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 link layer connections, transferring data encapsulated in frames between nodes, and detecting errors introduced by the communications channel.
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 baudBell202 tones and 9600baud G3RUHDFSK[4] are almost exclusively used on VHF and UHF. On HF the standard transmission mode is 300baud Bell103 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 frame syntax and procedures. (ISO3309)[5] 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.
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 source routing mechanism using digipeaters is available at the datalink level. Digipeaters act as simplexrepeaters, 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 onlynetwork 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, a variety of layer3 protocols can be used with AX.25, including the ubiquitous Internet Protocol (IP). This approach is used by AMPRNet, 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, 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.
AX.25 has also been implemented on personal computers. For example, the Linux kernel includes native support for AX.25.[6] 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 connecting 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.
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.
AX.25 is often used with a TNC that implements the KISS[7] 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 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 9,600bit/s, and typically 1,200bit/s),[9] 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 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 (or SAP); the SSID often assumes this role. Thus there can be only one service per AX.25 station SSID address, which is often kludged 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:
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 430MHz 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 1,200bit/s AFSK of Bell 202. Previously sufficient small 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 II 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 (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 or the Motorola68HC08 families) has been demonstrated.
It seems, however, that any new system that is not compatible with the current Bell202 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).
Ethernet is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1983 as IEEE 802.3. Ethernet has since been refined to support higher bit rates, a greater number of nodes, and longer link distances, but retains much backward compatibility. Over time, Ethernet has largely replaced competing wired LAN technologies such as Token Ring, FDDI and ARCNET.
In computer networking, Point-to-Point Protocol (PPP) is a data link layer communication protocol between two routers directly without any host or any other networking in between. It can provide loop detection, authentication, transmission encryption, and data compression.
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 data via a radio communication link.
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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.
In the IEEE 802 reference model of computer networking, the logical link control (LLC) data communication protocol layer is the upper sublayer of the data link layer of the seven-layer OSI model. The LLC sublayer acts as an interface between the medium access control (MAC) sublayer and the network 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 provides the functional and procedural means to transfer data between network entities and may also provide the means to detect and possibly correct errors that can occur in the physical layer.
In IEEE 802 LAN/MAN standards, the medium access control (MAC), also called media access control, is the layer that controls the hardware responsible for interaction with the wired or wireless transmission medium. The MAC sublayer and the logical link control (LLC) sublayer together make up the data link layer. The LLC provides flow control and multiplexing for the logical link, while the MAC provides flow control and multiplexing for the transmission medium.
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 station telemetry, text messages, announcements, queries, and other telemetry. APRS data can be displayed on a map, which can show stations, objects, tracks of moving objects, weather stations, search and rescue data, and direction finding data.
Link Access Procedure, Balanced (LAPB) implements the data link layer as defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the correct sequence. LAPB is specified in ITU-T Recommendation X.25 and ISO/IEC 7776. It implements the connection-mode data link service in the OSI Reference Model as defined by ITU-T Recommendation X.222.
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 convert baseband digital signals to audio tones.
The Bell 202 modem was an early (1976) modem standard developed by the Bell System. It specifies audio frequency-shift keying (AFSK) to encode and transfer data at a rate of 1200 bits per second (bit/s), half-duplex. It has separate sets of circuits for 1200 bit/s and 300 bit/s rates. These signalling protocols, also used in third-party modems, are referred to generically as Bell 202 modulation, and any device employing it as Bell-202-compatible.
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D-STAR is a digital voice and data protocol specification for amateur radio. The system was developed in the late 1990s by the Japan Amateur Radio League and uses minimum-shift keying in its packet-based standard. There are other digital modes that have been adapted for use by amateurs, but D-STAR was the first that was designed specifically for amateur radio.
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FX.25 is a protocol extension to the AX.25 Link Layer Protocol. FX.25 provides a Forward Error Correction (FEC) capability while maintaining legacy compatibility with non-FEC equipment. FX.25 was created by the Stensat Group in 2005, and was presented as a technical paper at the 2006 TAPR Digital Communications Conference in Tucson, AZ.
KISS 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 developed by Mike Cheponis and Phil Karn to allow transmission of AX.25 packet radio frames containing IP packets over an asynchronous serial link, for use with the KA9Q NOS program.
IL2P 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.
M17 is a digital radio modulation mode developed by Wojciech Kaczmarski 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.
↑ ISO/IEC 3309: "Information technology. Telecommunications and information exchange between systems. High-level Data Link Control (HDLC) procedures. Frame structure" (1984).
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