NAK is an abbreviation for negative acknowledgment or not acknowledged. It is a signal used in digital communications to ensure that data is received with a minimum of errors. Sometimes the NAK signal is called REJ (for rejection or rejected). An alternative signal is ARQ (automatic request for retransmission).
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The NAK, REJ, or ARQ signal is typically sent by the recipient of a message (destination) back to the sender (source) after the receipt of an unrecognizable block of data of specific size. When the source receives a NAK or REJ signal from the destination, the source either repeats the block of data until the destination station recognizes the protocol and responds with an ACK (acknowledgment or acknowledged) signal, or else stops transmission entirely. When the source receives an ARQ signal from the destination, the source retransmits the block of data.
NAK, REJ, or ARQ signals are special ASCII characters reserved so they are not confused with data characters.