RCVDL56DPFL/SP, RCV56DPFL/SP, and RCV336DPFL/SP Designer’s Guide
8-2
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resolution, coding scheme (Modified Huffman, Modified Read), recording width, recording length, and minimum scan line
time. The calling unit then responds with a Digital Command Signal (DCS) which informs the called unit which options are
chosen to complete this facsimile call.
After the DCS is transmitted, both the calling unit and the called unit set up for the high speed configuration that was chosen
and transmitted via the DCS. A Training Check (TCF) is transmitted by the calling unit to verify training and give an indication
of channel acceptability for the selected data rate. The TCF consists of a series of zeros for 1.5 seconds ±10%. Since the
called unit knows it will be receiving 1.5 seconds of zeros, the host can make a decision whether the line is good enough at
the chosen data rate or fallback to a slower speed.
After completing the TCF, the calling unit and the called unit re-configure for FSK, HDLC format. The called unit then
transmits either a Confirmation to Receive (CFR) or a Failure To Train (FTT). The CFR is a response informing the calling
unit of a successful pre-message procedure completion. A FTT informs the calling unit that the training signal was rejected
and requests re-training. If a FTT is received by the calling unit, the fax protocol jumps back to the transition of DCS and
continues until finally a CFR is received or the calling unit host decides to terminate the call.
8.1.3 Phase
C
Phase C occurs after both facsimile machines have set up for the high speed configuration that was decided upon in phase
B. The T.30 Error Correction Mode is addressed in a following section. The high speed message information is usually
compressed data using a Modified Huffman (MH) or Modified Read (MR) algorithm. The host processor must perform the
MH or MR compression before loading the data into the MDP. On the receive end, the host processor must perform the MH
or MR decompression.
The start of phase C is denoted by an End Of Line (EOL) 8-bit code. The data follows this first EOL character until the end of
the line. Another EOL character is transmitted to indicate a new line. A minimum transmission time of a total coded scan line
is measured from the beginning of the EOL to the beginning of the following EOL. If the transmitted data requires less time
than the minimum transmission time, fill bits must be transmitted. Six consecutive EOL character constitute a Return To
Control (RTC) command meaning end of document transmission. Figure 8-21 illustrates the phase C format.
8.1.4 Phase
D
The post-message phase D procedure uses FSK and HDLC format. The calling station will typically send an End Of
Message (EOM) signal. This FCF command (EOM) informs the called station that this is the end of the page and return to
Phase B. A Multi-Page Signaling (MPS) or End Of Procedure (EOP) signal may be sent instead of EOM. The MPS signal
informs the called unit that there are more pages in this facsimile transmission. EOP signals the end of the facsimile
transmission. Procedure Interrupt-EOM (PRI-EOM), Procedure Interrupt-MPS (PRI-MPS), and Procedure Interrupt-EOP
(PRI-EOP) indicate the same as EOM, MPS, and EOP, respectively, with the additional optional capability of requesting
operator intervention. If operator intervention is required, further facsimile procedures commence at the beginning of phase
B.
The called station might respond to an EOM, MPS, or EOP signal with a Message Confirmation (MCF) command. This FCF
command indicates to the calling unit that the complete message was received. One of the following FCF commands may be
sent instead of the MCF: Re-Train Positive (RTP), Re-Train Negative (RTN), Procedure Interrupt Positive (PIP), or
Procedure Interrupt Negative (PIN). RTP indicates that a complete message has been received and that additional
messages may follow after retransmission of TCF and CFR. RTN indicates that the previous message has not been
satisfactorily received, however, further receptions may be possible provided there is a retransmission of TCF and CFR. PIP
and PIN indicate that the previous message was received satisfactorily or not satisfactorily, respectively, and operator
intervention is required for further transmissions.
8.1.5 Phase
E
Call Release, or phase E, occurs after the last post-message signal of the procedure or under certain conditions such as a
time-out, procedural interrupt, or a Disconnect (DCN) command.
The DCN command indicates the initiation of phase E. This command requires no response.
8.2 ERROR CORRECTION MODE
8.2.1 General
The revised T.30 contains an Error Correction Mode (ECM) option. The ECM allows the phase C portion of the facsimile
transmission to be encoded in a HDLC framing format using a specified number of bits in the information field. The
transmitted high speed message is broken up into a number of frames identified by frame numbers. If an error is detected
during reception of the message, the called station records the frame number. After all the frames in the message has been
received, the called station transmits the frame numbers that were received in error. The calling station then re-transmits
only those frames in error. This continues until the entire message is received error free or the calling station decides not to
transmit any more frames.
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