5-2
5.1.7. Fax Control Reception
Fax control reception is initiated using the AT+FRH=n 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 V.21 carrier is detected when
a high speed carrier is expected, 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 to the DTE and returns to the command mode. If there is data in the buffer,
the modem sends the next frame of buffered data to the DTE, followed by <DLE><ETX>, and either the ERROR result code if
errors were detected or the OK result code if no errors were detected. The modem then returns to command mode.The
modem concludes an error is detected if carrier is lost for any period of time during or before the reception of a frame. If carrier
is lost for a time period longer than the time specified by the register S10, the modem finishes delivering the data in the
receive buffer (if any) to the DTE, sends <DLE><ETX>, generates the ERROR message, and returns to command mode. All
subsequent data received from the remote is discarded.If the modem detects a frame abort sequence (seven consecutive
ones with no zero insertion) while it is waiting for a frame beginning flag (it was receiving HDLC flags), it will wait for the HDLC
flags again until either carrier is lost, or the DTE aborts the process by sending an abort character or by dropping DTR with
&D2 in effect. If the frame abort sequence is detected while the modem is receiving a frame it finishes delivering the data in
the receive buffer (if any) to the DTE, sends <DLE><ETX>, generates the ERROR message, and returns to command mode.
The modem keeps looking for HDLC flags followed by more data from the remote, with the selected modulation scheme.If the
modem detects a receive buffer overflow condition, it concludes that there was an FCS error in that frame. The modem will
receive more frames only if a starting flag is detected and there is room in the receive buffer. If a starting flag is detected and
there is no room in the buffer, the modem discards all data in that frame.If the modem receives any character from the DTE
after the +FRH command (except flow control characters if software flow control is in effect), or if the modem detects a high-
to-low transition of the DTR signal while &D1 is in effect, it sends <DLE><ETX> to the DTE, generates OK result code, and
returns to command mode. The receive buffer is cleared and all data received from the remote is discarded. If the modem
detects a DTR drop while &D2 is in effect, it goes on-hook, sends <DLE><ETX> to the DTE, generates OK result code, and
returns to command mode. If the modem detects a DTR drop while &D3 is in effect, the modem performs a warm reset.
5.1.8. Fax Data Transmission
Fax data transmission is initiated by the AT+FTM=n 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
constant 1 bits for a minimum of one second and continues to transmit constant 1 bits until it receives a character from the
DTE.When data is received from the DTE, the modem deletes start and stop bits and deletes all <DLE><chr> character pairs
before transmitting the data to the remote. Each <DLE><DLE> sequence is transmitted as a single <DLE>. <DLE><ETX> is
considered as the end of stream marker, and is not transmitted. All data received from the DTE after the <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 last transmitted character was not an ASCII NULL, the modem generates the OK result code and
returns to the command mode. If the last character transmitted was an ASCII NULL, the modem generates the CONNECT
message to the DTE and waits for further data from the DTE while transmitting NULLs to the remote. If more data is received
before five seconds elapse, the modem continues to transmit the data as described in the previous paragraph. If five seconds
elapse and no data is received from the DTE, the modem drops carrier, goes on-hook, and generates the ERROR result code.
5.1.9. Fax Data Reception
Fax data reception is initiated using the AT+FRM=n 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 V.21 carrier is detected when
a high speed carrier is expected, 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 data receive mode.While in data
receive mode, the modem receives data from the remote, adds start and stop bits, 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.If carrier is lost for a time period longer than the time specified by the register S10, the modem finishes delivering
the data in the receive buffer (if any) to the DTE, sends <DLE><ETX>, generates the NO CARRIER result code, and returns to
the command mode. All subsequent data received from the remote is discarded.If the modem detects a receive buffer
overflow condition, it stops receiving from the remote until there is room in the receive buffer. The modem informs the DTE of
the buffer overflow after it sends to the DTE the last character that was stored in the buffer before the overflow occurred.If the
modem receives any character from the DTE after the +FRM command (except flow control characters if software flow control
is in effect), or if the modem detects a high-to-low transition of the DTR signal while &D1 is in effect, it sends <DLE><ETX> to
the DTE, generates the OK result code, and returns to command mode. The receive buffer is cleared and all data received
