AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`2.4£.2 [A DXE shall initiate a reset procedure whenever it receives an
`unexpected UA response frame or an unsolicited response frame with the F bit
`set to one.
`A DXE may also initiate the reset procedure upon receipt of a
`FRMR frame. Alternatively,
`the DXE may respond to a FRHR by terminating the
`connection with a DISC frame.
`l
`
`the link by sending a SABM frame and starting timer
`2Jn6.3 [A DXE shall reset
`Tl. Upon receiving a SABM frame from the DXE previously connected to, the
`receiver of a SABM frame should send a UA frame back at
`the earliest
`opportunity, set its send and receive state variables, VCS) and V(R),
`to zero
`and stop Tl unless it has sent a SABM or DISC itself.
`If the UA is correctly
`received by the initial DXE,
`it resets its send and receive state variables,
`V(S) and V(R), and stops timer T1. Any busy condition that previously existed
`will also be cleared.
`
`the DXE will enter the disconnected
`If a DH response is received,
`state and stop timer T1.
`If timer T1 runs out before a BA or DH response
`frame is recieved,
`the SABM will be retransmitted and timer T1 restarted.
`If
`timer Tl runs out N2 times,
`the DXE will enter the disconnected state, and any
`previously existing link conditions will be cleared.
`1
`Other commands or responses received by the DXE before completion of
`the reset procedure will be discarded.
`
`2Jn6.4 One DXE may request that the other DXE reset the link by sending a DM
`response frame. After the DH frame is sent,
`the sending DXE will then enter
`the disconnected state.
`'
`
`2.4.7 List of System Defined Parameters
`
`2.4.7.1‘ Timers
`
`To maintain the integrity of the AX.25 level 2 connection, use of
`I
`these timers is recommended.
`I
`
`2.4.7.l.1 Acknowledgement Timer T1
`
`is used to make sure a DXE doesn't wait
`The first timer, T1,
`forever for a response to a frame it sends. This timer cannot be expressed in
`absolute time, since the time required to send frames varies greatly with the
`signaling rate used at level 1. T1 should take at least twice the amount of
`time it would take to send a maximum length frame to the other DXE, and get
`the proper response frame back from the other DXE. This would allow time for
`the other DXE to do some processing before responding.
`l
`[
`If level 2 repeaters are to be used,
`the value of T1 should be
`adjusted according to the number of repeaters the frame is being transferred
`through.
`]
`
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`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`2.4.7.1.2 [Response Delay Timer T2
`
`The second timer, T2, may be implemented by the DXE to specify a
`maximum amount of delay to be introduced between the time an I frame is
`received,
`and the time the resulting response frame is sent. This delay may
`be introduced to allow a receiving DXE to wait a short period of time to
`determine if there is more than one frame being sent to it.
`If more frames
`are received,
`the DXE can acknowledge them at once (up to seven),
`rather than
`acknowledge each individual frame.
`The use of timer T2 is not mandatory, but
`is recommended to improve channel efficiency. Note that, on full-duplex
`channels, acknowledgements should not be delayed beyond k/2 frames to achieve
`maximum throughput. The R parameter is defined in 2.4.7.4, below.
`
`2.4.7.l.3 Inactive Link Timer T3
`
`is used whenever Tl isn't running to maintain
`The third timer, T3,
`link integrity.
`It is recommended that whenever there are no outstanding
`unacknowledged I frames or P-hit frames (during the information-transfer
`state), a RR or RNR frame with the P bit set to one be sent every T3 time
`units to query the status of the other DXE. The period of T3 is locally
`defined, and depends greatly on level 1 operation.
`T3 should be greater than
`T1, and may be very large on channels of high integrity.
`]
`
`2.4.7.2 Maximum Number of Retries (N2)
`1
`
`timer.
`
`The maximum number of retries is used in conjunction with the T1
`
`2.4.7.3 Maximum Number of Ogtets in an I Field (N1)
`
`The maximum number of octets allowed in the I field will be 256.
`There shall also be an integral number of octets.
`
`2.4.7.4 Maximum Number of I Frames Outstanding (k)
`
`The maximum number of outstanding I frames at a time is seven.
`
`I
`
`-A.-A—3r.:«-.;4-4-.—...
`
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`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`Appendix A - Glossary
`P
`
`Note: This appendix is not a part of the protocol.
`
`ADCCP -- Advanced data communication control procedure, a bit-oriented link-
`level protocol developed by ANSI.
`
`Address field -- The octets within an HDLC frame containing the identities of
`the sending and receiving stations.
