Error correction mode

Error correction mode (ECM) is an optional transmission mode built into Class 1 fax machines or fax modems. ECM automatically detects and corrects errors in the fax transmission process that are sometimes caused by telephone line noise. The page data is divided into what is known as Octets (small blocks of data ), once the reciever has received all the Octets it examines them ( using checksums I believe ) and then advises the transmitting fax of any Octets that are in error. The transmitter then need only resend the errored blocks rather than the whole page. This generally means an ECM coded fax will be more likely to suceed on a noisey line. ECM is the norm rather than the exception.