JMODEM is a file transfer protocol developed by Richard Johnson in 1988.
It is similar to the seminal XMODEM in most ways, but uses a variable-size packet in order to make better use of the available bandwidth on high-speed modems.
The block length will increase in 512-byte increments as long as there are no errors requiring retransmission.
JMODEM also included a basic RLE data compression system, which replaces strings of repeated characters with a counter.
JMODEM applied RLE on a block-by-block basis, as opposed to the file as a whole.