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`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`Preface
`
`Note: This preface is not a part of the protocol.
`
`This is the first edition of AX.25 Amateur Packet-Radio Link-Layer
`Protocol (Version 2.0, October 1984) published by the American Radio Relay
`League.
`It was approved by the ARRL Board of Directors in October, 1984.
`The
`ARRL was designated the international clearinghouse for information relating
`to packet radio with a view to encouraging common standards and regulations
`on behalf of the International Amateur Radio Union (IARU) by their
`Administrative Council at their meeting in Paris during July, 1984.
`
`Earlier implementations of this protocol were based on a paper given at
`the Second ARRL Amateur Radio Computer'Networking Conference in March 1983.
`Changes
`introduced since that paper are bracketed ([ ll
`a
`
`Major sections within this specification have been organized and
`numbered in a manner similar to that of International Telegraph and Telephone
`Consultative Committee (CCITT) Recommendation X.25.
`G
`qThis document defines a protocol
`to be used between two Amateur Radio
`stations in a point—to-point communications environment.
`It specifies only
`link-layer functions. Other than certain interface requirements to and from
`other layers, this protocol is not meant to specify any other layer.
`
`This protocol recognizes that the Amateur Radio operating environment is
`unique, and takes this into consideration.
`A key feature is the inclusion (at
`this layer) of repeater stations and repeater linking. These repeater
`stations simply extend the RF range of Amateur Radio stations.
`Since they do
`not
`impose any flow controls, data switching, orgrouting,
`their inclusion at
`this layer does not
`imply any network-layer functions.
`It is anticipated that
`repeater stations will be phased out
`(or at
`least their number greatly
`reduced) when a true network layer becomes operational.
`
`History
`
`Over the years there have been several link-layer protocols suggested
`for amateur packet radio. The first link—layer protocol to achieve widespread
`use was created by Douglas Lockhart, VE7APU, of the Vancouver (BC) Amateur
`Digital Communications Group (VADCG).
`It was based on the IBM SDLC protocol
`and implemented on a packet-radio controller board designed and built by
`VADCG. This protocol was used for the first few years_of amateur packet-radio
`activity. One of the limitations of the VADCG protocol was that it used a
`single octet (8 bits) for the station address. This restricted the number of
`stations to 254 or a smaller number, depending on how the addressing scheme
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`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`It also required that someone had to assign these arbitrary
`was implemented.
`addresses to each amateur in a local area.
`In early 1982,
`the Amateur Radio Research and Development Corporation
`(AMRAD) began a study of the link-layer protocols in commercial use at the
`The intent was to recommend a protocol that would not suffer from major
`time.
`limitations in a few years, after packet radio had grown.
`The result of this
`study was a recommendation for the use of a slightly modified version of the
`CCITT X.25 level 2 LAPB protocol standard, which could be considered a subset
`of
`the American National Standards Institute (ANSI) Advanced Data
`Communications Control Procedure (ADCCP), balanced mode.
`In June, 1982, a series of meetings was held by Al-{RAD and the Radio
`Amateur Telecommunications Society (RATS) of New Jersey.
`An exploratory
`meeting was held at Bell Laboratories.
`Two definitive meetings in which the
`prototype of AX.25 protocol was developed took place in Vienna, Virginia.
`Involved at
`those meetings were Gordon Beattie, NZDSY; Jon Bloom, KE3Z; Dave
`Borden, KSMMO; Terry Fox, WB4JFI; Paul Rinaldo, 1114111; and Eric Scace, KSNA.
`Both link- and network-layer protocols were defined at that time.
`Since both
`layers were based on the CCITT X.25 recommendation,
`it was decided to follow
`the pattern set by AT&T (131.25 for Bell X.25) and call this new protocol
`AX.25,
`for Amateur X.25.
`The link-layer protocol was then documented by Terry
`Fox and circulated to other packet-radio experimenters for comment. The
`network-layer proposal was held for further study. Eric Scace was able to
`provide invaluable insight
`into CCITT 11.25 as he was one of its authors.
`The next step in the evolution of AX.25 was taken in October of 1982.
`Thomas Clark, W3IWI, president of The Radio Amateur Satellite Corporation
`(AMSAT), hosted a gathering of most of the leaders in amateur packet radio at
`that
`time. AMRAD, AMSAT,
`the ARRL Ad Hoc Committee on Amateur Radio Digital
`Communication, Pacific Packet Radio Society (PPRS), St. Louis Amateur Packet
`Radio (SLAPB),
`and Tucson Amateur Packet Radio Corporation (TAPR) were
`represented. The AMRAD version 1.1 AX.25 link-layer protocol was slightly
`modified and adopted at this meeting.
`
`this document would not
`Had AX.25 remained merely an agreeable concept,
`exist today.
