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AX.25 Amateur Packet-Radio Link-Layer Protocol

AX.25 Amateur Packet-Radio Link-Layer Protocol

Version 2.2, 1998

Ver 2.2 Rev: 1998 (pdf format 3.0, 1.6M)


AX.25 Amateur Packet-Radio Link-Layer Protocol

Version 2.0, October 1984


    PID Table Updated November, 1997.

2 AX.25 Link-Layer Protocol Specification

2.1 Scope and Field of Operation

In order to provide a mechanism for the reliable transport of data between two signaling terminals, it is necessary to define a protocol that can accept and deliver data over a variety of types of communications links. The AX.25 Link- Layer Protocol is designed to provide this service, independent of any other level that may or may not exist.

This protocol conforms to ISO Recommendations 3309, 4335 (including DAD 1&2) and 6256 high-level data link control (HDLC) and uses some terminology found in these documents. It also conforms with ANSI X3.66, which describes ADCCP, balanced mode.

This protocol follows, in principle, the CCITT X.25 Recommendation, with the exception of an extended address field and the addition of the Unnumbered Information (UI) frame. It also follows the principles of CCITT Recommendation Q.921 (LAPD) in the use of multiple links, distinguished by the address field, on a single shared channel.

As defined, this protocol will work equally well in either half- or full-duplex Amateur Radio environments.

This protocol has been designed to work equally well for direct connections between two individual amateur packet-radio stations or an individual station and a multiport controller.

This protocol allows for the establishment of more than one link-layer connection per device, if the device is so capable.

This protocol does not prohibit self-connections. A self-connection is considered to be when a device establishes a link to itself using its own address for both the source and destination of the frame.

Most link-layer protocols assume that one primary (or master) device (generally called a DCE, or data circuit- terminating equipment) is connected to one or more secondary (or slave) device(s) (usually called a DTE, or data terminating equipment). This type of unbalanced operation is not practical in a shared-RF Amateur Radio environment. Instead, AX.25 assumes that both ends of the link are of the same class, thereby eliminating the two different classes of devices. The term DXE is used in this protocol specification to describe the balanced type of device found in amateur packet radio.

2.2 Frame Structure

Link layer packet radio transmissions are sent in small blocks of data, called frames. Each frame is made up of several smaller groups, called fields. Fig.1 shows the three basic types of frames. Note that the first bit to be transmitted is on the left side.

First Bit Sent
FlagAddressControlFCSFlag
01111110112/560 Bits8 Bits16 Bits01111110
Fig. 1A -- U and S frame construction

First Bit Sent
FlagAddressControlPIDInfo.FCSFlag
01111110112/560 Bits8 Bits8 BitsN*8 Bits16 Bits01111110
Fig. 1B -- Information frame construction

Each field is made up of an integral number of octets (or bytes), and serves a specific function as outlined below.

2.2.1 Flag Field

The flag field is one octet long. Since the flag is used to delimit frames, it occurs at both the beginning and end of each frame. Two frames may share one flag, which would denote the end of the first frame, and the start of the next frame. A flag consists of a zero followed by six ones followed by another zero, or 01111110 (7E hex). As a result of bit stuffing (see 2.2.6, below), this sequence is not allowed to occur anywhere else inside a complete frame.

2.2.2 Address Field

The address field is used to identify both the source of the frame and its destination. In addition, the address field contains the command/response information and facilities for level 2 repeater operation.

The encoding of the address field is described in 2.2.13.

2.2.3 Control Field

The control field is used to identify the type of frame being passed and control several attributes of the level 2 connection. It is one octet in length, and its encoding is discussed in 2.3.2.1, below.

2.2.4 PID Field

The Protocol Identifier (PID) field shall appear in information frames (I and UI) only. It identifies what kind of layer 3 protocol, if any, is in use.

The PID itself is not included as part of the octet count of the information field. The encoding of the PID is as follows:

HEX M      L
S      S
B      B
Translation
0x01 00000001 ISO 8208/CCITT X.25 PLP
0x06 00000110 Compressed TCP/IP packet. Van Jacobson (RFC 1144)
0x07 00000111 Uncompressed TCP/IP packet. Van Jacobson (RFC 1144)
0x08 00001000 Segmentation fragment
** yy01yyyy AX.25 layer 3 implemented.
** yy10yyyy AX.25 layer 3 implemented.
0xC3 11000011 TEXNET datagram protocol
0xC4 11000100 Link Quality Protocol
0xCA 11001010 Appletalk
0xCB 11001011 Appletalk ARP
0xCC 11001100 ARPA Internet Protocol
0xCD 11001101 ARPA Address resolution
0xCE 11001110 FlexNet
0xCF 11001111 NET/ROM
0xF0 11110000 No layer 3 protocol implemented.
0xFF 11111111 Escape character. Next octet contains more Level 3 protocol information.

Where:
A y indicates all combinations used.

