xmodem
     
        the "christensen" file transfer protocol, probably the most
        widely available protocol used for file transfer over serial
        lines e.g. between modems.  xmodem uses 128-byte packets
        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 modems.
        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
     
     
see also:
protocol serial line modem packet xmodem-1k crc 
checksum ymodem zmodem