2 AX.25 Link-Layer Protocol Specification

2.1 Scope and Field of Operation

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

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

2.2.2 Address Field

The encoding of the address field is described in 2.2.13.

2.2.3 Control Field

2.2.4 PID Field

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

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

2.2.6 Bit Stuffing

2.2.7 Frame-Check Sequence

2.2.8 Order of Bit Transmission

2.2.9 Invalid Frames

2.2.10 Frame Abort

2.2.11 Interframe Time Fill

2.2.12 Link Channel States

2.2.13 Address-Field Encoding

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

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.

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

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

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.

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.

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 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

2.3.2 Control-Field Formats and State Variables

2.3.2.1 Control-Field Formats

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.

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

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)

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

2.3.4 Control Field Coding for Commands and Responses

2.3.4.1 Information Command Frame Control 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.

2.3.4.2 Supervisory Frame Control Field