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 oRen 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.