Advanced Modem Concepts
E-2
Issue 2 May 1995
The %B command, S-Register S76 (Autorate), and S78
(Automode) work together:
G
If Automoding and Autorating are both off, a connection
can only be made if the remote modem agrees to use the
data rate and modulation set by %B.
G
If Automoding is on, modulations and data rates lower
than the maximums set by %B can be used (even if
Autorating is off).
G
If Automoding is off, but Autorating is on, then the
connection must use the modulation set by %B, but a
lower data rate can be used.
While some settings for %B allow only one specific modulation
to be used, other settings allow the modem to select between
two modulations. For example, %B28800 sets the modulation to
V.34, while %B14400 allows the modulation to be either V.34 or
V.32bis. When you set %B to a setting that allows a choice of
modulations, the following rules controls which modulation is
selected:
G
If Automoding is on, the modem may select either
modulation (depending on negotiations with the remote
modem).
G
If Automoding is off, only the higher of the two
modulations (V.34) can be used.
G
If you want the modem to only connect at the
lower
modulation, then %B should be set to the setting that
includes the ‘L’ before the desired maximum data rate.
(The ‘L’ means to use the ‘Lower’ modulation.)
Consider an example in which you want a maximum data rate of
19,200 bps. If you set %B19200 and Automoding is on, either
V.34 or V.32terbo can be selected. However, if Automoding is
off, V.34 must be used. If you require V.32terbo, you would need
to set %BL19200.
When operating at lower speeds (300 and 1200 bps), the
B command selects the modem’s modulation (BELL or CCITT).
The B command also controls the data rate when using V.23,
allowing you to choose between sending at 75 bps and receiving
at 1200 bps, or vice versa.
For more information on the B and %B commands and
S-Registers S76 and S78, refer to Chapter 4.
