AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`The number of repeater addresses is variable. All but the last
`repeater address will have the address extension bits of all octets set to
`zero, as will all but
`the last octet
`(SSID octet) of
`the last repeater
`address. The last octet of the last repeater address will have the address
`extension bit set to one,
`indicating the end of the address field.
`oted that various timers (see 2.4.7, below) may have
`It should be n
`e the additional delays encountered when a frame
`to be adjusted to accommodat
`-repeater chain,
`and the return acknowledgement
`must pass through a multiple
`th before reaching the source device.
`must travel through the same pa
`
`It is anticipated that multiple-repeater operation is a temporary
`method of interconnecting stations over large distances until such time that a
`layer 3 protocol
`is in use. Once this layer 3 protocol becomes operational,
`repeater chaining should be phased out.
`
`2.3 Elements of Procedure
`23.1 The elements of procedure are defined in terms of actions that occur on
`receipt of frames.
`
`23.2 Control-Field Formats and State Variables
`
`2.3.2.1 Control-Field Formats
`The control field is responsible for identifying the type of frame
`being sent, and is also used to convey commands and responses from one end of
`the link to the other in order to maintain proper link control.
`The control fields used in AX.25 use the CCITT X.25 control fields
`for balanced operation (LAPB), with an additional control field taken from
`ADCCP to allow connectionless and round-table operation.
`They are the
`There are three general
`types of AX.25 frames.
`Information frame (1 frame),
`the Supervisory frame (S frame), and the
`Unnumbered frame (U frame). Fig. 5 shows the basic format of the control
`field associated with these types of frames.
`
`___—__—__—_...__—....__...—_———..—_...-.._..._.__._—.-.——_
`|Control-Field I
`Control-Field Bits I
`I
`Type
`I765I4|3210|
`TmI"§§§§.E"'T-§«Z£§"T-E‘?‘I{£§§"TB'T
`‘tun;-EiliénunFEE}-—T§7§T§‘§TB"I'T
`‘I"-‘B’;-Z;MT-zT1T§fI§}i=TIf11TI"I'T
`
`—_—_....__.—__....—._.-_...__...__....—_.__._._...—.__——__...._...._
`
`Fig. 5 -- Control-field formats
`
`10
`
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`
`

`
`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`Where:
`
`1. Bit O is the first bit sent and bit 7 is the —last‘bit sent of the
`control field.
`
`[
`
`2. MS)
`
`is the send sequence number (bit
`
`1
`
`is the LS3).
`
`3. N01) is the receive sequence number (bit 5 is the LSB).
`
`I
`
`4.
`
`The "S" bits are the supervisory function bits, and their encoding is
`discussed in 2.3.4.2.
`‘
`
`5. The "M" bits are the unnumbered frame modifier bits and their
`encoding is discussed in 2.3.4.3.
`
`[
`
`Its function is described in
`6. The P/F bit is the Poll/Final bit.
`2.3.3.
`The distinction between command and response,
`and therefore
`the distinction between 1’ bit and I-‘ bit,
`is made by addressing rules
`discussed in 2.4.1.2.
`]
`
`2.3.2.1.1 Information-Transfer Format
`
`All I frames have hit 0 of the control field set to zero. MS) is
`the sender's send sequence number (the send sequence number of this frame).
`N0!)
`is the sender's receive sequence number (the sequence number of the next
`expected received frame).
`These numbers are described in 2.3.2.4.
`In
`addition, the PIP bit is. be used as described in 2.4.2.
`
`2.3 .2. 1 . 2 Supervisory Forms.‘
`
`Supervisory frames are denoted by having bit 0 of the control field
`set to one, and bit 1 of the control field set to~zero.
`S frames provide
`supervisory link control such as acknowledging or requesting retransmission of
`I frames, and link—leve1 window control. Since S frames do not have an
`information field,
`the sender.'s send variable and the receiver's receive
`variable are not
`incremented for S frames.
`In addition,
`the PIP bit
`is used
`as described in 2.4.2.
`
`2.3 . 2.1 .3 Unnumbered Format
`
`Unnumbered frames are distinguished by having both bits 0 and 1 of
`the control field set
`to one.
`U frames are responsible for maintaining
`additional control over
`the link beyond what
`is accomplished with S frames.
`-They are also responsible for establishing and terminating link connections.
`U frames also allow for the transmission and reception of information outside
`of the normal flow control.
`Some 1] frames may contain information and PID
`fields. The P/F bit is used as described in 2.4.2.
`
`11
`
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`

