Voice Call Establishment - Answer
Directed Answer (Answer as a specific modem type)
If the DTE wants to be only one kind of answerer (voice, FAX, or data),
it can configure the modem to answer exclusively in the chosen mode.
Voice
The modem is configured to answer in Voice 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.
FAX
The modem is configured to answer in Class 1 or Class 2 Fax Mode only,
and it assumes the caller is going to cooperate. This configuration has
the effect of disabling Voice Mode, forcing +FCLASS 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 Mode, forcing +FCLASS = 0, and forcing both +FAA and +FAE to 0.
Adaptive Answer (Answer with Voice/FAX/Data 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 (#CLS=8), causing
the modem to enter Online Voice Command Mode immediately upon going off-hook.
In Voice 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 a Class 1 or Class 2 answer mode by virtue of the #CLS=1 or
2 commands, 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 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 issuing +FCLASS 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/FAX/Data 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.
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