在图3-1所示的系统中，员工小郭要上Internet需要三层交换机、防火墙、路由器3台设备(每种设备赋予相同的可靠度)均正常运行。若要求整个系统可靠度为0.98，那么每一种设备的可靠度至少为(11)。

(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.