Fax Operation
V.3400
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, detec-
tion 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 later in this section.
Answer Command A
The A command instructs the modem to answer the call. For fax opera-
tion, 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), 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 later in this section. In accordance with T.30, to allow proper
interaction with a manually originated fax call, 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 transmis-
sion and terminate the call by going on hook.
