Source : Free On-Line Dictionary of Computing

YMODEM
     
        A file transfer {protocol} used between {modem}s.  YMODEM was
        developed by Chuck Forsberg as the successor to {XMODEM} and
        was itself succeeded by {ZMODEM}.  XMODEM used 128-byte
        {packet}s, YMODEM can also use 1 kilobyte packets.  Whereas
        YMODEM is a batch protocol, {YMODEM-G} is a non-stop version.
     
        File sizes are included in the YMODEM header when sending both
        binary and text files.  Thus files transferred via YMODEM
        should preserve their exact length.  File modification times
        may also be present in the YMODEM header.
     
        YModem can {fall back} to smaller packets when necessary but
        there is no backward compatibility with XModem's error
        detection.
     
        [Chuck Forsberg, "XMODEM/YMODEM Protocol Reference"].
     
        (1995-02-02)