The Free On-line Dictionary of Computing (30 December 2018):

XMODEM

    Ward Christensen's 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.

   John Mahr wrote the original XMODEM CRC error correction code.
   This implementation was backward compatible with Christensen's
   original checksum code.  It improved the error detection from
   98% to 99.97% and improved the reliability of transmitting
   binary files.

   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"].

   (2005-09-16)