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（本题）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.