Note:
All forms of yy11yyyy and yy00yyyy other than those listed above are reserved at this time for future level 3 protocols. The assignment of these formats is up to amateur agreement. It is recommended that the creators of level 3 protocols contact the ARRL Ad Hoc Committee on Digital Communications for suggested encodings.

2.2.5 Information Field

The information field is used to convey user data from one end of the link to the other. I fields are allowed in only three types of frames: the I frame, the UI frame, and the FRMR frame. The I field can be up to 256 octets long, and shall contain an integral number of octets. These constraints apply prior to the insertion of zero bits as specified in 2.2.6, below. Any information in the I field shall be passed along the link transparently, except for the zero-bit insertion (see 2.2.6) necessary to prevent flags from accidentally appearing in the I field.

2.2.6 Bit Stuffing

In order to assure that the flag bit sequence mentioned above doesn't appear accidentally anywhere else in a frame, the sending station shall monitor the bit sequence for a group of five or more contiguous one bits. Any time five contiguous one bits are sent the sending station shall insert a zero bit after the fifth one bit. During frame reception, any time five contiguous one bits are received, a zero bit immediately following five one bits shall be discarded.

2.2.7 Frame-Check Sequence

The frame-check sequence (FCS) is a sixteen-bit number calculated by both the sender and receiver of a frame. It is used to insure that the frame was not corrupted by the medium used to get the frame from the sender to the receiver. It shall be calculated in accordance with ISO 3309 (HDLC) Recommendations.

2.2.8 Order of Bit Transmission

With the exception of the FCS field, all fields of an AX.25 frame shall be sent with each octet's least-significant bit first. The FCS shall be sent most-significant bit first.

2.2.9 Invalid Frames

Any frame consisting of less than 136 bits (including the opening and closing flags), not bounded by opening and closing flags, or not octet aligned (an integral number of octets), shall be considered an invalid frame by the link layer. See also 2.4.4.4, below.

2.2.10 Frame Abort

If a frame must be prematurely aborted, at least fifteen contiguous ones shall be sent with no bit stuffing added.

2.2.11 Interframe Time Fill

Whenever it is necessary for a DXE to keep its transmitter on while not actually sending frames, the time between frames should be filled with contiguous flags.

2.2.12 Link Channel States

Not applicable.

2.2.13 Address-Field Encoding

The address field of all frames shall be encoded with both the destination and source amateur call signs for the frame. Except for the Secondary Station Identifier (SSID), the address field should be made up of upper-case alpha and numeric ASCII characters only. If level 2 amateur "repeaters" are to be used, their call signs shall also be in the address field.

The HDLC address field is extended beyond one octet by assigning the least-significant bit of each octet to be an "extension bit". The extension bit of each octet is set to zero, to indicate the next octet contains more address information, or one, to indicate this is the last octet of the HDLC address field. To make room for this extension bit, the amateur Radio call sign information is shifted one bit left.

2.2.13.1 Nonrepeater Address-Field Encoding
If level 2 repeaters are not being used, the address field is encoded as shown in Fig. 2. The destination address is the call sign and SSID of the amateur radio station to which the frame is addressed, while the source address contains the amateur call sign and SSID of the station that sent the frame. These call signs are the call signs of the two ends of a level 2 AX.25 link only.

First Octet Sent
Address Field of Frame
Destination Address Source Address
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14
Fig. 2 -- Nonrepeater Address-Field Encoding

A1 through A14 are the fourteen octets that make up the two address subfields of the address field. The destination subaddress is seven octets long (A1 thru A7), and is sent first. This address sequence provides the receivers of frames time to check the destination address subfield to see if the frame is addressed to them while the rest of the frame is being received. The source address subfield is then sent in octets A8 through A14. Both of these subfields are encoded in the same manner, except that the last octet of the address field has the HDLC address extension bit set.

There is an octet at the end of each address subfield that contains the Secondary Station Identifier (SSID). The SSID subfield allows an Amateur Radio operator to have more than one packet-radio station operating under the same call sign. This is useful when an amateur wants to put up a repeater in addition to a regular station, for example. The C bits (see 2.4.1.2, below) and H bit (see 2.2.13.2, below) are also contained in this octet, along with two bits which are reserved for future use.

Fig. 3A shows a typical AX.25 frame in the nonrepeater mode of operation.

OctetASCIIBin.DataHex Data
Flag011111107E
A1K1001011096
A280111000070
A3M100110109A
A4M100110109A
A5O100111109E
A6space0100000040
A7SSID11100000E0
A8W10101110AE
A9B1000010084
A1040110010068
A11J1001010094
A12F100011008C
A13I1001001092
A14SSID0110000161
ControlI001111103E
PIDnone11110000F0
FCSpart 1xxxxxxxxHH
FCSpart 2xxxxxxxxHH
Flag011111107E
Bit position76543210
Fig. 3A -- Nonrepeater AX.25 frame

The frame shown is an I frame, not going through a level 2 repeater, from WB4JFI (SSID=0) to K8MMO (SSID=0), with no level 3 protocol. The P/F bit is set; the receive sequence number (N(R)) is 1; the send sequence number (N(S)) is 7.

