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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".
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