Redundancy bit

The redundancy bit is a bit (or set of bits) that is sometimes deliberately introduced into the transmission or recording of information without being part of it, but that serves to detect possible mistakes.

For example, if we want to represent four {1,2,3,4} symbols and make a code without redundancies, two bits are needed to represent it.

• If by some mistake, we get another symbol of the alphabet, we have no way to detect it.
• If we add a bit to the previous code at the end, certain errors may be detected.
• If the bit we added put it to zero, and by an error there is a change in the last bit, it would be detected that that symbol received is an invalid symbol.

For example, if we receive a 001 instead of a 000, we would know that the received symbol is wrong since it does not exist in our alphabet.

Normally the value of the redundancy bit or bits is set with a value that is a function of one or more of the previous bits, so that it helps to identify communication errors in other bits. Usually this function is the sum or XOR of all the previous bits.