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Chapter 4. Modulation and Demodulation > 4.5 Error Detection and Correction

4.5 Error Detection and Correction

An error-correcting code is a technique for reducing the bit error rate on a communicaton channel. It involves transmitting additional bits, called redundancy, along with the data bits. These additional bits carry redundant information and are used by the receiver to correct most of the errors in the data bits. This method of reducing the error rate by having the transmitter send redundant bits (using an error-correcting code) is called forward error correction (FEC).

An alternative is for the transmitter to use a smaller amount of redundancy, which the receiver can use to detect the presence of an error, but there is insufficient redundancy to identify/correct the errors. This approach is used in telecommunication systems based on SONET and SDH to monitor the bit error rate in the received signal. It is also widely used in data communication systems, where the receiver requests the transmitter to resend the data blocks that are detected to be in error. This technique is called automatic repeat request (ARQ).


  

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