AX.25
Encyclopedia
AX.25 is a data link layer
protocol derived from the X.25
protocol suite and designed for use by amateur radio
operators. It is used extensively on amateur packet radio
network
s.
AX.25 occupies the first
, second
, and often the third layer
s of the OSI networking model
, and is responsible for transferring data (encapsulated in packets) between node
s and detecting errors introduced by the communications channel.
AX.25 is thus comparable to the combination of Ethernet
and TCP
in the services it provides. However, as AX.25 is a pre-OSI-model protocol, the specification was not written to cleanly separate into OSI layers. This is not necessarily a problem, and can have some advantages.
In practice, it is not uncommon to find an AX.25 data link layer as the transport for some other network layer, such as IPv4, with TCP used on top of that. Note that, like Ethernet, AX.25 frames are not engineered to support switching. For this reason, AX.25 supports a somewhat limited form of source routing
. Although possible to build AX.25 switches in a manner not unlike how Ethernet switches work, this has not yet been accomplished.
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.
, which contains a microprocessor
and an implementation of the protocol in firmware
. These devices allow network resources to be accessed using only a dumb terminal and a transceiver
.
More recently, AX.25 implementations have appeared for personal computers. For example, the Linux kernel
includes native support for AX.25 networking.
Technically, AX.25 does not completely define a physical layer implementation, although it does define a physical-layer state machine. In practice AX.25 is almost exclusively used with 300 baud
Bell 103 tone
s on HF
, 1200 baud Bell 202 tone
s and 9600 baud G3RUH DFSK on VHF and UHF, although the NRZI encoding differs from the telephone modem standards (it uses NRZS, not NRZM). These combinations are so ubiquitous that most users do not differentiate between the AX.25 data layer and the underlying physical layer. This problem is exacerbated by the fact that the designers of the AX.25 protocol did not use consistent terminology and did not themselves clearly differentiate between tasks belonging to the various layers. The more recent AX.25 publications attempt to better delineate physical and data link layers.
that implements the KISS framing as a low-cost alternative to using expensive and uncommon HDLC 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 PC 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.
stations, without any additional network layers. This is sufficient for keyboard-to-keyboard contacts between stations and for accessing local bulletin board system
s and DX
clusters.
A simple source routing
mechanism using digipeaters is available at this level of operation. Digipeaters act as simplex
repeater
s, receiving and retransmitting packets from local stations. They allow multi-hop connections to be established between two stations unable to communicate directly.
The AX.25 specification does define a complete, albeit point to point only network layer
protocol, but this has seen little use outside of keyboard-to-keyboard or keyboard-to-BBS connections. NET/ROM, ROSE, and TexNet exist to provide routing between nodes. In principle, any layer 3
protocol can be used with AX.25, including the ubiquitous Internet protocol
.
In recent years, the Automatic Packet Reporting System 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.
The AX.25 protocol identifies each message by sender and destination station call-sign plus SSID value
in range 0 through 15.
At ITU
WARC2003 meeting earlier radio amateur station call-sign specification was amended so that
earlier maximum length of 6 character was raised to 7 characters. However AX.25 has a built in hard limit
of 6 characters, which means a 7 character call-sign cannot be used in an AX.25 network.
AX.25 lacks an explicit port (or SAP
); the SSID often assumes this role. Thus there can be only one service per AX.25 station SSID address, which is often kludge
d around with varying degrees of success.
Some amateurs, notably Phil Karn
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
(FEC) and automatic data compression
.
However, a viable widely-adopted successor to AX.25 has yet to emerge. Likely reasons may include:
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
(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 USD 5 instead of USD 30 and a system cost might stay below USD 50, transmitter included. However, in recent years, the ability to receive as well as send using cheap microcontrollers (such as the Atmel AVR
or the Motorola
68HC08 families) have 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. However, as existing Bell 202 modem chips become obsolete without replacement, this consideration becomes less overwhelming.
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
protocol derived from the X.25
X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
protocol suite and designed for use by amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
operators. It is used extensively on amateur packet radio
Packet radio
Packet radio is a form of packet switching technology used to transmit digital data via radio or wireless communications links. It uses the same concepts of data transmission via Datagram that are fundamental to communications via the Internet, as opposed to the older techniques used by dedicated...
network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
s.
AX.25 occupies the first
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
, second
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
, and often the third layer
Network layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
s of the OSI networking model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
, and is responsible for transferring data (encapsulated in packets) between node
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
s and detecting errors introduced by the communications channel.
AX.25 is thus comparable to the combination of Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
and TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
in the services it provides. However, as AX.25 is a pre-OSI-model protocol, the specification was not written to cleanly separate into OSI layers. This is not necessarily a problem, and can have some advantages.
