Hardware compression built into modems 'buffers' the data in compressed 'windows' while it is being sent/received over the connection and then decompresses before sending the data over COM (serial) port. The data is only in a compressed state 'modem to modem', not the complete connection. This extra overhead and buffering in the modem itself and uncompressed data being sent over the COM (serial) port increases latency.
compress - To make data take up less space. Archiving programs
do this, which means that files will take less time to transfer
with modems. Many modems now have the ability to automatically
compress the information they send and receive. [See also ar-
chive, data compression].
compression ratio - The ratio of the original size of data that
is sent to the compressed size. For example, a 3:1 compression
ratio means that the original data takes up 3 times the amount of
space as the compressed data, and a modem would transfer the data
3 times more quickly than if it was not compressed.
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error - When there is line noise and one or more characters are
changed. This is especially noticeable when downloading or
uploading a program. In this case the error must be detected,
and the data must be re-sent (or else the file will be
destroyed). [line noise].
error control - The ability of a modem to notice errors in trans-
mission, and have any incorrect data re-sent.
error correction - (Error control is a more correct term,
since the modem does not correct incorrect data, it
just has it sent again).
error free - When referring to data transmission, error free
refers to communications equipment in which data is transmitted
perfectly. This is actually an impossible situation, but it is
possible to have data that is very, very close to error-free.