2.2.13.1.1 Destination Subfield Encoding

Fig. 3 shows how an amateur call sign is placed in the destination address subfield, occupying octets A1 thru A7.

OctetASCIIBin.DataHex Data
A1W10101110AE
A2B1000010084
A340110100068
A4J1001010094
A5F100011008C
A6I1001001092
A7SSIDCRRSSID0
Bit Position76543210
Fig. 3 -- Destination Field Encoding

Where:

  1. The top octet (A1) is the first octet sent, with bit 0 of each octet being the first bit sent, and bit 7 being the last bit sent.
  2. The first (low-order or bit 0) bit of each octet is the HDLC address extension bit, which is set to zero on all but the last octet in the address field, where it is set to one.
  3. The bits marked "r" are reserved bits. They may be used in an agreed-upon manner in individual networks. When not implemented, they should be set to one.
  4. The bit marked "C" is used as the command/response bit of an AX.25 frame, as outlined in 2.4.1.2 below.
  5. The characters of the call sign should be standard seven-bit ASCII (upper case only) placed in the leftmost seven bits of the octet to make room for the address extension bit. If the call sign contains fewer than six characters, it should be padded with ASCII spaces between the last call sign character and the SSID octet.
  6. The 0000 SSID is reserved for the first personal AX.25 station. This establishes one standard SSID for "normal" stations to use for the first station.
2.2.13.2 Level 2 Repeater-Address Encoding
If a frame is to go through level 2 amateur packet repeater(s), there is an additional address subfield appended to the end of the address field. This additional subfield contains the call sign(s) of the repeater(s) to be used. This allows more than one repeater to share the same RF channel. If this subfield exists, the last octet of the source subfield has its address extension bit set to zero, indicating that more address-field data follows. The repeater-address subfield is encoded in the same manner as the destination and source address subfields, except for the most-significant bit in the last octet, called the "H" bit. The H bit is used to indicate whether a frame has been repeated or not.

In order to provide some method of indicating when a frame has been repeated, the H bit is set to zero on frames going to a repeater. The repeater will set the H bit to one when the frame is retransmitted. Stations should monitor the H bit, and discard any frames going to the repeater (uplink frames), while operating through a repeater. Fig. 4 shows how the repeater- address subfield is encoded. Fig. 4A is an example of a complete frame after being repeated.

OctetASCIIBin.DataHex Data
A15W10101110AE
A16B1000010084
A1740110100068
A18J1001010094
A19F100011008C
A20I1001001092
A21SSIDHRRSSID1
Bit Order -->76543210
Fig. 4 -- Repeater-address encoding
Where:
  1. The top octet is the first octet sent, with bit 0 being sent first and bit 7 sent last of each octet.
  2. As with the source and destination address subfields discussed above, bit 0 of each octet is the HDLC address extension bit, which is set to zero on all but the last address octet, where it is set to one.
  3. The "R" bits are reserved in the same manner as in the source and destination subfields.
  4. The "H" bit is the has-been-repeated bit. It is set to zero whenever a frame has not been repeated, and set to one by the repeater when the frame has been repeated.

OctetASCIIBin.DataHex Data
Flag011111107E
A1K1001011096
A280111000070
A3M100110109A
A4M100110109A
A5O100111109E
A6space0100000040
A7SSID11100000E0
A8W10101110AE
A9B1000010084
A1040110010068
A11J1001010094
A12F100011008C
A13I1001001092
A14SSID0110000060
A15W10101110AE
A16B1000010084
A1740110100068
A18J1001010094
A19F100011008C
A20I1001001092
A21SSID11100011E3
ControlI001111103E
PIDnone11110000F0
FCSpart 1xxxxxxxxHH
FCSpart 2xxxxxxxxHH
Flag011111107E
Bit position76543210
Fig. 4A -- AX.25 frame in repeater mode

The above frame is the same as Fig. 3A, except for the addition of a repeater-address subfield (WB4JFI, SSID=1). The H bit is set, indicating this is from the output of the repeater.

2.2.13.3 Multiple Repeater Operation
The link-layer AX.25 protocol allows operation through more than one repeater, creating a primitive frame routing mechanism. Up to eight repeaters may be used by extending the repeater-address subfield. When there is more than one repeater address, the repeater address immediately following the source address subfield will be considered the address of the first repeater of a multiple-repeater chain. As a frame progresses through a chain of repeaters, each successive repeater will set the H bit (has-been-repeated bit) in its SSID octet, indicating that the frame has been successfully repeated through it. No other changes to the frame are made (except for the necessary recalculation of the FCS). The destination station can determine the route the frame took to each it by examining the address field.

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.

