The Free On-line Dictionary of Computing (30 December 2018):

YMODEM

   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"].

   (1995-02-02)