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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"].
This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)


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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References in periodicals archive ?
Protocols Supported: Zmodem, XModem, Telnet, FTP Push, Asynchronous ASCII and CBB.
All popular file transfer protocols including ASCII, Xmodem, Ymodem, Zmodem, CompuServe-B and Kermit are supported as well.
Some application programs use wait-for-acknowledge protocols, such as the XMODEM file transfer protocol.
Delrina WinComm PRO is a Windows application that supports all the popular file transfer protocols such as Zmodem, Xmodem, Ymodem, Compuserve B+ and Kermit, plus Hilgraeve's ultra-fast HyperProtocol.
If asked which modem format you use, Xmodem usually works on most systems.
Data transfer uses xmodem and a variant called Telink, 128-byte block ACK/NAK protocols, neither of which is streaming, bidirectional, or windowing, and which discriminate between email and file transfer at the session and data transfer level.
Having gained access to the system, users can then transfer files back to their local host, where the files can then be downloaded to a Mac or PC with Kermit, XMODEM, ZMODEM, or other transfer protocol.
File transfer protocols--for example, XMODEM, YMODEM, ZMODEM, Kermit (from Columbia University, popular with users of mainframe computers, especially in academic settings), and SEAlink (System Enhancement Associates, Wayne, N.J.)--differ in packet size, frequency of handshaking exchanges required, adaptability to telephone line conditions, and error detection/correction techniques.