Group 3 fax machines incorporate a means for identifying the optional features that are available on the receiving fax machine during the initial connection and identification phase. Better quality fax machines including the LaserWriter Pro 810 and LaserWriter Select 360 fax options support the ECM error correction method in both the group 3 and PostScript fax modes.
ECM is a method for error correction where the facsimile data to be transmitted is divided into data units each having a certain length, so that when any error occurs during data transmissions the applicable data unit is resent. Both the sending and receiving fax machines must support ECM for the option to be enabled.
The Group 3 raster fax data is sent in packets with a CRC byte. If the CRC byte is bad then the receiving station will keep requesting retransmission of the packet until it is successfully received.
The above information applies to Group 3 fax jobs. All PostScript fax jobs transmitted to other PostScript fax printers will always use ECM error correction since a single error can prevent a PostScript job from printing.