s expected but to 7 to allow FEC (Forward Error Figure 60.4 shows the ECM transmitted frame structure. Each Correction) data to be included. The 7th bit, bringing the number of frame contains 256 octets of image plus its frame number. A block states to be decoded at the receiver to 128, is generated from 2 of can have up to 256 frames, equivalent to around 0.5Mbits of data, the data bits in each symbol interval. This introduced redundancy sufficient for most facsimile pages. In this case, if there are no Facsimile Synchronisation Variable length coded data string of zeros FCD frame FCD frame FCD frame RCP frame RCP frame RCP frame Frame Facsimile coded data Figure 60.4 ECM transmitted frame structure 60/10 Facsimile transmission errors, there are no additional turn rounds above a standard G3 but the cycle is repeated for the then outstanding error frames. When transmissions. the receiver has all frames correct it finishes printing the page and If the pages are of greater data content than 0.5Mbit (about 50s at returns MCF for the next block to follow. 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 60.4.4.2 ECM performance page and block number and the actual number of frames in that block, for checking and recovery purposes. 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 60.4.4.1 ECM receiver operation to the image redundancy remaining in the MH and MR coding. The reception at the transmitter of MCF confirms that the document has The receiver knows by the FCS which frames are correctly received. been satisfactorily received but not necessarily that it is free of Often, in a thermal paper receiver, the machine will print the copy errors. as received until such time as an error is recognised. It then stops There are