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A file transfer protocol used between modems. YMODEM was developed by Chuck Forsberg as the successor to XMODEM and was itself succeeded by ZMODEM. XMODEM used 128-byte packets, YMODEM can also use 1 kilobyte packets. Whereas YMODEM is a batch protocol, YMODEM-G is a non-stop version.

File sizes are included in the YMODEM header when sending both binary and text files. Thus files transferred via YMODEM should preserve their exact length. File modification times may also be present in the YMODEM header.

YModem can fall back to smaller packets when necessary but there is no backward compatibility with XModem's error detection.

[Chuck Forsberg, "XMODEM/YMODEM Protocol Reference"].
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A file transfer protocol that adds batch file processing to Xmodem. Multiple files can be sent at the same time. It is faster than Xmodem and sends the file name before sending the data. Ymodem-G transmits without acknowledgment when error-free channels or modems with built-in error correction are used, but transmission is cancelled upon any error.
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