Negative-acknowledge character

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For the eutectic alloy see NaK. See Rakion for the creature Nak. For Nack the Weasel, see Fang the Sniper.

In telecommunications, a negative-acknowledge character (NAK) is a transmission control character sent by a station as a negative response to the station with which the connection has been set up.

Note 1: In binary synchronous communication protocol, the NAK is used to indicate that an error was detected in the previously received block and that the receiver is ready to accept retransmission of that block.

Note 2: In multipoint systems, the NAK is used as the not-ready reply to a poll.

Sources: Federal Standard 1037C and from MIL-STD-188

Also used as an acronym in internet forums and chats (mainly technology) for "I don't agree" or "That's wrong".

In the ASCII code, the NAK character is 0x15 (hexadecimal).