RCVDL56DPFL/SP, RCV56DPFL/SP, and RCV336DPFL/SP Designer’s Guide
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8.2.2 ECM Frame Structure
In Error Correction Mode, one frame of facsimile data consists of 256 or 64 octets of data. Each page may contain 1 to 256
frames. Also, 1 to 256 pages may be transmitted. The ECM frame structure is illustrated in Figure 8-22. Following the high
speed training sequence, the flag, address field, and control field is transmitted. In ECM, Flag = 7E, Address = FF, and
Control = B0. The Facsimile Control Field for the Facsimile Coded Data block (FCD) is 60. The frame number follows the
FCF for FCD, followed by the facsimile data. Pad bits such as EOL, Tag, and Align bits follow the facsimile data. Finally, the
FCS check and the ending flag is transmitted.
After 256 frames, a Return Control for Partial page (RCP) block is transmitted three times. The RCP block consists of the
same Flag, Address Field, and Control field followed by the FCF for RCP. The FCS immediately follows with the ending flag.
After the third RCP, a maximum of 50 ms of flags are transmitted.
An ECM message protocol example is shown in Figure 8-23. The bold arrows are high speed transmissions and the other
arrows are FSK transmissions. The example is self-explanatory. If more information is needed, refer to the T.30 ECM
specification.
In this paragraph, the Q refers to the NULL, EOP, MPS, or EOM Facsimile Control Field commands. The Partial Page
Signals (PPS-Q) and Partial Page Request (PPR) frame structures are shown in Figure 8-24. The PPS-Q frame begins with
the same Flag, Address field, and Control field. Two FCF commands follow. The first FCF transmitted is to indicate PPS. The
second FCF is either NULL, EOP, MPS, or EOM. The page count followed by the block count, followed by the total number
of frames in the block are transmitted next. The FCS and ending flag are finally transmitted.
The PPR frame structure also begins with the same Flag, Address, and Control field. The FCF for PPR is the next octet. The
FIF consists of 256 or 64 bits depending on how many frames were transmitted. The contents of FIF is either a 0 or a 1. The
bit number corresponds to the frame number and a 0 indicates the frames was received correctly and a 1 indicates an
incorrect frame was received.
8.3 SIGNAL RECOGNITION ALGORITHM
A method of determining whether a high speed message or FSK handshaking is being received by the MDP is necessary
when implementing the T.30 recommendation. When the calling unit transmitter and called unit receiver configure for V.29 or
V.27 ter, sometimes the high speed message may not be received (typically due to a noisy line). In this case, the calling unit
transmitter will try to send the message up to three times before re-negotiating in FSK signaling. The called unit receiver
must, therefore, be able to distinguish between a high speed message and FSK handshaking.
The algorithm shown in Figure 8-25 can be used to perform the signal recognition. The use of the P2DET and PNDET status
bits may also be incorporated for qualifying high speed PSK carrier.
High speed PSK reception in high noise environments can be optimized by setting the RTH bits so that the level of the noise
is below the MDP's receiver threshold. This can be accomplished by measuring the received signal level via the AGC gain
word RAM access while receiving the V.21 channel 2 carrier. Then, after configuring to a high speed configuration, RTH
should be set to a value which will provide a threshold range centered around the expected receive level.
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