AMTOR (amateur teleprinting over radio) is a digital communications method used by radio amateurs, in which the frequency of errors is reduced by handshaking or character repetition. There are two modes in AMTOR, known as automatic repeat request (ARQ) or Mode A, and forward error correction (FEC) or Mode B.
In ARQ mode, handshaking is used between the source (transmitting station) and destination (receiving station). Characters are sent by the source in groups of three. After receiving each group, the destination sends an ACK (acknowledged) or NAK (not acknowledged) signal back to the source. An ACK signal means that the destination received all three characters according to protocol, so the source sends the next group. A NAK signal means that one or more of the characters did not conform to protocol at the destination, so the source sends the same group again.
In FEC mode, every character is sent by the source twice. The destination tests each pair of characters to see if they are in the proper format. If one or both of the characters are in the right format, then the character is displayed. If neither character has the correct format, the destination displays or prints a blank space or an underscore.
AMTOR was adapted in the 1980s by Peter Martinez, an English amateur radio operator, from SITOR (simplex telex over radio), an error-reducing digital communications method used in maritime communications.