It should be noted that various timers (see 2.4.7, below) may have to be adjusted to accommodate the additional delays encountered when a frame must pass through a multiple-repeater chain, and the return acknowledgement must travel through the same path before reaching the source device.

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

2.3.1

The elements of procedure are defined in terms of actions that occur on receipt of frames.

2.3.2 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.

There are three general types of AX.25 frames. They are the Information frame (I 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
Type
Control-Field Bits
7654 3210
I FrameN(R)P N(S)0
S FrameN(R)P/FSS01
U FrameMMMP/FMM11
Fig. 5 -- Control-field formats

Where:

  1. Bit 0 is the first bit sent and bit 7 is the last bit sent of the control field.
  2. N(S) is the send sequence number (bit 1 is the LSB).
  3. N(R) is the receive sequence number (bit 5 is the LSB).
  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.
  6. The P/F bit is the Poll/Final bit. Its function is described in 2.3.3. The distinction between command and response, and therefore the distinction between P bit and F bit, is made by addressing rules discussed in 2.4.1.2.

2.3.2.1.1 Information-Transfer Format

All I frames have bit 0 of the control field set to zero. N(S) is the sender's send sequence number (the send sequence number of this frame). N(R) 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 P/F bit is to be used as described in 2.4.2.

2.3.2.1.2 Supervisory Format

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- level 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 P/F 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 U frames may contain information and PID fields. The P/F bit is used as described in 2.4.2.
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

2.3.2.4.1 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 frames. It contains the sequence number of the I frame being sent. Just prior to the transmission of the I frame, N(S) is updated to equal the send state variable.

2.3.2.4.3 Receive State Variable V(R)

The receive state variable is a variable that is internal to the DXE. It contains the sequence number of the next expected received I frame. 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 implicitly acknowledging the proper reception of all frames up to and including N(R)-1.

2.3.3 Functions of Poll/Final (P/F) Bit

The P/F bit is used in all types of frames. It is used in a command (poll) mode to request an immediate reply to a frame. The reply to this poll 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 2.4.2.

2.3.4 Control Field Coding for Commands and Responses

The following commands and responses, indicated by their control field encoding, are to be use by the DXE:
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.

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-layer connection.

Control Field Bits
76543210
N(R) P N(S) 0
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.

Control Field Type Control Field Bits
76543210
Receive ReadyRR N(R)P/F0001
Receive Not ReadyRNR N(R)P/F0101
RejectREJ N(R)P/F1001
Fig. 7 -- S frame control fields

The Frame identifiers:
C or SABMLayer 2 Connect Request
D or DISCLayer 2 Disconnect Request
I Information Frame
RR Receive Ready. System Ready To Receive
RNR or NRReceive Not Ready. TNC Buffer Full
RJ or REJReject Frame. Out of Sequence or Duplicate
FRMR Frame Reject. Fatal Error
UI Unnumbered Information Frame. "Unproto"
DM Disconnect Mode. System Busy or Disconnected.

2.3.4.2.1 Receive Ready (RR) Command and Response

Receive Ready is used to do the following:
  1. to indicate that the sender of the RR is now able to receive more I frames.
  2. to acknowledge properly received I frames up to, and including N(R)-1, and
  3. to clear a previously set busy condition created by an RNR command having been sent.

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 not 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 N(R). Any frames that were sent with a sequence number of N(R)-1 or less are acknowledged. Additional I frames may be appended to the retransmission of the N(R) frame if there are any.

Only one reject frame condition is allowed in each direction at a time. The reject condition is cleared by the proper reception of I frames up 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.

Control Field Type Control Field Bits
76543210
Set Asynchronous Balanced ModeSABMRes 001P1111
DisconnectDISCCmd 010P0011
Disconnected ModeDMRes 000F1111
Unnumbered AcknowledgeUARes 011F0011
Frame RejectFRMRRes 100F0111
Unnumbered InformationUIEither 000P/F0011
Fig. 8 -- U frame control fields

2.3.4.3.1 Set Asynchronous Balanced Mode (SABM) Command

The SABM command is used to place 2 DXEs in the asynchronous balanced mode. This is a balanced mode of operation known as LAPB where both devices are treated as equals.

Information fields are not allowed in SABM commands. Any 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.

2.3.4.3.2 Disconnect (DISC) Command

The DISC command is used to terminate a link session between two stations. No information field is permitted in a DISC command frame.

Prior to acting on the DISC frame, the receiving DXE confirms 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.

Any unacknowledged I frames left when this command is acted upon will remain unacknowledged.

2.3.4.3.3 Frame Reject (FRMR) Response

2.3.4.3.3.1

The FRMR response frame is sent to report that the receiver of a frame cannot successfully process that frame and that the error condition is 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 an invalid or not implemented command or response frame.
  2. The reception of an I frame whose information field exceeds the agreed-upon length. (See 2.4.7.3, 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.
  4. The reception of a frame with an information field where one is not allowed, or the reception of a U or S frame whose length is 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 command frame sent with the P bit set to one. Bit W (described in 2.3.4.3.3.2) should be set to one.
  6. The reception of an unexpected UA or DM response frame. Bit W should be set to one.
  7. The reception of a frame with an invalid N(S). Bit W should be set to one.

