结构化设计方法使用的图形工具是(19)，图中矩形表示(20)。如果两矩形之间有直线相连，表示它们存在(21)关系。

(60)

(73)

(72)

(71)

(70)

(68)

In low-speed network，it is usually adequate to wait for congestion to occur and then react to it bytelling 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 offeredand the service expected．

(69)

(67)

电视信号的标准也称电视的制式，制式的区别在于其帧频的不同、分辨率的不同、信号带宽及载频的不同、彩色空间的转换关系不同等。我国电视采用的制式是(58)，对应帧频是(59)帧/妙，对应分辨率为(60)。

(66)

(65)

(64)