`
`AX.25 Amateur Packet-Radio Link—Layer Protocol
`
`2.3.2.2 Control-Field Parameters
`
`2.3.2.3 Sequence Numbers
`
`Every AX.25 I frame shall be assigned, modulo 8, a sequential number
`from 0 to 7. This will allow up to seven outstanding I frames per level 2
`connection at a time.
`
`2.3.2.4 Frame Variables and Sequence Numbers
`
`2.3.2.4.l
`
`Send State Variable V(S)
`
`.The send state variable is a variable that is internal to the DXE
`and is never sent.
`It contains the next sequential number
`to be assigned to
`the next transmitted I frame. This variable is updated upon the transmission
`of each I frame.
`
`2.3.2.4.2 Send Sequence Number N(S)
`
`The send sequence number is found in the control field of all I
`It contains the sequence number of the 1 frame being sent.
`Just
`frames.
`prior to the transmission of the I frame, N(S) is updated to equal the send
`state variable.
`
`2.3.2.h.3 Receive State Variable V(R)
`
`The receive state variable is a variable that is internal to the
`It contains the sequence number of
`the next expected received I frame.
`DXE.
`This variable is updated upon the reception of an error-free I frame whose
`send sequence number equals the present received state variable value.
`
`2.3.2.4.4 Received Sequence Number N(R)
`
`The received sequence number is in both I and S frames. Prior to
`sending an I or S frame,
`this variable is updated to equal
`that of the
`received state variable,
`thus implictly acknowledging the proper reception of
`all I frames up to and including N(R)-1.
`
`2.3.3 Functions of Poll/Final (P/F) Bit
`
`It is used in a command
`The P/F bit is used in all types of frames.
`I
`The reply to this poll
`(poll) mode to request an immediate reply to a frame.
`is indicated by setting the response (final) bit in the appropriate frame.
`Only one outstanding poll condition per direction is allowed at a time.
`The
`procedure for P/F bit operation is described in ZJL2.
`]
`
`2.3.4 Control Field Coding for Commands and Responses
`
`The following commands and responses,
`encoding, are to be used by the DXE:
`
`indicated by their control field
`
`12
`
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`
`

`
`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`2.3.4.1 Information Command Frame Control Field
`
`The function of the information (I) command is to transfer across a
`[
`data link sequentially numbered frames containing an information field.
`I
`
`The information-frame control field is encoded as shown in Fig.6.
`These frames are sequentially numbered by the N(S) subfield to maintain
`control of their passage over the link—1ayer connection.
`
`Control Field Bits
`
`l7s5l4l321I0|
`
`Fig. 6 -- I frame control field
`
`2.3.4.2 Supervisory Frame Control Field
`
`The supervisory frame control fields are encoded as shown in Fig. 7.
`
`us...—_......._.______——..._..._.._._..._..—________._._._.__———___...__
`
`1 0 I
`I 7 6 5 I 4 I 3 2 I
`I Control Field Bits
`I ----------------------------------------------I
`IReceive Ready Jr
`RR
`I N(R)
`IP/Fl 0 0 I 0 1
`I
`IReceive Not Ready RNR I N(R)
`IP/FI 0 1
`I 0 1
`I
`IReject
`REJ
`I N(R)
`IP/Fl
`l 0 I 0 1
`I
`—.....______———.._.......—......._.__...._.___.___..._._———.——._..._.._......_..__
`
`Fig.
`
`7 - S frame control fields
`
`2.3.4.2.1 Receive Ready (RR) Command and Response
`
`Receive Ready is used to do the following:
`
`1.
`
`2.
`
`3.
`
`to indicate that the sender of the RR is now able to receive more I
`frames,
`
`to acknowledge properly received I frames up to, and including
`N(R)-1, and
`
`to clear a previously set busy condition created by an RNR command
`having been sent.
`
`13
`
`Page 000031
`
`

`
`AX.25 Amateur Packet-Radio Link—Layer Protocol
`
`The status of the DXE at the other end of the link can be requested
`by sending a RR command frame with the P-bit set to one.
`
`2.3.4.2.2 Receive Not Ready (RNR) Command and Response
`
`Receive Not Ready is used to indicate to the sender of I frames
`that the receiving DXE is temporarily busy and cannot accept any more I
`frames.
`Frames up to N(R)-1 are acknowledged. Any I frames numbered N(R) and
`higher that might have been caught between states and not acknowledged when
`the RNR command was sent are 3% acknowledged.
`
`The RNR condition can be cleared by the sending of a UA, RR, REJ,
`or SABM frame.
`
`The status of the DXE at the other end of the link can be requested
`by sending a RNR command frame with the P bit set to one.
`
`2.3.4.2.3 Reject
`
`(REJ) Command and Response
`
`The reject frame is used to request retransmission of I frames
`starting with MR). Any frames that were sent with a sequence number of
`MR)-1 or less are acknowledged. Additional I frames may be appended to the
`retransmission of the N01) frame if there are any.
`
`Only one reject frame condition is allowed in each direction at a
`The reject condition is cleared by the proper reception of I frames up
`time.
`to the I frame that caused the reject condition to be initiated.
`
`The status of the DXE at the other end of the link can be requested
`by sending a REJ command frame with the P bit set to one.
`
`2.3.4.3 Unnumbered Frame Control Fields
`
`Unnumbered frame control fields are either commands or responses.
`
`Fig. 8 shows the layout of U frames implemented within this protocol. _
`
`-
`
`’
`
`"
`
`14
`
`
`
`Page 000032
`
`

