When the network needs to control the congestion, the signal will be delivered to the receiver at the fast speed, and was brought to the initiator by the acknowledgementframe.
当网络需要预防拥塞发生时，标记将以最快的速度到达接收端，并且被确认帧带回到发送端。
2
If the sender receives a positive acknowledgement about a frame, it knows the frame has arrived safely.
如果发送端收到了有关帧的肯定应答，它就知道发送的帧已经安全到达。
3
On the other hand, a negative acknowledgement means that something has gone wrong, and the frame must be transmitted again.