ATM信元中，UNI信头和NNI信头的差别是(21)。当网络出现阻塞时，交换机要(22)，信元路上传输时，ATM信元要装配成传输帧，如果高层没有数据发送，则必须插入空信元或(23)，以便将ATM层信元速率适配成传输线路的速率。ATM网络是基于(24)方式的数据交换网络，若要实现IP　Over　ATM，则需要(25)技术。

(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.