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 (71) between sending the notification and notification arriving at the source, thousands of additional (72) may arrive. In ATM network , a major tool for preventing (73) is (74) control. When a host wants a new virtual (75) ,it must describe the traffic to be offered and the service expected.

A．packets

B．cells

C．message

D．files