Error control coding provides the means to protect data from errors. Data transferred from one place to the other has to be transferred reliably. Unfortunately, in many cases the physical link can not guarantee that all bits will be transferred without errors. It is then the responsibility of the error control algorithm to detect those errors and in some cases correct them so upper layers will see an error free link.
Two error control strategies have been popular in practice. They are the FEC (Forward Error Correction) strategy, which uses error correction alone, and the ARQ (Automatic Repeat Request) strategy which uses error detection combined with retransmission of corrupted data. The ARQ strategy is generally preferred for several reasons. The main reason is that the number of overhead bits needed to implement an error detection scheme is much less then the number of bits needed to correct the same error.
The FEC strategy is mainly used in links where retransmission is impossible or impractical. The FEC strategy is usually implemented in the physical layer and is transparent to upper layers of the protocol. When the FEC strategy is used, the transmitter sends redundant information along with the original bits and the receiver makes its best to find and correct errors. The number of redundant bits in FEC is much larger then in ARQ.