Noise on a wire or radio link can flip bits, and the receiver must notice before it trusts the data. Parity and CRC add extra check bits so errors can be detected, and Hamming codes can even correct a single flipped bit. Ethernet uses CRC on every frame, while memory and storage systems use error-correcting codes. Detection is not correction; most links just drop a bad frame and let higher layers retransmit, and exams test this difference with numericals.