Apple Macintosh PowerBook Internal Modem/Ethernet - Manual (Page 5)

Determining the speed
A modem communicates with the computer to which it is connected (by
means of a cable), and with another modem (by means of a telephone line).
The computer-to-modem communication speed is generally faster than the
modem-to-modem speed, often considerably faster. The “advertised” modem
speed, such as 33,600 bits per second (bps), is the maximum speed for
modem-to-modem connections.
Initially, your modem tries to connect to another modem at its fastest speed,
which is 33,600 bps. If the remote modem cannot connect at this speed or if
the telephone line does not provide a completely clear connection, your
modem falls back to the next highest speed. Fallback continues until the two
modems agree on the highest speed supported by both using the particular
telephone line connection.
IMPORTANT The actual connection speed of your modem may vary, depending
on the specific telephone line in use, the quality of the connection, and other
factors that affect communications.
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 normal 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.
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