An invalid N(R) is defined as one which points to an I frame that 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.

An invalid or not implemented command or response is defined as a 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.

Information Field Bits
2322212019181716 15141312111098 76543210
0000ZYXW V(R)C
R
V(S)0 Rejected Frame
Control Field
Fig. 9 -- FRMR frame information field

Where:

  1. 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.
  2. V(S) is the current send state variable of the device reporting the rejection (bit 9 is the low bit).
  3. The CR bit is set to zero to indicate the rejected frame was a command, or one if it was a response.
  4. V(R) is the current receive state variable of the device reporting rejection (bit 13 is the low bit).
  5. If W is set to 1, the control field received was invalid or not implemented.
  6. If X is set to 1, the frame that caused the reject condition was 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.
  7. If Y is set to 1, the control field received and returned in bits exceeded the maximum allowed under this recommendation in 2.4.7.3, below.
  8. If A is set to 1, the control field received and returned in bits 1 to 8 contained an invalid N(R).
  9. Bits 8, and 20 to 23 are set to 0.

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.

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 a 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 not 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 DM frame when in the disconnected state or a RR (or RNR, if appropriate) frame in the information transfer state.

2.3.5 Link Error Reporting and Recovery

There are several link-layer errors that are recoverable without 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 frame does not match the receive state variable, V(R), a send sequence error has occurred, 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).

2.3.5.3 Reject (REJ) Recovery
REJ is used to request a retransmission of I frames following the detection of a N(S) sequence error. Only one outstanding "sent REJ" condition is allowed at a time. This condition is cleared when the requested I frame has been received.

A DXE receiving the REJ command will clear the condition by resending all outstanding I frames (up to the window size), starting with the one indicated in N(R) of the REJ frame.

2.3.5.4 Time-out Error Recovery

2.3.5.4.1 T1 Timer Recovery

If a DXE, due to a transmission error, does not receive (or receives and discards) a single I frame or the last I frame in a sequence of I frames, it will not detect a send-sequence- number error, and therefore will not transmit a REJ. The DXE which transmitted the unacknowledged I frame(s) shall, following the completion of time-out period T1, take appropriate recovery action to determine when I frame retransmission should begin as described in 2.4.4.9, below. This condition is cleared by the reception of an acknowledgement for the sent frame(s), or by the link being reset. See 2.4.6.

2.3.5.4.2 Timer T3 Recovery

Timer T3 is used to assure the link is still functional during periods of low information transfer. Whenever T1 is not running (no outstanding I frames), T3 is used to periodically poll the other DXE of a link. When T3 times out, a RR or RNR frame is transmitted as a command and with the P bit set. The waiting acknowledgement procedure (2.4.4.9, below) is then executed.
2.3.5.5 Invalid Frame or FCS Error
If an invalid frame is received, or a frame is received with an FCS error, that frame will be discarded with no action taken.
2.3.5.6 Frame Rejection Condition
A frame rejection condition occurs when an otherwise error-free frame has been received with one of the conditions listed in 2.3.4.3.3 above.

Once a rejection error occurs, no more I frames are accepted (except for the examination of the P/F bit) until the error is resolved. The error condition is reported to the other DXE by sending a FRMR response frame. See 2.4.5.

2.4 Description of AX.25 Procedures

The following describes the procedures used to setup, use, and disconnect a balanced link between two DXE stations.

2.4.1 Address Field Operation

2.4.1.1 Address Information
All transmitted frames shall have address fields conforming to 2.2.13, above. All frames shall have both the destination device and the source device addresses in the address field, with the destination address coming first. This allows many links to share the same RF channel. The destination address is always the address of the station(s) to receive the frame, while the source address contains the address of the device that sent the frame.

The destination address can be a group name or club call sign if the point-to-multipoint operation is allowed. Operation with destination addresses other than actual amateur call signs is a subject for further study.

2.4.1.2 Command/Response Procedure
AX.25 Version 2.0 has implemented the command/response information in the address field. In order to maintain compatibility with previous versions of AX.25, the command/response information is conveyed using two bits.

An upward-compatible AX.25 DXE can determine whether it is communicating with a DXE using an older version of this protocol by testing the command/response bit information located in bit 7 of the SSID octets of both the destination and source address subfields. If both C bits are set to zero, the device is using the older protocol. The newer version of the protocol always has one of these two bits set to one and the other set to zero, depending on whether the frame is a command or a response.

The command/response information is encoded into the address field as shown in Fig. 10.

Frame TypeDest. SSID C-BitSource SSID C-Bit
Previous versions00
Command (V.2.0)10
Response (V.2.0)01
Previous versions11
Fig. 10 -- Command/Response encoding

Since all frames are considered either commands or responses, a device shall always have one of the bits set to one, and the other bit set to zero.

