Error correction mode

Last updated March 30, 2019

Error correction mode (ECM) is an optional transmission mode built into Class 1/2/2.0 fax machines or fax modems. ECM automatically detects and corrects errors in the fax transmission process that are sometimes caused by telephone line noise.

A fax modem enables a computer to transmit and receive documents as faxes on a telephone line. A fax modem is like a data modem but is designed to transmit and receive documents to and from a fax machine or another fax modem. Some, but not all, fax modems do double duty as data modems. As with other modems, fax modems can be internal or external. Internal fax modems are often called fax boards.

A telephone line or telephone circuit is a single-user circuit on a telephone communication system. This is the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network, and usually also implies a single telephone number for billing purposes reserved for that user. Telephone lines are used to deliver landline telephone service and Digital subscriber line (DSL) phone cable service to the premises. Telephone overhead lines are connected to the public switched telephone network.

Noise is unwanted sound judged to be unpleasant, loud or disruptive to hearing. From a physics standpoint, noise is indistinguishable from sound, as both are vibrations through a medium, such as air or water. The difference arises when the brain receives and perceives a sound.

The page data is divided into what is known as Octets (small blocks of data). Once the receiver has received all the Octets it examines them (using check-sums) and then advises the transmitting fax of any Octets that are in error. The transmitter then need only resend the blocks in error rather than the whole page. This generally means an ECM coded fax will be more likely to succeed in transmitting a copy of the page(s) on a noisy line. ECM is the norm rather than the exception. Some fax machines have the capability to enable or disable this function.

In telecommunications a block is one of:

The receiver in information theory is the receiving end of a communication channel. It receives decoded messages/information from the sender, who first encoded them. Sometimes the receiver is modeled so as to include the decoder. Real-world receivers like radio receivers or telephones can not be expected to receive as much information as predicted by the noisy channel coding theorem.

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