In practice, it is not uncommon to find an AX.25 data link layer as the transport for some other network layer, such as IPv4, with TCP used on top of that. Note that, like Ethernet, AX.25 frames are not engineered to support switching. For this reason, AX.25 supports a somewhat limited form of source routing
Source routing
In computer networking, source routing allows a sender of a packet to partially or completely specify the route the packet takes through the network...
. Although possible to build AX.25 switches in a manner not unlike how Ethernet switches work, this has not yet been accomplished.
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.
Implementations
Traditionally, amateur radio operators have connected to AX.25 networks through the use of a terminal node controllerTerminal node controller
A terminal node controller 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 convert baseband digital signals to audio tones.The TNC was...
, which contains a microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
and an implementation of the protocol in firmware
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...
. These devices allow network resources to be accessed using only a dumb terminal and a transceiver
Transceiver
A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...
.
More recently, AX.25 implementations have appeared for personal computers. For example, the Linux kernel
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
includes native support for AX.25 networking.
Technically, AX.25 does not completely define a physical layer implementation, although it does define a physical-layer state machine. In practice AX.25 is almost exclusively used with 300 baud
Baud
In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...
Bell 103 tone
Bell 103 modem
The Bell 103 modem or Bell 103 dataset was the second commercial modem for computers, released by AT&T in 1962. It allowed digital data to be transmitted over regular unconditioned telephone lines at a speed of 300 bits per second. It followed the introduction of the 110 baud Bell 101 dataset in...
s on HF
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
, 1200 baud Bell 202 tone
Bell 202 modem
The Bell 202 modem was an early modem developed by Bell System. It specifies audio frequency-shift keying to encode and transfer data at a rate of 1200 bits per second, half-duplex and at a rate of 1800 bits per second full duplex using differential phase-shift keying modulation...
s and 9600 baud G3RUH DFSK on VHF and UHF, although the NRZI encoding differs from the telephone modem standards (it uses NRZS, not NRZM). These combinations are so ubiquitous that most users do not differentiate between the AX.25 data layer and the underlying physical layer. This problem is exacerbated by the fact that the designers of the AX.25 protocol did not use consistent terminology and did not themselves clearly differentiate between tasks belonging to the various layers. The more recent AX.25 publications attempt to better delineate physical and data link layers.
KISS-mode framing
AX.25 is often used with a TNCTerminal node controller
A terminal node controller 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 convert baseband digital signals to audio tones.The TNC was...
that implements the KISS framing as a low-cost alternative to using expensive and uncommon HDLC 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 PC 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 radioPacket radio
Packet radio is a form of packet switching technology used to transmit digital data via radio or wireless communications links. It uses the same concepts of data transmission via Datagram that are fundamental to communications via the Internet, as opposed to the older techniques used by dedicated...
stations, without any additional network layers. This is sufficient for keyboard-to-keyboard contacts between stations and for accessing local bulletin board system
Bulletin board system
A Bulletin Board System, or BBS, is a computer system running software that allows users to connect and log in to the system using a terminal program. Once logged in, a user can perform functions such as uploading and downloading software and data, reading news and bulletins, and exchanging...
s and DX
DXing
DXing is the hobby of tuning in and identifying distant radio or television signals, or making two way radio contact with distant stations in amateur radio, citizens' band radio or other two way radio communications. Many DXers also attempt to receive written verifications of reception from the...
clusters.
A simple source routing
Source routing
In computer networking, source routing allows a sender of a packet to partially or completely specify the route the packet takes through the network...
mechanism using digipeaters is available at this level of operation. Digipeaters act as simplex
Simplex communication
Simplex communication refers to communication that occurs in one direction only. Two definitions have arisen over time: a common definition, which is used in ANSI standard and elsewhere, and an ITU-T definition...
repeater
Repeater
A repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...
s, receiving and retransmitting packets from local stations. They allow multi-hop connections to be established between two stations unable to communicate directly.
The AX.25 specification does define a complete, albeit point to point only network layer
Network layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
protocol, but this has seen little use outside of keyboard-to-keyboard or keyboard-to-BBS connections. NET/ROM, ROSE, and TexNet exist to provide routing between nodes. In principle, any layer 3
Network Layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
protocol can be used with AX.25, including the ubiquitous Internet protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
.
In recent years, the Automatic Packet Reporting System has become a popular application.
For tunneling
Tunneling protocol
Computer networks use a tunneling protocol when one network protocol encapsulates a different payload protocol...
of AX.25 packets over IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
, AXIP and AXUDP are used to encapsulate
Encapsulation (networking)
In computer networking, encapsulation is a method of designing modular communication protocols in which logically separate functions in the network are abstracted from their underlying structures by inclusion or information hiding within higher level objects....
AX.25 into IP or UDP
User Datagram Protocol
The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
packets.
Limitations
At the speeds commonly used to transmit packet radio data (rarely higher than 9,600 bit/s, 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.The AX.25 protocol identifies each message by sender and destination station call-sign plus SSID value
in range 0 through 15.