The use of the command/response information in AX.25 allows S frames to be either commands or responses. This aids maintenance of proper control over the link during the information transfer state.

2.4.2 P/F Bit Procedures

The next response frame returned by the DXE to a SABM or DISC command with the P bit set to 1 will be a UA or DM response with the F bit set to 1.

The next response frame returned to an I frame with the P bit set to 1, received during the information transfer state, will be a RR, RNR, or REJ response with the F bit set to 1.

The next response frame returned to a supervisory command frame with the P bit set to 1, received during the information transfer state, will be a RR, RNR, or REJ response frame with the F bit set to 1.

The next response frame returned to a S or I command frame with the P bit set to 1, received in the disconnected state, will be a DM response frame with the F bit set to 1.

The P bit is used in conjunction with the time-out recovery condition discussed in 2.3.5.4, above.

When not used, the P/F bit is set to zero.

2.4.3 Procedures For Link Set-Up and Disconnection

2.4.3.1 LAPB Link Connection Establishment
When one DXE wishes to connect to another DXE, it will send a SABM command frame to that device and start timer (T1). If the other DXE is there and able to connect, it will respond with a UA response frame, and reset both of its internal state variables (V(S) and V(R)). The reception of the UA response frame at the other end will cause the DXE requesting the connection to cancel the T1 timer and set its internal state variables to 0.

If the other DXE doesn't respond before T1 times out, the device requesting the connection will re-send the SABM frame, and start T1 running again. The DXE should continue to try to establish a connection until it has tried unsuccessfully N2 times. N2 is defined in 2.4.7.2, below.

If, upon reception of a SABM command, the DXE decides that it cannot enter the indicated state, it should send a DM frame.

When receiving a DM response, the DXE sending the SABM should cancel its T1 timer, and not enter the information- transfer state.

The DXE sending a SABM command will ignore and discard any frames except SABM, DISC, UA, and DM frames from the other DXE.

Frames other than UA and DM in response to a received SABM will be sent only after the link is set up and if no outstanding SABM exists.

2.4.3.2 Information-Transfer Phase
After establishing a link connection, the DXE will enter the information-transfer state. In this state, the DXE will accept and transmit I and S frames according to the procedure outlined in 2.4.4, below.

When receiving a SABM command while in the information- transfer state, the DXE will follow the resetting procedure outlined in 2.4.6 below.

2.4.3.3 Link Disconnection

2.4.3.3.1

While in the information-transfer state, either DXE may indicate a request to disconnect the link by transmitting a DISC command frame and starting timer T1 (see 2.4.7).

2.4.3.3.2

A DXE, upon receiving a valid DISC command, shall send a UA response frame and enter the disconnected state. A DXE, upon receiving a UA or DM response to a sent DISC command, shall cancel timer T1, and enter the disconnected state.

2.4.3.3.3

If a UA or DM response is not correctly received before T1 times out, the DISC frame should be sent again and T1 restarted. If this happens N2 times, the DXE should enter the disconnected state.
2.4.3.4 Disconnected State

2.4.3.4.1

A DXE in the disconnected state shall monitor received commands and react upon the reception of a SABM as described in 2.4.3.1 above and will transmit a DM frame in response to a DISC command.

2.4.3.4.2

In the disconnected state, a DXE may initiate a link set-up as outlined in connection establishment above (2.4.3.1). It may also respond to the reception of a SABM and establish a connection, or it may ignore the SABM and send a DM instead.

2.4.3.4.3

Any DXE receiving a command frame other than a SABM or UI frame with the P bit set to one should respond with a DM frame with the F bit set to one. The offending frame should be ignored.

2.4.3.4.4

When the DXE enters the disconnected state after an error condition or if an internal error has resulted in the DXE being in the disconnected state, the DXE should indicate this by sending a DM response rather than a DISC frame and follow the link disconnection procedure outlined in 2.4.3.3.3, above. The DXE may then try to re-establish the link using the link set-up procedure outlined in 2.4.3.1, above.
2.4.3.5 Collision Recovery

2.4.3.5.1 Collisions in a Half-Duplex Environment

Collisions of frames in a half-duplex environment are taken care of by the retry nature of the T1 timer and retransmission count variable. No other special action needs to be taken.

2.4.3.5.2 Collisions of Unnumbered Commands

If sent and received SABM or DISC command frames are the same, both DXEs should send a UA response at the earliest opportunity, and both devices should enter the indicated state.

If sent and received SABM or DISC commands are different, both DXEs should enter the disconnected state and transmit a DM frame at the earliest opportunity.

2.4.3.5.3 Collision of a DM with a SABM or DISC

When an unsolicited DM response frame is sent, a collision between it and a SABM or DISC may occur. In order to prevent this DM from being misinterpreted, all unsolicited DM frames should be transmitted with the F bit set to zero. All SABM and DISC frames should be sent with the P bit set to one. This will prevent any confusion when a DM frame is received.
2.4.3.6 Connectionless Operation
In Amateur Radio, there is an additional type of operation that is not feasible using level 2 connections. This operation is the round table, where several amateurs may be engaged in one conversation. This type of operation cannot be accommodated by AX.25 link-layer connections.

