FEC - Forward Error Correction, is a technique for transmitting digital data in a manner that minimizes errors, without having to retransmit the data if an error occurs.
Most modern communications protocols use a form of ARQ to eliminate transmission errors. However, under some circumstances, the overhead of retransmission required by ARQ is unacceptable. For example, a base station on Earth trying to control a robotic device on another planet cannot afford to retransmit a command; by the time the retransmitted command reaches the robot, the robot may have already made a fatal mistake. FEC is a way to try to avoid this problem.
The idea behind FEC is to include enough redundant data in a transmission to allow the receiver to correct for common errors. Each piece of data in a transmission is represented by a "code word". These code words are chosen in such a way that a corrupted code word will still be recognizable by the receiver, and can be reverted to its original form.
An example might be a spam message that refers to "Vi@gra"; here the "@" sign can be easily converted by the human mind back into the letter "a", and the word "Viagra" recognized. In this example, the code word would be the letter "a", and the corrupted code word the "@" symbol. The error correction is in the viewer's mind; it knows that the "@" symbol is very close to the letter "a". The use of "@" might be considered a form of data corruption, and the underlying "a"-ness of the image provides the redundancy the the mind needs to correct the error.
By the same token, software can find an original symbol from one that was corrupted during the transmission process, and turn it back into the symbol intended by the transmitter.
FEC is used extensively in applications such as space exploration, and is also used to provide reliable communications over HF frequencies, where noise and other interference make digital communications problematic.
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