At ITU
Itu
Itu is an old and historic municipality in the state of São Paulo in Brazil. The population in 2009 was 157,384 and the area is 641.68 km². The elevation is 583 m. This place name comes from the Tupi language, meaning big waterfall. Itu is linked with the highway numbered the SP-75 and are flowed...
WARC2003 meeting earlier radio amateur station call-sign specification was amended so that
earlier maximum length of 6 character was raised to 7 characters. However AX.25 has a built in hard limit
of 6 characters, which means a 7 character call-sign cannot be used in an AX.25 network.
AX.25 lacks an explicit port (or SAP
Service Access Point
A Service Access Point is an identifying label for network endpoints used in Open Systems Interconnection networking.When using the OSI Network Layer , the base for constructing an address for a network element is an NSAP address, similar in concept to an IP address...
); the SSID often assumes this role. Thus there can be only one service per AX.25 station SSID address, which is often kludge
Kludge
A kludge is a workaround, a quick-and-dirty solution, a clumsy or inelegant, yet effective, solution to a problem, typically using parts that are cobbled together...
d around with varying degrees of success.
Some amateurs, notably Phil Karn
Phil Karn
Phil Karn is an engineer from Baltimore, 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 Mellon University in 1979. From 1979 until 1984, Phil Karn worked at Bell Labs in Naperville,...
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
Forward error correction
In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
(FEC) and automatic data compression
Data compression
In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....
.
However, a viable widely-adopted successor to AX.25 has yet to emerge. Likely reasons may include:
- a large existing deployment of recycled narrowbandNarrowbandIn radio, narrowband describes a channel in which the bandwidth of the message does not significantly exceed the channel's coherence bandwidth. It is a common misconception that narrowband refers to a channel which occupies only a "small" amount of space on the radio spectrum.The opposite of...
FM radios and especially existing APRSAutomatic Position Reporting SystemAutomatic Packet Reporting System is an amateur radio-based system for real time tactical digital communications of information of immediate value in the local area. In addition, all such data is ingested into the APRS Internet system and distributed globally for ubiquitous and immediate access...
applications, - easy availability of cheap, low-power FM transmitters, especially for the 430 MHz UHF 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 codings potentially including forward error correction takes more effort than the 1,200 bit/s AFSKFrequency-shift keyingFrequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
of Bell 202. Previously sufficient small 8-bit8-bitThe first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...
microprocessors with 128 bytes of RAM would not be enough, and new ones might cost USD 30 instead of USD 3. Phil Karn did demo decoding of this new modulation of his by running it on a Pentium IIPentium IIThe Pentium II brand refers to Intel's sixth-generation microarchitecture and x86-compatible microprocessors introduced on May 7, 1997. Containing 7.5 million transistors, the Pentium II featured an improved version of the first P6-generation core of the Pentium Pro, which contained 5.5 million...
machine - some 10 years later, mid-level embedded microprocessors are capable enough to do the same with under USD 50 system cost. - HSMM-MESH (High-speed Multimedia MESH networking) has a bandwidth of up to 10Mbs and uses inexpensive commercially available hardware and standard Ethernet networking practices and protocols.
Despite these limitations, an extension to the AX.25 protocol, supporting forward error correction, has been created by the TAPR
Tucson Amateur Packet Radio
TAPR is an international amateur radio organization. It was founded in Tucson, Arizona, in 1981 by a group of amateurs interested in developing a terminal node controller for amateur use. Thus, the group was named Tucson Amateur Packet Radio, Inc. After developing one of the first widely...
. This extension is called FX.25
FX.25 Forward Error Correction
FX.25 is a protocol extension to the AX.25 Link Layer Protocol. FX.25 provides a Forward Error Correction capability while maintaining legacy compatibility with non-FEC equipment...
.
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
RSSI
In telecommunications, received signal strength indicator is a measurement of the power present in a received radio signal.RSSI is a generic radio receiver technology metric, which is usually invisible to the user of the device containing the receiver, but is directly known to users of wireless...
(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 USD 5 instead of USD 30 and a system cost might stay below USD 50, transmitter included. However, in recent years, the ability to receive as well as send using cheap microcontrollers (such as the Atmel AVR
Atmel AVR
The AVR is a modified Harvard architecture 8-bit RISC single chip microcontroller which was developed by Atmel in 1996. The AVR was one of the first microcontroller families to use on-chip flash memory for program storage, as opposed to one-time programmable ROM, EPROM, or EEPROM used by other...
or the Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
68HC08 families) have 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. However, as existing Bell 202 modem chips become obsolete without replacement, this consideration becomes less overwhelming.
Further reading
- AMPRNet – a project to construct a global, radio-based network using TCP/IP over AX.25 links
- Linux-AX25.org – Site dedicated to packet radio on Linux
- AX.25 Layer 2 – This web site has been established to be a concise repository for AX.25 layer 2 design activities.
- APRS - Bob Bruninga's official APRS website.