A Negative Acknowledgement(NAK) is a transmission control character used in character-oriented transmissions.
The negative acknowledgement is transmitted from the data sink, which is the receiving station, to the data source, the transmitting station, when it detects an erroneous data packet and requests a retransmission. In this case, the data sink sends a NAK signal to the data source, which then sends the same data packet a second time.
To avoid further delays, the time in which the NAK signal must be sent back is limited by a time-out. In the Automatic Repeat Request ( ARQ) the NAK signal is used for error correction. In the ASCII character set it has the decimal ASCII value 021, hexadecimal it is 15. The positive acknowledgement( ACK) has the ASCII value 006.