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4.3 Automatic Fallback
The modem incorporates three
automatic fallback features that can
be controlled by the user:
• Operating-Mode Fallback—
Provides compatibility with
modes that use any of several
different operating modes.
• Two options for automatic data
rate adjustment: Rate
Renegotiation Procedure
(RRP) and Speed Fallback—
These features allow the
modem to automatically select
the highest possible data rate,
depending on the signal
quality.
4.3.1 O
PERATING
This section applies to dial-line
operation only.
When the modem originates or
answers a call, it determines from
the "handshake" routine with the
other modem the most efficient
operating mode that is supported
by the other modem and
automatically "falls back" to that
mode. The modem supports two
alternatives for operating-mode
fallback: V.22 bis/Bell 212A/Bell
103 fallback operation (default
setting) and V.22
bis/V.22/V.21/V.23 fallback
operation. Operating-mode
fallback functions when the
modem is in either manual or
automatic originate mode or
manual or automatic answer mode.
CHAPTER 4: General Operation and Special Features
-M
F
ODE
ALLBACK
For communication within the
United States, use the default
setting for Operating-Mode
Fallback—V.22bis/Bell 212A/Bell
103.
V.22 bis/V.22/V.21/V.23 Fallback
Operation
To configure the modem for V.22
bis/V.22/V.21/V.23 fallback
operation, refer to Appendix B,
Fig. B-7. Change the Operating
Mode Fallback parameter shown in
the diagram from 0 (the factory
setting) to 1. (This is the only way
the Operating Mode Fallback
setting can be changed.)
Changing the Operating Mode
Fallback parameter to 1 has the
following effects:
• The modem will be configured
for V.22 bis 2400-bps/V.22
1200- and 600-bps/V.21 300-
bps/V.23 1200/75 bps fallback
operation. As a result, 600 and
1200/75 bps will appear as
additional selections on the
SPEED LIMIT LCD screens.
• Fallback to V.22 bis Bell 212A
and Bell 103 compatibility is
disabled.
• An AT command is available (in
Hayes mode only) that allows
control over operating mode as
follows: F0—Multi-mode
operation (default); F1—V.21,
300 bps; F2—V.22, 600 bps;
F3—V.23, 1200/75 bps; F4—
77
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