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 (64) between sending the notification and notification arriving at the source, thousands of additional (65) may arrive. In ATM network , a major tool for pre- venting (66) is (67) control. When a host wants a new virtual (68) , it must describe the traffic to be offered and the service expected.

65（）

A．packets

B．cells

C．message

D．files