AX.25

AX.25 is a data link layer protocol 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 not uncommon 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 possible to build AX.25 switches in a manner not unlike how Ethernet switches work, this has not yet been accomplished.