From Just Solve the File Format Problem
modem connection to a BBS.YMODEM is a file transfer protocol. It belongs to a category of protocols that work on top of an already-established connection, typically a
As far as we can tell, YMODEM was not particularly popular. This may have been because there were several incompatible variants of it, and/or because of the small window of time between it and the introduction of ZMODEM.