zmodem

Zmodem (made by Omen Technology Inc) was one of the best ways to transfer files back in the days of the BBS. It included some compression, error detection, error correction, crash recovery, batch mode, and an awesome UI (if you were using the gz).

The ZMODEM protocol is a full-duplex protocol that supports both 128 and 1024 data block transfers. ZMODEM selects the optimal block size based on the quality of the line determined by the number of pre-transmission requests received. ZMODEM also enables an interrupted file transfer to be restarted from the point of failure, support for multiple file transfers and extended CRC error checking.

There was also an enhancement to Z-Modem called ZedZap which provided both larger block sizes (better for reliable links, especially those with hardware error correction) and smaller block sizes (better for less reliable links!), automatic sliding windows, data streaming and various other bits and pieces.

Was available in the Frontdoor Terminal program, which also used IEMSI to handle automatic triggering of downloads.