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

      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
     XMODEM-1K)+and+different+error+detection+({CRC">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
     [Chuck Forsberg, "XMODEM/YMODEM Protocol Reference"].

Contact=webmaster@dict.org Specification=RFC 2229