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CHAPTER 5: Command Mode
codings except under an unusual line condition called impulse noise.
AT#T0 turns Trellis coding off and AT#T1 turns Trellis coding on
(factory default).
Fallback Modes When On-Line #F
If line conditions deteriorate, the Series II Modem automatically drops
its transmission speed (“fallback”). The #F command controls the
different ways the Modem falls back. During operation, if the error rate
becomes too great, the modem performs a retrain. If after the retrain,
the error rate is still too high for 33600 bps operation, the modem
initiates a retrain at 4800 bps. If after the first retrain the modem
returns on line at 33600 bps, the modem then starts a counter and a
timer. If three retrains occur within a two minute period, the modem
falls back to 4800 bps.
Entering AT#F0 (or AT#F) allows no fallback when on-line. AT#F1
allows the Modem to fallback (based on the error rate or if three
retrains have occurred within a two minute period) from 33.6K to 31.2K
to 28.8K to 26.4K to 24K to 21.6K to 19.2K to 16.8K to 14.4K to 12K to
9.6K to 4.8K bps. The AT#F2 command enables incremental fall back
from 33.6K to 4.8K, but also enables incremental fall forward (from
4.8K to 33.6K) if the phone line improves. AT#F2 is the factory default
setting.
Enable/Disable Lowspeed Fast Connect $FC (336+ Only)
With the Series II Modem 336+ (MD1640A), entering
AT$FC0&W0<CR> sets up the data pump so that it will not transmit a 5
second 2100 Hz signal. Entering AT$FC0&W0<CR> allows the modem
to transmit a 5 second 2100 Hz signal and is the factory default.
Cleardown at Disconnect &CD
The &CD command is used for control of cleardown at disconnect in
V.32, V.32bis, V.34 and V.34 enhanced modes.
A cleardown usually adds 1-2 seconds to the time it takes for the modem
to go on-hook after a disconnect. A cleardown makes a disconnect by
the remote modem easier to detect and a subsequent re-connection
more likely. Enter AT&CD0&W0<CR> to execute a cleardown at
disconnect (default); and enter AT&CD1&W0<CR> to disable the
cleardown function.