Error correction mode

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PART 3 (60.4.4 – 60.5)

In CCITT ECM the transmitted signal is separated into HDLC frames so that on receive the FCS bytes allow the data accuracy to be checked. G3 transmission is half duplex and when the trans­mission stops the receiver replies giving the condition for each of the frames. The transmitter then selectively repeats the frames which the receiver advised were in error.

Figure 60.4 shows the ECM transmitted frame structure. Each frame contains 256 octets of image plus its frame number. A block can have up to 256 frames, equivalent to around 0.5Mbits of data, sufficient for most facsimile pages. In this case, if there are no errors, there are no additional turn rounds above a standard G3 transmissions.

If the pages are of greater data content than 0.5Mbit (about 50s at 9.6kbit/s) a partial page is generated at 256 frames. This partial page is corrected before proceeding with the next block. Following the image transmission a PPS is sent at 300bit/s to tell the receiver the page and block number and the actual number of frames in that block, for checking and recovery purposes.

60.4.4.1 ECM receiver operation

The receiver knows by the FCS which frames are correctly received. Often, in a thermal paper receiver, the machine will print the copy as received until such time as an error is recognised. It then stops with the data going to memory until corrected data is available. In most practical cases this procedure, and the availability in the receiver of sufficient memory for the next page/partial page, avoids waiting between pages due to printer unavailability.

With the page/partial page correctly received the receiver replies with MCF and the transmitter continues with the next data.

Figure 60.5 shows ECM operation with errors. If errors are de­tected the receiver sends PPR which has an information field of 256 bits, one for each frame of the transmitted block. The receiver sets each bit of PPR to 'O' for a correctly received frame and waits for the transmitter to selectively repeat the unacceptable frames. Pages/partial pages are often corrected with a single retransmission but the cycle is repeated for the then outstanding error frames. When the receiver has all frames correct it finishes printing the page and returns MCF for the next block to follow.

60.4.4.2 ECM performance

The overhead for G3 ECM is in the transmit direction and is approximately 5% when there are no errors. Normally in non-ECM facsimile transmission a limited number of errors are tolerated due to the image redundancy remaining in the MH and MR coding. The reception at the transmitter of MCF confirms that the document has been satisfactorily received but not necessarily that it is free of errors. There are 2 main user advantages with ECM:

1. The sender is 100% confident that acknowledged pages are
complete and received free of error.

2. That as errors are efficiently corrected transmission time (and
cost) can be reduced by using higher transmission rates together
with MMR coding.

The transmitter has the opportunity from studying the pattern of errors in PPR to reduce the frame size to 64 octets or to change the data rate to maximise throughput. The selective retransmission and large window with minimum turnarounds providing efficient long haul communication even including 3 satellite links.

Дата добавления: 2015-03-31