AX.25

AX.25 (Amateur X.25) is a data link layer derived from the X.25 protocol suite and designed for use by amateur radio operators. It is used extensively on amateur packet radio networks.

AX.25 v2.0 and later occupies the data link layer, the second layer of the OSI model. It is mainly responsible for establishing connections and transferring data encapsulated in frames between nodes and detecting errors introduced by the communications channel. As AX.25 is a pre-OSI-model protocol, the original specification was not written to cleanly separate into OSI layers. This was rectified with version 2.0 (1984), which assumes compliance with OSI level 2.

In practice, it is common to find an AX.25 data link layer as the transport for some other network layer, such as IPv4, with TCP used on top of that. Note that, like Ethernet, AX.25 frames are not engineered to support switching. For this reason, AX.25 supports a somewhat limited form of source routing. Although it is possible to build AX.25 switches similar to the way Ethernet switches work, this has not yet been accomplished.

AX.25 does not define a physical layer implementation. In practice 1200 baud Bell 202 tones and 9600 baud G3RUH DFSK are almost exclusively used on VHF and UHF. On HF the standard transmission mode is 300 baud Bell 103 tones, although very little use of AX.25 on HF exists today. At the physical layer, AX.25 defines only a "physical layer state machine" and some timers related to transmitter and receiver switching delays.

At the datalink level, AX.25 specifies HDLC (ISO 3309) frames transmitted with NRZI encoding. Media access control follows the Carrier sense multiple access approach with collision recovery (CSMA/CR).

...
Wikipedia