The major change in WXMODEM is the use of a sliding window to improve throughput on high-latency links. To do so, the ACK messages were followed by the packet number they were ACKing or NAKing. The receiver does not have to ACK every packet, it is allowed to ACK any number between one and four packets. An ACK with the fourth packet sequence number is assumed to ACK all four packets. A error causes a NAK to be sent immediately, with all packets from that number and after being re-sent.