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Fax Operation
Modem 3600 9-5
After dialing the specified number in fax mode, the modem generates
1100 Hz CNG tones in accordance with T.30 while trying to detect
either CED (answer back tone) or the HDLC preamble of the first T.30
frame. The CNG tones cease after detecting either of these two signals.
To allow proper interaction with manually answered fax stations,
detection of CED is not required. After detecting 1650 Hz (i.e., the
HDLC preamble of the first frame), the modem enters V.21 Channel 2
receive state with HDLC framing and returns a CONNECT result code.
This is because dialing in fax mode (+FCLASS=1) implies an initial
+FRH=3 command described in the “Fax Transmit and Receive Modes”
section on pa ge9-7.
Answer Command A
The A command instructs the modem to answer the call. For fax
operation, set the modem for Service Class 1 operation before the A
command is issued; use the +FCLASS=1 command.
If the modem is in the online command state when an answer command
is issued (e.g., a connection is already established), the modem returns
an ERROR result code. Otherwise, it answers the call and generates a
2100 Hz CED (answer back) tone in accordance with T.30. The modem
then enters V.21 Channel 2 transmit state with HDLC framing and
returns a CONNECT result code. This is because answering a call in
fax mode (+FCLASS=1) implies an initial +FTH=3 command
described in the “Fax Transmit and Receive Modes” section on
page 9-7. To allow proper interaction with a manually originated fax
call (in accordance with T.30), the modem does not require detection of
CNG before generating CED or entering HDLC transmit mode.
If the DTE sends a character while executing the answer command and
before CED is completed, the call is released and the modem will send
the NO CARRIER result code.
On Hook H
In fax mode, the H command instructs the modem to stop any
transmission and terminate the call by going on hook.