AX.25

AX.25 is a packet protocol developed by amateur radio operators and commonly used for many packet-based systems. It is used as a transport layer for several applications including APRS, Winlink, BBSs, and more.

Packet Format

AX.25 is derived from HDLC frames and consists of an address field, a control field, a PID field (information frames only), and a payload field.

• The address field consists of 7-byte addresses. The first 6 bytes are 7-bit ASCII, bit shifted one bit to the left such that the least significant bit is always zero. The seventh byte contains, from most to least significant bit: the command-response bit, two optional bits (usually set to 11), a 4-bit SSID (interpreted as a number from 0 to 15, used to distinguish multiple stations with the same callsign), and a final bit to indicate whether there is another address (0) or not (1). The first address is the destination, followed by the source, followed by optional digipeaters. Usually a maximum of two digipeaters are used, although occasionally packets will have more than two digipeaters specified.
• The control field consists of 1 or 2 bytes and indicates the type of frame being transmitted by the packet. These are generally categorized into information frames (I frames, start with 0), supervisory frames (S frames, start with 10), and unnumbered frames (U frames, start with 11). The remaining bits are used to further specify the frame type.
• The PID field consists of 1 byte which indicates the layer 3 protocol for information (I) and unnumbered information (UI) frames.
• The payload consists of a variable number of bytes and is determined by the application the protocol is being used for.