Error Detection or Error Correction?

Error Detection or Error Correction

We have mentioned that it is possible to use codes that not only detect the presence of errors but also enable errors to be corrected. Since the details of such codes require yet more complex mathematics than that required to understand CRCs, we will not dwell on them here. However, it is worth considering the merits of correction versus detection.

At first glance, it would seem that correction is always better, since with detection we are forced to throw away the message and, in general, ask for another copy to be transmitted. This uses up bandwidth

and may introduce latency while waiting for the retransmission. However, there is a downside to correction: It generally requires a greater number of redundant bits to send an error-correcting code that is as strong (that is, able to cope with the same range of errors) as a code that only detects errors. Thus, while error detection requires more bits to be sent when errors occur, error correction requires more bits to be sent all the time. As a result, error correction tends to be most useful when (1) errors are quite probable, as they may be, for example, in a wireless environment, or (2) the cost of retransmission is too high, for example, because of the latency involved in retransmitting a packet over a satellite link.

The use of error-correcting codes in networking is sometimes referred to as forward error cor- rection (FEC) because the correction of errors is handled “in advance” by sending extra information, rather than waiting for errors to happen and dealing with them later by retransmission.