From The Free On-line Dictionary of Computing: (http://www.foldoc.org/, Editor Denis Howe)
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.