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 (1) between sending the notification and notification arriving at the source, thousands of additional (2) may arrive. In ATM network, a major tool for preventing (3) is (4) control. When a host wants a new virtual (5) ,it must describe the traffic to be offered and the service expected.

2（）

A．packets

B．cells

C．message

D．files

弗里德赖希（Friedreich）共济失调发病年龄为（）

A.40岁左右

B.5岁以下

C.5～15岁

D.40岁以后

E.20～40岁