users to connect to the system using a terminal program. Once logged in, the user can perform functions such as ...
s and
DX cluster
A DX cluster is a network of computers, each running a software package dedicated to gathering, and disseminating, information on amateur radio DX (long-distance contact) activities.
The computers comprising the network are called nodes, the ne ...
s.
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
bit/s
In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time.
The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
, and typically 1,200 bit/s), the use of additional network layers with AX.25 is impractical due to the data overhead involved. This is not a limitation of AX.25 ''per se'', but places constraints on the sophistication of applications designed to use it.
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
ITU
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
WARC2003 the radio amateur station callsign specification was amended so that the earlier maximum length of six characters was raised to seven characters. However AX.25 has a built-in hard limit of six characters, which means a seven-character callsign cannot be used in an AX.25 network.
AX.25 lacks an explicit ''
port
A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ham ...
'' (or
SAP
Sap is a fluid transported in xylem cells (vessel elements or tracheids) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant.
Sap is distinct from latex, resin, or cell sap; it is a separa ...
); the SSID often assumes this role. Thus there can be only one service per AX.25 station SSID address, which is often
kludge
A kludge or kluge () is a workaround or quick-and-dirty solution that is clumsy, inelegant, inefficient, difficult to extend and hard to maintain. This term is used in diverse fields such as computer science, aerospace engineering, Internet sla ...
d around with varying degrees of success.
Some amateurs, notably
Phil Karn
Phil Karn (born October 4, 1956) is a retired American engineer from Lutherville, Maryland. He earned a bachelor's degree in electrical engineering from Cornell University in 1978 and a master's degree in electrical engineering from Carnegie Mell ...
KA9Q, have argued that AX.25 is not well-suited to operation over noisy, limited-bandwidth radio links, citing its lack of
forward error correction
In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea is ...
(FEC) and automatic
data compression
In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compression ...
. However, a viable widely adopted successor to AX.25 has yet to emerge. Likely reasons may include:
* a large existing deployment of recycled
narrowband
Narrowband signals are signals that occupy a narrow range of frequencies or that have a small fractional bandwidth. In the audio spectrum, narrowband sounds are sounds that occupy a narrow range of frequencies. In telephony, narrowband is usua ...
FM radios and especially existing APRS applications,
* easy availability of cheap, low-power FM transmitters, especially for the 430 MHz
UHF
Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...
band, 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
line coding
In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained co ...
s potentially including ''forward error correction'' takes more effort than the 1,200 bit/s
AFSK
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather ball ...
of
Bell 202. Previously sufficient small
8-bit
In computer architecture, 8-bit Integer (computer science), integers or other Data (computing), data units are those that are 8 bits wide (1 octet (computing), octet). Also, 8-bit central processing unit (CPU) and arithmetic logic unit (ALU) arc ...
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 II
The Pentium II brand refers to Intel's sixth-generation microarchitecture (" P6") and x86-compatible microprocessors introduced on May 7, 1997. Containing 7.5 million transistors (27.4 million in the case of the mobile Dixon with 256 KB ...
machine 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
RSSI
In telecommunications, received signal strength indicator (RSSI) is a measurement of the Electric power, power present in a received radio signal.
RSSI is usually invisible to a user of a receiving device. However, because signal strength can var ...
(Received Signal Strength Indication) to know when not to send. Transmitting interleaved
Reed-Solomon FEC signal in some smart modulation needs a lot fewer resources than reception of the same signal, thus a sufficient microprocessor might cost just US$5 instead of US$30 and a system cost might stay below US$50, transmitter included. However, in recent years, the ability to receive as well as send using cheap microcontrollers (such as the
Atmel AVR
AVR is a family of microcontrollers developed since 1996 by Atmel, acquired by Microchip Technology in 2016. These are modified Harvard architecture 8-bit Reduced instruction set computer, RISC single-chip microcontrollers. AVR was one of the f ...
or the
Motorola
Motorola, Inc. () was an American Multinational corporation, multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent p ...
68HC08
The 68HC08 (HC08 in short) is a broad family of 8-bit microcontrollers originally from Motorola Semiconductor, later from Freescale Semiconductor.
HC08's are fully code-compatible with their predecessors, the Motorola 68HC05. Like all Motorola pr ...
families) has been demonstrated.
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 (Improved Layer 2 Protocol) 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. It is used exclusively on amateur packet radio networks.
IL2P occupies the da ...
(IL2P).
See also
*
Packet radio
In digital radio, packet radio is the application of packet switching techniques to digital radio communications. Packet radio uses a packet switching protocol as opposed to circuit switching or message switching protocols to transmit digital dat ...
*
Automatic Packet Reporting System
Automatic Packet Reporting System (APRS) is an amateur radio-based system for real time digital communications of information of immediate value in the local area. Data can include object Global Positioning System (GPS) coordinates, weather st ...
(APRS)
*
FX.25 Forward Error Correction
*
Improved Layer 2 Protocol IL2P (Improved Layer 2 Protocol) 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. It is used exclusively on amateur packet radio networks.
IL2P occupies the da ...
(IL2P)
References
Further reading
*
*
AMPRNeta project to construct a global, radio-based network using
TCP/IP
The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suit ...
over AX.25 links
Linux-AX25.orga site dedicated to packet radio on Linux
AX.25 Layer 2a web site established to be a concise repository for AX.25 layer 2 design activities
APRSBob Bruninga's official APRS website
Tadd Torborg KA2DEW - Terrestrial Amateur Radio Packet Network site
AX.25 Specification
{{DEFAULTSORT:Ax.25
Packet radio
Link protocols
X.25