Apple Express Fax/Modem - Manual (Page 25)

Determining the speed
Initially, your modem tries to connect at 14400 bps, its fastest speed. If the
remote modem can’t connect at 14400 bps, your modem falls back to the next
highest speed. Fallback continues until the two modems agree on their
capabilities.
Error control
Error control is used by modems to check data for errors. Your modem and
the remote modem must agree on an error control method. Your modem will
try to use a method called V.42 and MNP class 2–4. If the remote modem
doesn’t support this type of error control, both fall back until an agreement is
reached.
Data compression
Data compression increases the speed of your modem by eliminating gaps
and redundancies from the data. For instance, if you connect at 14400 bps
with data compression enabled, the speed can increase up to 4 times the
non-compression speed.
Your modem uses a type of data compression called V.42 bis or MNP class 5. If
the remote modem doesn’t support these types of data compression, it will
not use any.
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