Xmodem

The first widely used file transfer protocol for personal computers, developed by Ward Christensen for CP/M machines. Xmodem programs supported the earlier checksum method and the subsequent CRC method of error detection. Xmodem transmits 128-byte blocks. Xmodem-1K improves speed with 1KB blocks. Xmodem-1K-G transmits without acknowledgment for error-free channels or when modems are self correcting, but transmission is cancelled upon any error.

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(communications)

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"].