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.
