在计算机网络中从源到目标的信息流动的各个阶段都可能受到安全威胁，如果图3-2左边所示为信息流动的正常情况，那么图中右边所示则为信息流动受到危害的情况，该危害属于(33)。

(143)

(142)

(141)

(140)

(138)

In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.

 Spread spectrum simply means that data is sent in small pieces over a number of the(71)frequencies available for use at any time in the specified range. Devices using(72)spread spectrum(DSSS)communicate by(73)each byte of data into several parts and sending them concurrently on different(74). DSSS uses a lot of the available(75), about 22 megahertz(MHz).

(137)

(136)

(135)

For each blank, choose the best answer from the four choices and write down on the answer sheet.

(66)is a protocol that a host uses to inform. a router when it joins or leaves an Internet multicast group.

(67)is an error detection code that most data communication networks use.

(68)is an interior gateway protocol that uses a distance vector algorithm to propagate routing information.

(69)is a transfer mode in which all types of information are organized into fixed form. cells on an asynchronous or non-periodic basis over a range of media.

(70)is an identifier of a web page.