Apple GeoPort Telecom Adapter Kit - Manual (Page 39)

Determining the speed
Initially, your modem tries to connect at its fastest speed,
which is either 28800 or 14400 bits per second (bps). If the
remote modem cannot connect at this speed, your modem
falls back to the next highest speed. Fallback continues until
the two modems agree on the highest speed supported by
both.
Error control
Error control is used by modems to ensure data accuracy.
Your modem and the remote modem must agree on an
error control method. Your modem will try to use one of
two types of error control, V.42 or MNP class 2–4. If the
remote modem does not support either of these types of
error control, no error control is used.
Data compression
Data compression increases the speed of your modem by
eliminating gaps and redundancies in the data. With data
compression enabled, the speed can increase up to 4 times
the non-compression speed.
Your modem can use two different types of data
compression, V.42 bis or MNP class 5. If the remote modem
does not support either of these types of data compression,
no data compression is used.
39 Using Your Modem