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Advanced Modem Concepts
Changing Modem Data Rates
and Modulations
Your modem is capable of communicating with other modems at
several different data rates, from 300 bits per second (bps) up to
28,800 bps. When a connection with another modem is first
being established, there are certain negotiations that take place
between the two modems. This handshaking process
determines the rate at which the two modems will communicate,
and the modulation (protocol or rules) that will govern their
conversation.
When two modems are first connecting, the %B command
controls the maximum data rate and modulation that may be
used. Two S-Registers that also affect the data rate and
modulation are S76 (Autorate control) and S78 (Automode
control).
The %B command sets the modulation and maximum data rate.
The modem can use lower rates if Automoding is turned on.
Automoding (S78) allows the modem to automatically detect
the remote modem's modulation and then connect using the
same modulation.
Automoding is only used while a connection is being set up.
Once a connection is made, the modulation does not change.
The data rate, however, can change during the connection if
Autorating is turned on.
Autorating (S76) allows the modem to adjust its data rate
based on changes it detects in line signal quality (during
connection establishment and when the modem is online).
E
Issue 2
May 1995
E-1
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