RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
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9-25
9.2 ERROR CORRECTION MODE
9.2.1 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.
The error detection is performed by comparing the CRC or FCS. Using ECM, the data rate can be as fast as 14,400 bps,
therefore, the host microprocessor cannot keep up implementing HDLC without the use of a serial I/O device. The modem
provides HDLC features at speeds up to 14,400 bps.
9.2.2 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 9-23. 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 9-24. 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 9-25. 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.
Summary of Contents for RFX144V24-S23
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