XMODEM
<communications> 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)
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