Source : Free On-Line Dictionary of Computing

XMODEM
     
        The "Christensen" file transfer {protocol}, probably the most
        widely available protocol used for file transfer over {serial
        line}s (e.g. between {modem}s).  XMODEM uses 128-byte {packet}s
        with error detection, allowing the receiver to request
        retransmission of a corrupted packet.  XModem is fairly slow
        but reliable.
     
        Several variations have been proposed with increasing packet
        sizes (e.g. {XMODEM-1K}) and different error detection ({CRC}
        instead of {checksum}) to take advantage of faster {modem}s.
        Sending and receiving programs can negotiate to establish the
        best protocol they both support.
     
        Standard XMODEM specifies a one-second timeout during the
        reception of characters in the data block portion of a packet.
     
        Chuck Forsberg improved upon XMODEM by developing {YMODEM} and
        {ZMODEM}.
     
        [Chuck Forsberg, "XMODEM/YMODEM Protocol Reference"].
     
        (1995-02-02)