Correspondingly a negative-acknowledgement (NAK or NACK[1]) is a signal that is sent to reject a previously received message or to indicate some kind of error.
ASCII also provides a NAK code point (0x15, binary 0001 0101) which can be used to indicate the receiving device cannot, or will not, comply with the message.
[3] While some protocols send an acknowledgement per each packet received, other protocols such as TCP and ZMODEM allow many packets to be transmitted before sending an acknowledgement for the set of them, a procedure necessary to fill high bandwidth-delay product links with a large number of bytes in flight.
Binary Synchronous Communications (Bisync) and Adaptive Link Rate (for Energy-Efficient Ethernet) are examples.
Bisync does not use a single ACK character but has two control sequences for alternate even/odd block acknowledgement.