当前磁盘读写位于柱面号20，此时有多个磁盘请求，以下列柱面号顺序送至磁盘驱动器：10、22、20、2、40、6、38。寻道时，移动一个柱面需6ms，采用改进电梯调度算法所需寻道时间为(21)。

(143)

(142)

(141)

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(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.