In computers and digital communication systems, informa￾tion is almost always represented in a binary form as a se￾quence of bits each having the values 0 or 1. This sequence of
bits is transmitted over a channel from a sender to a receiver.
In some applications the channel is a storage medium like a
CD, where the information is written to the medium at a cer￾tain time and retrieved at a later time. Due to physical limita￾tions of the channel, some of the transmitted bits may be cor￾rupted (the channel is noisy) and thus make it difficult for the
receiver to reconstruct the information correctly.