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AT Command Reference Manual
1048 7-3
7.2.2 Call Establishment - Answer
Directed Answer (Answer as a specific modem type)
If the DTE wants to be only one kind of answerer (i.e., voice/audio, fax, or data), it can configure the modem to answer
exclusively in the chosen mode.
Voice/Audio
The modem is configured to answer in Voice/Audio Mode only and assumes the caller will cooperate. After going off-hook,
the voice VCON is issued, no answer tone is generated, and the modem is immediately placed in Online Voice Command
Mode. The DTE typically responds by sending a greeting message of some type, and DTMF tone recognition/reporting can
be enabled. Eventually, an incoming voice message can be recorded by the host. (Unpredictable results occur if the caller is
not prepared for a voice call.)
Fax
The modem is configured to answer in Fax Class 1 or Fax Class 2 Mode only, and it assumes the caller is going to
cooperate. This configuration has the effect of disabling Voice/Audio Mode, fFCLASS to either 1 or 2, and forcing
both +FAA and +FAE to 0.
Data
The modem is configured to answer in Data Mode only and assumes the caller is going to cooperate. This configuration has
the effect of disabling Voice/Audio Mode, fFCLASS = 0, and forcing both +FAA and +FAE to 0.
Adaptive Answer (Answer with Voice/Data/Fax Discrimination)
In normal operation, it is desirable for a modem supporting fax and voice to provide the ability to discriminate between the
two when answering unsolicited or unattended calls. (It is most often the case that a fax is received or a voice message
recorded when nobody is present.)
Data/Fax Discrimination
If the DTE wishes to allow for a data or fax call, the +FCLASS and +FAA or +FAE commands can be configured for adaptive
answer between data and Class 1 or Class 2 fax.
Voice/Fax Discrimination
This is the most important discrimination capability needed from the user's standpoint. The modem must be configured for
Voice/Audio (#CLS=8), causing the modem to enter Online Voice Command Mode immediately upon going off-hook. In
Voice/Audio Mode, the DTE automatically receives indications of DTMF tones and Calling Tones. The DTE can now switch
to Voice Transmit Mode in order to play a greeting message, perhaps one which instructs the caller how to enter specific
DTMF sequences to switch modes. The DTE can then react to the response, or the lack thereof, to such a message. The
modem supports switching to Class 1 or Class 2 answer mode by virtue of the #CLS=1 or #CLS=2 command, and if such a
switch is made and fails, the modem reports the failure but does not hang up, allowing the DTE further experimentation time.
If the user wishes to switch to Class 1 or Class 2, but also wants the DTE to indeed hang up the line if the fax fails, the
+FCLASS command should be used instead of the #CLS command. The only difference between these commands is that
iFCLASS cancels the modem's memory of voice, whereas #CLS causes the modem to remain off-hook, even if a
fax or data handshake fails, until it receives an H command.
Voice/Data/Fax Discrimination
The DTE can try data modem operation after an answer by changing the #CLS setting to 0. A data handshake attempt can
be added based upon DTMF responses or lack thereof.