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XMODEM is the "Christensen" (developed by Ward 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, XMODEM-1K for example has uses 1 kilobyte packets, 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.

Reference: Chuck Forsberg, "XMODEM/YMODEM Protocol Reference".

This article was originally based on material from the Free On-line Dictionary of Computing, which is used under the GFDL.

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XMODEMXMODEM is the "Christensen" (developed by Ward Christensen) file transfer protocol, probably the most widely available protocol used for file transfer over serial lines (e. between modems). XMODEM uses 128-byte packets with error detection, allowing the r  
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