RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
1070
9-7
1070F9-7
PREAMBLE
FLAG
01111110
FLAG
01111110
ADDRESS
11111111
CONTROL
1100X000
FCF
8 BIT
8 BIT
8 BIT
8 BIT
8 BIT
16 BIT
32 BIT
FIF
FCS
BINARY CODED INFO
FCS: Frame check sequence; CCITT V.41 CRC-16 is used
CSI FRAME
(OPTIONAL)
DIGITAL ID FRAME
(MANDATORY)
NSF FRAME
(OPTIONAL)
Figure 9-7. HDLC Frame Structure
After the modem has been configured for FSK, the Digital Identification Signal (DIS) is transmitted by the called unit. The
DIS informs the calling unit about the called unit's capabilities such as group capability (G1, G2, G3), data rate, vertical
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-configures 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.
The fallback criterion based upon the modem performance information during TCF reception, for example, may consist of
monitoring the number of bit errors or EQM number. If EQM is to be used as a fallback criterion for an acceptable BER
performance, the minimum metric EQM number should be used for V.17/V.33 modes and the hard decision EQM number
should be used for V.29 and V.27 ter modes (see Table 4-1 for RAM access codes and Section 4.2 for scaling information).
Refer to the EQM and BER vs. S/N charts in Section 13 for a guideline on EQM numbers. The EQM numbers shown in the
charts are the decimal equivalent to the 16-bit hexadecimal numbers divided by 256.
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