What Causes Errors?
Line noise on electrical media (e.g., coaxial cable and twisted pair) consists of unwanted electrical signals
Line noise on optical media (e.g., fiber optic cable) consists of undesirable light)
Noise is introduced by equipment or by natural
Noise degrades the performance of a communications line
Noise manifests itself as missing bits, extra bits, or bits whose states have been flipped (0 is changed into 1 and vise versa)