compression

compression: Enables or disables data compression performed by the modem, also called hardware compression. Compression reduces the amount of time required to transfer data.

You may want to disable data compression if your data communication program is configured to compress data (resulting in software compression) or if you transfer a lot of compressed files (with extensions like ZIP, ARC, SIT, LHA, or SEA). Compression types supported by some modems actually slows transfer of data that has already been compressed.