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Chapter 3 - AT Commands
&CDn Cleardown at Disconnect
n = 0 or 1
Default: &CD0
In the V.32, V.32bis, and V.34 protocols, a cleardown is sent by one of the modems to
signal the other modem that it is going to hang up. A cleardown is rate renegotiation in
which the modem that is hanging up tells the other modem that it has no speeds that it can
connect at. It is designed to help the remote modem detect a hangup. However, it does take
a second or two to send the cleardown. If you find this delay unacceptable, you can disable
cleardown by using the &CD command.
&CD0 enables cleardown at disconnect.
&CD1 disables cleardown at disconnect.
>DTn DTMF Detection
n = 0 or 1
Default: >DT0
Normally, when it goes off-hook, a fax modem ignores tones from the other end of the
connection unless they are fax or modem handshake tones, in which case it responds in
kind. The >DT1 command causes the modem to report DTMF (touch-tone) tones and fax
calling tones to software for further processing. A typical use would be for a program that
routes calls to specific communications applications depending on the type of call or upon a
DTMF tone added to a dialing string.
>DT0 disables DTMF detection.
>DT1 enables DTMF detection when the modem goes off-hook.
#Fn Fallback Modes
n = 0, 1, or 2
Default: #F2
The #F command controls whether the modem will fall back to a lower speed because of an
unacceptable error rate. If line conditions deteriorate, the modem has the ability to fall back
from its original speed to the next lower speed when operating in V.34, V.32terbo, V.32bis,
or V.32 mode. If the error rate becomes too great, the modem performs a retrain at the next
lower speed. If, after the retrain, the error rate is still too high, the modem performs a retrain
at the next lower speed, down to 4800 bps.
If the modem returns online at the original speed after the first retrain, the modem starts a
counter and a timer. If three retrains occur within a two-minute period, the modem falls back
to 4800 bps.
The fallback and fall-forward speeds for the modem are 33.6K, 31.2K, 28.8K, 26.4K, 24.0K,
21.6K, 19.2K, 16.8K, 14.4K, 12K, 9.6K, and 4.8K bps.
#F0 (or #F) disables fallback when online.
#F1 lets the modem fall back from the starting speed to a low speed of 4800 bps in 2400
bps decrements.
#F2 lets the modem fall back when line conditions deteriorate, but also lets it fall forward in
2400 bps increments if line conditions improve.