`
`AMRAD -— Amateur Radio Research and Development Corporation, a nonprofit
`organization involved in packet-radio development.
`(P.0. Drawer 6148,
`McLean, VA 22106.)
`
`AMSAT -- The Radio Amateur Satellite Corporation, a nonprofit organization
`involved in packet radio via OSCAR satellites.
`(P.0. Box 27, Washington, DC
`20044.)
`
`ANSI -- American National Standards Institute.
`10018.)
`1
`
`(1430 Broadway, New York, NY
`
`Balanced -- A relationship where two stations communicate with one another as
`equals;
`that is, neither is primary (master) or secondary (slave).
`
`Bit -- Binary digit.
`
`01'
`
`CCITT -- International Telegraph and Telephone Consultative Committee, a part
`of the International Telecommunication Union.
`-
`
`Collision -- A condition when two or more transmissions occur at the same time
`and cause interference to the intended receiver(sL
`
`logical coupling of two packet-radio stations
`the link layer,
`Connection -- At
`for information-transfer and control purposes.
`
`Control field -- An 8-bit pattern in an HDLC frame containing commands or
`responses, and sequence numbers.
`
`CRC —- Cyclic Redundancy Check, a mathematical operation in which the results
`are sent with a transmission block to enable receiving stations to check the
`integrity of the data.
`(Reference ISO 3309 Annex.)
`
`DCE ~- Data Circuit-Terminating Equipment, a master station in unbalanced mode
`at the link layer. Compare DTE and DXE.
`
`Domain -- In packet radio,
`service area.
`
`the combination of a frequency and a geographical
`
`DTE —— Data Terminal Equipment, a slave station in unbalanced mode at the link
`layer. Compare DCE and DXE.
`
`31
`
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`
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`

`
`
`
`
`
`l_4.D_”’..s..m_..u_.,.......&-_-_+__,:
`
`AX.25 Amateur Packet-Radio Link—Layer Protocol
`
`DXE —— Data Switching Equipment, a peer (neither master or slave) station in
`balanced mode at the link layer. Compare DCE and DTE.
`
`Destination -- The station that is the intended receiver of the frame sent
`over a radio link, either directly or via a repeater.
`
`FCS -— Frame Check Sequence.
`
`See CRC.
`
`Field -- In link-layer packet radio, a subdivision of a frame, consisting of
`one or more octets.
`
`Flag -- In HDLC, a bit pattern (0l1l1ll0) used to initiate and terminate a
`frame.
`
`Flow control -- Measures to restrict the data transmission rate to one which
`can be accommodated by the receiver.
`
`Frame -- The data structure of the high-level data link control procedure
`(HDLC) .
`
`Full duplex -- A physical or logical connection between two points over which
`data may travel in both directions simltaneously.
`
`Half duplex -- A physical or logical connection between two points over which
`data may travel in both directions alternately (one way at a time).
`
`Hex -- Hexadecimal, a base-16 number system that uses the symbols: 0, 1, 2, 3,
`4, 5, 6, 7, 8, 9, A, B, C, D, E, and F.
`
`HLC -- High-level data link control procedures, as specified in ISO 3309 and
`other international standards.
`
`Information field -- Any sequence of bits which, when used, contains the
`intelligence to be conveyed in a frame.
`
`ISO -- International Organization for Standardization.
`
`"Data commmnication —- High-
`ISO 3309 -- An international standard entitled,
`level data link control procedures —— Frame structure."
`‘
`
`a link-layer protocol for X.25 unbalanced-mode
`LAP -- Link Access Procedure,
`comunications between DTE and DCE.
`
`LAPB —— Link Access Procedure, Balanced, a link-layer protocol for X.25
`balanced-mode communications between peer stations.
`
`Layer -- In communications protocols, one of the strata or levels in a
`reference model. For example, the ISO open systems interconnection (OSI)
`reference model has seven functional layers.
`
`Level —— See layer.
`
`LSB -- Least-significant bit.
`
`32
`
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`

`
`AX.25 Amateur Packet-Radio Link—Layer Protocol
`
`MSB —- Most-significant bit.
`
`Octet 7- A group of eight bits.
`interconnection, a communications protocol reference model
`International Organization
`
`'
`
`e involving radio
`ontrol
`
`OSI -- Open systems
`“'developed by the
`
`'
`Packet Radio
`transmission of short
`
`PID —— Protocol Identifi
`bit patterns signify t
`
`layer protocol, an octet whose
`er -- Within AX.25 link-
`rotocol in use, if any.