`It is due to yeoman efforts in software development of the AX.25
`protocol
`implementations that packet radio flourishes today. Shortly after
`the October, 1982, meeting at AMSAT, AX.25 packet signals began to appear.
`The rapidity with which AX.25 was
`integrated into the TAPR TNC board was due
`to the unstinting efforts of the TAPP. software development crew, primarily
`such stalwart packet-radio enthusiasts as Dave Henderson, KD4NL; Margaret
`Morrison, KV7D; and Harold Price, NKSK. The original VADCG board was soon
`running AX.25 as well,
`thanks to Hank Magnuski,
`I(A6l*I, who modified the
`original Lockhart software to execute the new protocol.
`The first public release of the AX.25 link-layer protocol was in a paper
`given at
`the Second Amateur Radio Computer Networking Conference,
`in March,
`1983.
`Some corrections and changes have been made since then by the ARRL Ad
`Hoc Committee on Amateur Radio Digital Communication.
`In July, 1983, West
`Coast packet groups met to form WESTNET —- to link packet-radio repeaters from
`San Diego to San Francisco. The WESTNET group decided to extend the AX.25
`1ink—layer address field to accommodate up to eight repeaters.
`This
`modification was accepted by the ARRL Committee at their November, 1983,
`
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`AX.25 Amateur Packet-Radio Link—Layer Protocol
`
`meeting in Washington, DC. Unresolved at that meeting was the handling of the
`polllfinal bit. When the Committee met again at Trenton, NJ,
`in April, 1984,
`Phil Karn, KA9Q, proposed a solution to the poll/final bit problem. His
`proposal was published in Q§§_and packet-radio club newsletters, and later
`adopted by the Committee.
`'
`
`It is fitting, given the history of collective effort by packet-radio
`pioneers,
`that this document
`is the work of many. Special
`thanks are due to
`Chuck Green, NDADI; Lyle Johnson, WA7GXD; Phil Karn, KA9Q; Paul Newland, AD7I;
`Harold Price, NK6K; and Eric Scace, K3NA, for their perceptive and helpful
`comments.
`
`ARRL Ad Hoc Committee on Digital Communication
`
`This protocol was finalized and approved for submission to the ARRL
`Board of Directors by the Ad Hoc Committee on Digital Communication.
`Committee approval of this protocol does not necessarily imply that all
`committee members voted for its approval. At the time it approved of this
`protocol,
`the Comittee was comprised of:
`
`Paul L. Rinaldo, W4RI, Chairman
`Marshall Quiat, AGDX, Board Liaison
`Dennis Connors, KDZS
`Terry Fox, WB4JFI
`Lyle Johnson, WA7GXD
`Douglas Lockhart, VE7APU
`Wally Linstruth. WAGJPR
`Henry S. Magnuski, KAGM
`Paul Newland , AD7I
`Eric-L. Scace, K3NA
`
`I
`
`‘
`
`vii
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`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`Contents
`
`I 0 I s 1 0 I 0 I I 0 I
`
`2i
`
`AX.25 Link-Layer Protocol Specification
`
`2.1
`
`Scope and Field of Operation
`
`2.2 Frame Structure .
`2.2.1 Flag Field .
`Address Field
`Control Field
`
`.
`
`.
`.
`
`.
`.
`.
`PID Field
`Information Field
`.
`Bit Stuffing .
`.
`Frame—Check Sequence .
`Order of Bit Transmission
`Invalid Frames .
`.
`.
`.
`0 Frame Abort
`.
`.
`.
`1
`Interframe Time Fill
`
`2.2.13.1 Nghrepeater Address-Field Encoding .
`2.2.l3.1.l Destination Subfield Encoding
`.13.2 Level 2 Repeater-Address Encoding .
`.
`.
`.
`.13.3 Multiple Repeater Operation
`
`2.2
`2 2
`.
`
`Page
`
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`Page 000014
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`2.3 Elements of Procedure .
`2.3.2 Control-Field Formats and State Variables
`n
`c
`u
`20302:].
`c°ntr°1—Fie1d Formats n
`o
`u
`u
`1
`2.3.2.1.1 Information-Transfer Format
`2.3.2.1.2 Supervisory Format
`.
`.
`.
`.
`2.3.2.1.3 Unnumbered Format
`2.3.2.2 Control-Field Parameters
`2.3.2.3 Sequence Numbers
`2.3.2.4 Frame Variables and Sequence Numbers
`2.3.2.4.1 Send State Variable V(S)
`2.3.
`2.4.2 Send Sequence Number N(S)
`2.3.2
`.4.3 Receive State Variable V(R)
`2.3.2
`.4.4 Received Sequence Number N(R).
`0
`a
`I
`n
`I
`I
`I
`0
`2.3.3 Functions of P
`611/Final (P/F) Bit
`
`I
`
`.
`
`.