单选题 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 ______ between sending the notification and notification arriving at the source, thousands of additional ______ may arrive. In ATM network, a major tool for preventing ______ is ______ control. When a host wants a new virtual ______, it must describe the traffic to be offered and the service expected.

【正确答案】 A

【答案解析】当拥塞发生时，便通告信源要放慢速度，在低速网络中，等待通告时间通常是足够的。但在高速网络中，这种方法工作起来有时会很差，这是因为，在发出通告与通告到达信源这段时间内(interval)，可能又有成千上万个数据包(packets)已经到达。在ATM网络中，主要的拥塞(congestion)避免工具是通信量(traffic)控制。当一台主机想建立一条新的虚电路(circuit)时，必需要描述其通信量和服务要求。