`he specific network—layer p
`(clo Hank Magnuski, KAGM, 311 Standord
`
`the exchange of
`
`PPRS -- Pacific Packet Radio Society.
`Avenue, Menlo Park, CA 9h025.)
`lationship; the master
`ter station in a master-slave re that the slave cannot.
`Primary -- The mas 1 and is able to perform actions
`maintains contro
`Compare secondary.
`-- A formal set of rules and procedures for
`Protocol
`information with
`
`in a network.
`eledhmmunication Society.
`RATS —- Radio Amateur T Little Falls, NJ 07424.)
`NZDSY, 45 Union Ave.,
`r -- In packet radio,,an Amateur Radio station that recei and (if the
`Repeate
`rity by pfirforming a cyclic redundancy check,
`tests their integ
`e frame without readdressing.
`CRC is good) retransmits th
`y -- The slave in a master-slave relationship. Compare primary.
`(clo Pete Eaton, WB9FLW, 35 Norspur,
`SLAPR -- St. Louis Amateur Packet Radio.
`Route 4, Edwardsville, IL 62025.)
`
`(clo J. Gordon Beattie Jr”
`
`ves frames,
`
`Secondar
`
`frame over a direct radio link or via a
`
`Source -- The station transmitting the
`repeater.
`
`o Corporation, a nonprofit organization
`TAPR -— Tucson Amateur Packet Radi
`(PAL Box 22888, Tucson, AZ 85734J
`involved in packet-radio development.
`ommunications Group.
`VADCG -— Vancouver Amateur Digital C
`1, Canada.)
`VE7APU, 9531 Odlin Road, Richmond, BC, V6X IE
`the range of frame numbers within the
`layer packet radio,
`um number of frames the sender may
`Window -- In link-
`d to set the maxim
`control field use
`dgement from the receiver.
`eceives an acknowle
`transmit before it r
`X.25 -- A CCITT Recommendation entitled, "Interface between Data Terminal
`'ng Equipment
`(DCE) for Terminals
`Equipment
`(DTE) and Data Circuit-Terminati
`Operating in the Packet Mode on Public Data Networks."
`
`(clo Doug Lockhart,
`
`"“"""--IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
`
`Page 000051
`
`33
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`

`
`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`Appendix B —— References
`
`Note: This appendix is not a part of the protocol.
`
`"Operations Systems Network
`American Telephone and Telegraph Company,
`Comunications Protocol Specification BX.25 - Issue 2."
`
`ANSI X3.66, "Advanced Data Communication Control Procedure," (ADCCP).
`
`CCITT Recommendation X.25, "Interface between Data Terminal Equipment(DTE)
`and Data-Circuit Terminating Equipment
`(DCE) for Terminals Operating in the
`Packet Mode on Public Data Networks."
`
`‘Data communication —— High-level data link control procedures ~-
`ISO 3309;
`Frame structure."
`
`ISO 7205,
`
`"Reference Model of Open Systems Architecture."
`
`ISO 7776, "Information Processing Systems - Data Communications - 2nd DP 7776
`REVISED - Description of the 1984 X.25 LAPB - Compatible DTE Data Link
`Procedures."
`
`34
`
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`
`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`Appendix C -— Implementation Notes
`
`Note: This appendix is not a part of the protocol.
`
`Several implementations of packet-radio devices existed prior to the
`publication of this document.
`In order to maintain full compatibility with
`those devices,
`implementors of new AX.25 systems may wish to consider the
`following points.
`
`The use of the P/F bit in conjuction with the command/response bits,
`described in section 2.3.3 and in the state tables of appendix A, was not
`standardized until
`the publication of this document.
`AX.25 devices produced
`prior to the publication of this standard may handle the PIP bit
`in a
`different manner and the C bits not at all.
`No known implementation of AX.25
`that.existed before this standard uses the C bits in any way. Since this
`standard requires that the C bits always be complements of one another, the
`presence of an AX.25 device of nonstandard protocol can be detected by
`examining these bits and determining that they are both the same (that is,
`both 0 or both 1).
`
`the standard AX.25 DXE should relax the requirements on its use of
`standard,
`the P/F bit. Specifically,
`the other DXE may or may not reply to a poll (P
`bit equals 1).
`If it does feply,
`the F bit may or may not be set}
`The
`question of whether or not a given frame is a command or a response is
`somewhat confused without the C bits, and the standard DXE should assume that,
`in the absence of proper C bit content, the other DXE is incapable of proper
`P/F bit operation.
`
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`
`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`Appendix D - State Tables
`
`36
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`Page 000054