Onnowpurbo: /* Further reading */

2016-03-02T21:52:44Z

Onnowpurbo: New page: '''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 ...

2009-12-20T01:02:19Z

New page: '''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 ...

New page

'''AX.25''' is a [[data link layer]] [[protocol (computing)|protocol]] derived from the [[X.25]] protocol suite and designed for use by [[amateur radio]] operators. It is used extensively on amateur [[packet radio]] [[computer network|network]]s.

AX.25 occupies the [[physical layer|first]], [[data link layer|second]], and often the [[network layer|third layer]]s of the [[OSI model|OSI networking model]], and is responsible for transferring data (encapsulated in [[Packet (information technology)|packet]]s) between [[node (networking)|node]]s and detecting errors introduced by the [[communications channel]]. It 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. 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]].

==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]].

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 modem|Bell 103 tone]]s on [[High frequency|HF]] and 1200 and 9600 baud [[Bell 202 modem|Bell 202 tone]]s on [[VHF]], 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, a situation that 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 attempts to better delineate physical and data link layers.

===KISS-mode framing===
AX.25 is often used with a [[terminal node controller|TNC]] 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 (telecommunications)|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 system]]s and [[DXing|DX]] clusters.

A simple [[source routing]] mechanism using [[digipeater]]s is available at this level of operation. Digipeaters act as [[simplex communication|simplex]] [[repeater]]s, receiving and retransmitting [[Packet (information technology)|packet]]s from local stations. They allow multi-hop connections to be established between two stations unable to communicate directly.

The AX.25 specification does defines 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 (network protocol)|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 protocol|tunneling]] of AX.25 packets over [[Internet Protocol|IP]], AXIP and AXUDP are used to [[encapsulation (networking)|encapsulate]] AX.25 into IP or [[User Datagram Protocol|UDP]] packets.

==Limitations==
At the speeds commonly used to transmit packet radio data (rarely higher than 9,600 [[bits per second|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]] 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 ''[[TCP and UDP ports|port]]'' (or [[Service Access Point|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]], 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 [[Automatic Position Reporting System|APRS]] 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 coding]]s potentially including ''forward error correction'' takes more effort than the 1,200 bit/s [[Frequency-shift keying|AFSK]] of [[Bell 202]]. Previously sufficient small [[8 bit]] 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 II]] machine - some 10 years later, mid-level embedded microprocessors are capable enough to do the same with under USD 50 system cost.
* a general decline in the use of Amateur Radio (particularly packet radio) would make adoption marginal and impractical.

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 error correction|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 3 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.

==See also==
*[[Packet Radio]]
*[[Automatic Packet Reporting System]] (APRS)

==Further reading==

*[http://www.ampr.org AMPRNet] – a project to construct a global, radio-based network using [[TCP/IP]] over AX.25 links
*[http://www.linux-ax25.org Linux-AX25.org] – Site dedicated to packet radio on Linux
*[http://www.ax25.net/ AX.25 Layer 2] – This web site has been established to be a concise repository for AX.25 layer 2 design activities.
*[http://www.aprs.org/ APRS] - Bob Bruninga's official APRS website.

← Older revision		Revision as of 21:52, 2 March 2016
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	*[http://www.ax25.net/ AX.25 Layer 2] – This web site has been established to be a concise repository for AX.25 layer 2 design activities.		*[http://www.ax25.net/ AX.25 Layer 2] – This web site has been established to be a concise repository for AX.25 layer 2 design activities.
	*[http://www.aprs.org/ APRS] - Bob Bruninga's official APRS website.		*[http://www.aprs.org/ APRS] - Bob Bruninga's official APRS website.

			==Pranala Menarik==

			* [[RaspberryPi: TNC Packet Radio]]

AX.25 - Revision history

Onnowpurbo: /* Further reading */

Onnowpurbo: New page: '''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 ...