The way round-table activity is implemented is technically outside the AX.25 connection, but still using the AX.25 frame structure.

AX.25 uses a special frame for this operation, called the Unnumbered Information (UI) frame. When this type of operation is used, the destination address should have a code word installed in it to prevent the users of that particular round table from seeing all frames going through the shared RF medium. An example of this is if a group of amateurs are in a round-table discussion about packet radio, they could put PACKET in the destination address, so they would receive frames only from others in the same discussion. An added advantage of the use of AX.25 in this manner is that the source of each frame is in the source address subfield, so software could be written to automatically display who is making what comments.

Since this mode is connectionless, there will be no requests for retransmissions of bad frames. Collisions will also occur, with the potential of losing the frames that collided.

2.4.4 Procedures for Information Transfer

Once a connection has been established, as outlined above, both devices are able to accept I, S, and U frames.
2.4.4.1 Sending I Frames
Whenever a DXE has an I frame to transmit, it will send the I frame with N(S) of the control field equal to its current send state variable V(S). Once the I frame is sent, the send state variable is incremented by one. If timer T1 is not running, it should be started. If timer T1 is running, it should be restarted.

The DXE should not transmit any more I frames if its send state variable equals the last received N(R) from the other side of the link plus seven. If it were to send more I frames, the flow control window would be exceed, and errors could result.

If a DXE is in a busy condition, it may still send I frames as long as the other device is not also busy.

If a DXE is in the frame-rejection mode, it will stop sending I frames.

2.4.4.2 Receiving I Frames

2.4.4.2.1

If a DXE receives a valid I frame (one with a correct FCS and whose send sequence number equals the receiver's receive state variable) and is not in the busy condition, it will accept the received I frame, increment its receive state variable, and act in one of the following manners:
  1. If it has an I frame to send, that I frame may be sent with the transmitted N(R) equal to its receive state variable V(R) (thus acknowledging the received frame). Alternately, the device may send a RR frame with N(R) equal to V(R), and then send the I frame.
  2. If there are no outstanding I frames, the receiving device will send a RR frame with N(R) equal to V(R). The receiving DXE may wait a small period of time before sending the RR frame to be sure additional I frames are not being transmitted.

2.4.4.2.2

If the DXE is in a busy condition, it may ignore any received I frames without reporting this condition other than repeating the indication of the busy condition.

If a busy condition exists, the DXE receiving the busy condition indication should poll the sender of the busy indication periodically until the busy condition disappears.

A DXE may poll the busy DXE periodically with RR or RNR frames with the P bit set to one.

The reception of I frames that contain zero-length information fields shall be reported to the next level but no information field will be transferred.

2.4.4.3 Reception of Out of Sequence Frames
When an I frame is received with a correct FCS, but its send sequence number, N(S), does not match the current receiver's receive state variable, the frame should be discarded. A REJ frame shall be sent with a receive sequence number equal to one higher (modulo 8) than the last correctly received I frame if an uncleared N(S) sequence error condition has not been previously established. The received state variable and poll bit of the discarded frame should be checked and acted upon, if necessary, before discarding the frame.
2.4.4.4 Reception of Incorrect Frames
When a DXE receives a frame with an incorrect FCS, an invalid frame, or a frame with an improper address, that frame shall be discarded.
2.4.4.5 Receiving Acknowledgement
Whenever an I or S frame is correctly received, even in a busy condition, the N(R) of the received frame should be checked to see if it includes an acknowledgement of outstanding sent I frames. The T1 timer should be cancelled if the received frame actually acknowledges previously unacknowledged frames. If the T1 timer is cancelled and there are still some frames that have been sent that are not acknowledged, T1 should be started again. If the T1 timer runs out before an acknowledgement is received, the device should proceed to the retransmission procedure in 2.4.4.9.
2.4.4.6 Receiving Reject
Upon receiving a REJ frame, the transmitting DXE will set its send state variable to the same value as the REJ frame's received sequence number in the control field. The DXE will then retransmit any I frame(s) outstanding at the next available opportunity conforming to the following:
  1. If the DXE is not transmitting at the time, and the channel is open, the device may commence to retransmit the I frame(s) immediately.
  2. If the DXE is operating on a full-duplex channel transmitting a UI or S frame when it receives a REJ frame, it may finish sending the UI or S frame and then retransmit the I frame(s).
  3. If the DXE is operating in a full-duplex channel transmitting another I frame when it receives a REJ frame, it may abort the I frame it was sending and start retransmission of the requested I frames immediately.
  4. The DXE may send just the one I frame outstanding, or it may send more than the one indicated if more I frames followed the first one not acknowledged, provided the total to be sent does not exceed the flow-control window (7 frames).

