Xmodem


XMODEM

(communications)Ward Christensen's file transferprotocol, probably the most widely available protocol usedfor 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 corruptedpacket. XModem is fairly slow but reliable.

Several variations have been proposed with increasing packetsizes (e.g. XMODEM-1K) and different error detection (CRCinstead of checksum) to take advantage of faster modems.Sending and receiving programs can negotiate to establish thebest protocol they both support.

John Mahr wrote the original XMODEM CRC error correction code.This implementation was backward compatible with Christensen'soriginal checksum code. It improved the error detection from98% to 99.97% and improved the reliability of transmittingbinary files.

Standard XMODEM specifies a one-second timeout during thereception of characters in the data block portion of a packet.

Chuck Forsberg improved upon XMODEM by developing YMODEM andZMODEM.

[Chuck Forsberg, "XMODEM/YMODEM Protocol Reference"].

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.