RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
9-8
1070
During TCF, EQMs that represent acceptable BERs can be used to determine whether or not to fallback. If a fallback is
necessary, EQM can be used to choose the appropriate fallback mode for a desired BER.
9.1.3 Phase
C
Phase C occurs after both facsimile machines have set up for the high speed configuration decided upon in phase B. The
T.30 Error Correction Mode is addressed in a following section. This 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 modem. 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 9-8 illustrates the phase C format.
9.1.4 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.
9.1.5 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.
9.1.6 Flowcharts
Figure 9-9 through Figure 9-15 illustrate how to implement certain phase B procedures. The examples show how the modem
can be set up using the internal HDLC framing capabilities.
Figure 9-9 describes how to transmit FSK/HDLC signals such as DIS, DCS, DTC, CFR, FTT, etc. Three subroutines are
called out: the low speed configuration, the transmit preamble, and interrupt-driven transmit routine. A Programmable
Interrupt set-up is required in the receiver to determine if it is the end of the frame and if the frame was received correctly.
Figure 9-10 describes how to set this up. The Low Speed Configuration subroutine is shown on Figure 9-11. Since the HDLC
function is being used, the parallel data mode is enabled. The Transmit Preamble routine is shown on Figure 9-12. When the
modem is configured in HDLC mode by setting RTSP, the modem will automatically transmit flags. The Interrupt-Driven
Transmit routine is illustrated in Figure 9-13.
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