In low-speed network, it is usually adequate to wait for congestion to occur and then react to it by telling the source of packets to slow down. In high-speed networks, this approach often works poorly, because in the (61) between sending the notification and notification arriving at the source, thousands of additional (62) may arrive. In ATM network, a major tool for preventing (63) is (64) control. When a host wants a new virtual (65) , it must describe the traffic to be offered and the service expected.

A．collision

B．congestion

C．drop

D．delay

实现比例积分（PI）调节器采用反馈环节的传递函数为（）。

A.A

B.B

C.C

D.D