If the DXE receives a REJ frame with the poll bit set, it should respond with either a RR or RNR frame with the final bit set before retransmitting the outstanding I frame(s).

2.4.4.7 Receiving a RNR Frame
Whenever a DXE receives a RNR frame, it shall stop transmission of I frames until the busy condition has been cleared. If timer T1 runs out after the RNR was received, the waiting acknowledgement procedure listed in 2.4.4.9, below, should be performed. The poll bit may be used in conjunction with S frames to test for a change in the condition of the busied-out DXE.
2.4.4.8 Sending a Busy Indication
Whenever a DXE enters a busy condition, it will indicate this by sending a RNR response at the next opportunity. While the DXE is in the busy condition, it may receive and process S frames, and if a received S frame has the P bit set to one, the DXE should send a RNR frame with the F bit set to one at the next possible opportunity. To clear the busy condition, the DXE should send either a RR or REJ frame with the received sequence number equal to the current receive state variable, depending on whether the last received I frame was properly received or not.
2.4.4.9 Waiting Acknowledgement
If timer T1 runs out waiting the acknowledgement from the other DXE for an I frame transmitted, the DXE will restart timer T1 and transmit an appropriate supervisory command frame (RR or RNR) with the P bit set. If the DXE receives correctly a supervisory response frame with the F bit set and with an N(R) within the range from the last N(R) received to the last N(S) sent plus one, the DXE will restart timer T1 and will set its send state variable V(S) to the received N(R). It may then resume with I frame transmission or retransmission, as appropriate. If, on the other hand, the DXE receives correctly a supervisory response frame with the F bit not set, or an I frame or supervisory command frame, and with an N(R) within the range from the last N(R) received to the last N(S) sent plus one, the DXE will not restart timer T1, but will use the received N(R) as an indication of acknowledgement of transmitted I frames up to and including I frame numbered N(R)-1.

If timer T1 runs out before a supervisory response frame with the F bit set is received, the DXE will retransmit an appropriate supervisory command frame (RR or RNR) with the P bit set. After N2 attempts to get a supervisory response frame with the F bit set from the other DXE, the DXE will initiate a link resetting procedure as described in 2.4.6, below.

2.4.5 Frame Rejection Conditions

A DXE shall initiate the frame-reset procedure when a frame is received with the correct FCS and address field during the information-transfer state with one or more of the conditions in 2.3.4.3.3, above.

Under these conditions, the DXE will ask the other DXE to reset the link by transmitting a FRMR response as outlined in 2.4.6.3, below.

After sending the FRMR frame, the sending DXE will enter the frame reject condition. This condition is cleared when the DXE that sent the FRMR frame receives a SABM or DISC command, or a DM response frame. Any other command received while the DXE is in the frame reject state will cause another FRMR to be sent out with the same information field as originally sent.

In the frame rejection condition, additional I frames will not be transmitted, and received I frames and S frames will be discarded by the DXE.

The DXE that sent the FRMR frame shall start the T1 timer when the FRMR is sent. If no SABM or DISC frame is received before the timer runs out, the FRMR frame shall be retransmitted, and the T1 timer restarted as described in the waiting acknowledgement section (2.4.4.9) above. If the FRMR is sent N2 times without success, the link shall be reset.

2.4.6 Resetting Procedure

2.4.6.1
The resetting procedure is used to initialize both directions of data flow after a nonrecoverable error has occurred. This resetting procedure is used in the information- transfer state of an AX.25 link only.
2.4.6.2
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 FRMR by terminating the connection with a DISC frame.
2.4.6.3
A DXE shall reset the link by sending a SABM frame and starting timer T1. 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, V(S) and V(R), to zero and stop T1 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.

If a DM response is received, the DXE will enter the disconnected state and stop timer T1. If timer T1 runs out before a UA or DM response frame is received, the SABM will be retransmitted and timer T1 restarted. If timer T1 runs out N2 times, the DXE will enter the disconnected state, and any previously existing link conditions will be cleared.

Other commands or responses received by the DXE before completion of the reset procedure will be discarded.

2.4.6.4
One DXE may request that the other DXE reset the link by sending a DM response frame. After the DM 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 these timers is recommended.

2.4.7.1.1 Acknowledgement Timer T1

The first timer, T1, is used to make sure a DXE doesn't wait 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 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.

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.

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 k parameter is defined in 2.4.7.4, below.

2.4.7.1.3 Inactive Link Timer T3

The third timer, T3, is used whenever T1 isn't running to maintain link integrity. It is recommended that whenever there are no outstanding unacknowledged I frames or P-bit 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)
The maximum number of retries is used in conjunction with the T1 timer.
2.4.7.3 Maximum Number of Octets 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.
This document was originally downloaded from the TAPR archives and FTP site, as ax25.doc. HTML markup was done by Bill Buthod, N5RRS. Last updated: 27 Dec 1997
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