`
`AX.25 Amateur Packet—Radio Link-Layer Protocol
`
`I Cmd
`ISet Asynchronous
`IBa1anced Mode-SABMI
`
`I 0 0 1
`I
`
`I P I
`I
`I
`
`1
`
`1
`
`I
`
`DM
`
`I
`
`I
`
`I
`
`I
`
`1 1
`
`I
`
`I
`I
`
`I
`
`I Res
`Unnumbered
`I
`I Acknowledge-UA I
`
`1
`
`I 0 l
`I
`
`I F I 0 0 I
`I
`I
`I
`
`1 1
`
`1 1
`IEitherI 0 0 0 IP/Fl 0 0 I
`Unnumbered
`I
`I
`I
`I
`I
`I
`I
`Information-UI
`I
`_..._.__.__________.______.___—_———.-.....__....._——-.....~._.......—..-.—..._
`
`Fig. 8 - U frame control fields
`
`2.3.4.3.l Set Asynchronous Balanced Mode (SABM) Command
`1
`
`in the asynchronous
`The SABH command is used to place 2 DXEs
`balanced mode. This is a balanced mode of operation known as LAPB where both
`devices are treated as equals.l
`
`Any
`Information fields are.not allowed in SABM commands.
`outstanding I frames left when the SABM command is issued will remain
`unacknowledged.
`
`The DXE confirms reception and acceptance of a SABM command by
`[
`sending a UA response frame at the earliest opportunity.
`If the DXE is not
`capable of accepting a SABM command, it should respond with a DM frame if
`possible.
`I
`
`2.3.4.3.2 Disconnect
`
`(DISC) Comand
`
`The-DISC command is used to terminate a link session between two
`
`stations. No information field is permitted in a DISC command frame.
`
`the receiving DXE confirms
`Prior to acting on the DISC frame,
`acceptance of the DISC by issuing a UA response frame at
`its earliest
`opportunity.
`The DXE sending the DISC enters the disconnected state when it
`receives the UA response.
`'
`I
`
`Any unacknowledged I frames left when this command is acted upon
`will remain unacknowledged.
`
`15
`
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`
`

`
`AX.25 Amateur Packet-Radio Link—Layer Protocol
`
`2.3.4.3.3 Frame Reject
`(FRM) Response
`a
`to report that the receiver of
`2.3.4.3.3.1 The FRMR response frame is sent
`is
`frame
`cannot successfully process that frame and that the error condition
`not
`correctable
`by
`sending
`the offending
`frame
`again.
`Typically this
`condition will appear when a frame without an FCS error has been received with
`one of the following conditions:
`1.
`The
`reception of
`response frame.
`The
`reception of an I frame whose information field exceeds
`agreed—upon length.
`(See 2.4.753, below.)
`3. The reception of an improper N(R). This usually happens when the
`N(R) frame has already been sent and acknowledged,
`or when N(R)
`is out of sequence with what was expected.
`
`implemented
`
`command or
`
`the
`
`an
`
`invalid or not
`
`2.
`
`~ 4.
`
`is
`reception of a frame with an information field where one
`The
`allowed,
`or the reception of a-D or S frame whose length is
`‘not
`incorrect.
`Bits W and Y described in 2.3.4.3.3.2 should both be
`set to one to indicate this condition.
`
`[
`
`5. The reception of a supervisory frame with the F bit set to one,
`except during a timer recovery condition (see 2.4.4.9), or except
`as a reply to a comand frame~sent with the P bit set to one.
`Bit W (described in 2.3.4.3.3.2) should be set to one.
`
`reception of an unexpected UA or DM response frame.
`'6. The
`should be set to one.
`
`Bit
`
`W
`
`7. The reception of a frame with an invalid N(S).
`set to one.
`
`Bit W should be
`
`is defined as one which points to an I frame that
`An invalid N(R)
`previously has been transmitted and acknowledged,
`or an I frame which has not
`been transmitted and is not
`the next sequential I frame pending transmission.
`_
`:
`An
`invalid N(S)
`is defined as an N(S) that
`is equal to the
`last
`transmitted N(R)+k and is equal to the received state variable V(R),. where
`k
`is
`the maximum number of outstanding information frames as defined in 2.4.7.4
`below.
`
`implemented comand or response is defined as a
`An invalid or not
`frame with a control field that is unknown to the receiver of this frame.
`2.3.4.3.3.2 When a FRMR frame is sent,
`an information field is added to the
`frame
`that
`contains
`additional
`information indicating where
`the
`problem
`occurred.
`This‘ information field is three octets long and is shown in Fig.
`9.
`]
`
`
`
`16
`
`Page 000034
`
`

