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Redundancy check

A redundancy check is extra symbols added to a message for the purposes of error detection or error correction.

A check digit is a digit used as a redundancy check. The UPC barcode has a check digit computed by multiplying digits by alternately 3 and 1 and adding them up. For instance:

0+0+1+0+6+9 =6
so the check digit, which is the 2 at the end, is correct.

A longitudinal redundancy check is computed by exclusive-oring all the bytes in a message.

See also: Cyclic redundancy check.