Origination of fax calls is made using the ATD command. Upon completion of the dial
function, a calling tone at 1100 Hz is transmitted, with a cadence of 0.5 seconds on and 3
seconds off. The modem automatically enters mode +FRH=3 and sends the CONNECT
message to the DTE when FSK flags are detected from the remote.
4.1.5 Fax Answering
Answering of fax calls is identical to answering of data calls with the exception that the
modem enters the fax handshaking mode instead of the data handshaking mode after going
off-hook. If +FAE=0, the modem, after sending answer tone, automatically enters fax mode
(+FTH=3), sends the CONNECT message to the DTE, and starts sending FSK flags. If
+FAE=1, the modem determines whether the caller is a data modem or fax modem and sends
the DATA or FAX result code, respectively, to the DTE.
4.1.6 Fax Control Transmission
Fax control transmission is initiated by the +FTH=<mod> command. After this command is
issued, the modem generates the CONNECT message and transmits carrier in the modulation
scheme specified by the parameter n. The modem then transmits HDLC flags for a minimum
of 1 second. The modem continues to transmit the HDLC flags until it receives a character
from the DTE.
When characters are received from the DTE, the modem adds start and end flags, performs
zero-bit insertion, generates FCS, and deletes <DLE><chr> character pairs before
transmitting the characters to the remote fax machine. Each <DLE><DLE> sequence is
transmitted as a single <DLE>. <DLE><ETX> is considered as the end of frame marker and
is not transmitted. All data received from the DTE after <DLE><ETX> is ignored by the
modem until the modem generates either the CONNECT, OK, or ERROR result code.
If no more data is in the transmit buffer and the final bit was a 1 (bit 4 of the second byte
received from the DTE), the modem generates the OK result code and returns to the command
mode. If the final bit was a 0, the modem generates the CONNECT message and waits for
further data from the DTE while transmitting HDLC flags. If no more data is received before
5 seconds elapse, the modem drops carrier, goes on-hook, and generates the ERROR result code.
4.1.7 Fax Control Reception
Fax control reception is initiated using the AT+FRH=<mod> command. After this command
is issued, the modem looks for carrier in the modulation scheme specified by the parameter n.
If no carrier is detected before the period of time specified by register S7 expires, the modem
generates the NO CARRIER message and returns to command mode. If a carrier is detected
that is not the one specified by the parameter n, the modem generates the +FCERROR
message and returns to the command mode. If the specified carrier is detected, the modem
generates the CONNECT message and enters the HDLC receive mode.
In HDLC receive mode, the modem receives HDLC frames from the remote fax machine,
strips the flags, performs zero-bit deletion, performs error checking, and handles
<DLE><chr> character pairs before passing the data to the DTE. The modem prefixes each
<DLE> character with another <DLE> character before sending it to the DTE. After the last
byte in the frame, the modem sends <DLE><ETX> to the DTE marking the end of the frame.
The modem then generates either the OK message if no errors were detected or the ERROR
message if errors were detected (FCS is incorrect), and returns to command mode.
While in command mode, the modem continues to receive data in the selected modulation
scheme, and sends the data after sending the CONNECT message to the DTE when the DTE
reissues the +FRH command with the same parameter. If the DTE issues the +FRH command
with a different parameter, the modem clears all buffers and proceeds as described previously.
If carrier is lost while in command mode and the DTE reissues the +FRH command with the
same parameter, and there is no data in the buffer, the modem sends the ERROR result code