`
`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`I 0 o o olzlrlxlwl V(R)ICl v(s)Iol Rejected FrameI
`I
`I
`I
`I
`I
`I
`IRI
`I
`I Control Field I
`
`Fig. 9 -— FRM frame information field
`
`Where:
`
`The rejected frame control field carries the control field of the
`frame that caused the reject condition.
`It is in bits 0-7 of the
`information field.
`
`V(S) is the current send state variable of the device reporting
`the rejection (bit 9 is the low bit).
`
`The CR bit is set to zero to indicate the rejected frame was a
`command, or one if it was a response.
`
`receive state variable of
`is the current
`VTR)
`reporting rejefition (bit 13 is the low bit).
`
`the device
`1
`
`If W is set to 1, the control field received was invalid or not
`implemented.
`'I
`
`the frame that caused the reject condition was
`If X is set to 1,
`considered invalid because it was a U or S frame that had an
`information field that is not allowed. Bit W must be set to 1 in
`addition to the X bit.
`
`the information field of a received frame
`If Y is set to l,
`exceeded the maximum allowed under this recommendation in
`2.4.7.3 , below.
`
`If Z is set to 1,
`1 to 8 contained
`
`the control field received and returned in bits
`an invalid N(R).
`
`9.
`
`Bits 8, and 20 to 23 are set to O.
`
`I
`
`2.3.4.3.4 Unnumbered Acknowledge (UA) Response
`
`The UA response frame is sent to acknowledge the reception and
`acceptance of a SABM or DISC command frame.
`A received command is not
`actually processed until
`the UA response frame is sent.
`Information fields
`are not permitted in a UA frame.
`
`17
`
`Page 000035
`
`

`
`AX.25 Amateur Packet-Radio Link-Layer Protocol
`
`2.3.4.3.5 Disconnected Mode (DM) Response
`
`The disconnected mode response is sent whenever a DXE receives a
`[
`frame other than a SABM or UI frame while in;a disconnected mode.
`It is also
`sent
`to request a set mode command, or to indicate it cannot accept
`a
`connection at
`the moment. The DM response does not have an information field.
`
`Whenever a SABM frame is received, and it is determined that a
`connection is not possible, a DM frame shall be sent. This will indicate that
`the called station cannot accept a connection at that time.
`
`While a DXE is in the disconnected mode, it will respond to any
`command other than a SABM or UI frame with a DM response with the P/F bit set
`to 1.
`
`2.3.4.3.6 Unnumbered Information (UI) Frame
`
`The Unnumbered Information frame contains PID and information
`fields and is used to pass information along the link outside the normal
`information controls. This allows information fields to go back and forth on
`the link bypassing flow control.
`Since these frames are pg; acknowledgeable,
`if one gets obliterated, there is no way to recover it. A received UI frame
`with the P bit set shall cause a response to be transmitted. This response
`shall be a DH frame when in the disconnected state or a RR (or RNR,
`if
`appropriate) frame in the information transfer state.
`1
`
`2.3.5 ,Link Error Reporting and Recovery
`
`link-layer errors that are recoverable without
`There are several
`terminating the connection. These error situations may occur as a result of
`malfunctions within the DXE, or if transmission errors occur.
`
`2.3.5.1 DXE Busy Condition
`
`When a DXE becomes temporarily unable to receive I frames, such as
`when receive buffers are full, it will send a Receive Not Ready (RNR) frame.
`This informs the other DXE that this DXE cannot handle any more I frames at
`the moment. This condition is usually cleared by the sending of a UA, RR,
`REJ, or SABM command frame.
`
`2.3.5.2 Send Sequence Number Error
`
`If the send sequence number, N(S), of an otherwise error—free
`received I frame does not match the receive state variable, VCR), a send
`sequence error has occured, and the information field will be discarded.
`The
`receiver will not acknowledge this frame, or any other I frames, until N(S)
`matches V(R).
`
`The control field of the erroneous I frame(s) will be accepted so
`that link supervisory functions such as checking the P/F bit can still be
`performed. Because of this updating, the retransmitted I frame may have an
`updated P bit and N(R).
`
`18
`
`Page 000036