问题 单项选择题

IEEE 802.11采用了类似于802.3 CSMA/CD协议的CSMA/CA协议,之所以不采用 CSMA/CD协议的原因是 (65)

A.CSMA/CA协议的效率更高

B.CSMA/CD协议的开销更大

C.为了解决隐蔽终端问题

D.为了引进其他业务

答案

参考答案:C

解析: CSMA/CD解决了在Ethernet上的各个工作站如何在线缆上进行传输的问题,利用它检测和避免当两个或两个以上的网络设备需要进行数据传送时网络上的冲突。在802.11无线局域网协议中,冲突的检测存在一定的问题,这个问题称为“Near/Far”现象,这是由于要检测冲突,设备必须能够一边接收数据信号一边传送数据信号,而这在无线系统中是无法办到的。
CSMA/CD和CSMA/CA的主要差别对比如下:
CSMA/CD:带有冲突检测的载波监听多路访问,可以检测冲突,但无法“避免”;
CSMA/CA:带有冲突避免的载波侦听多路访问,发送包的同时不能检测到信道上有无冲突,只能尽量“避免”;
1.两者的传输介质不同,CSMA/CD用于总线式以太网,而CS-MA/CA则用于无线局域网802.11a/b/S/n等;
2.检测方式不同,CSMA/CD通过电缆中电压的变化来检测,当数据发生碰撞时,电缆中的电压就会随着发生变化;而CSMA/CA采用能量检测(ED)、载波检测(CS)和能量载波混合检测三种检测信道空闲的方式;
3.WLAN中,对某个节点来说,其刚刚发出的信号强度要远高于来自其他节点的信号强度,也就是说它自己的信号会把其他的信号给覆盖掉;
4.本节点处有冲突并不意味着在接收节点处就有冲突。

阅读理解

阅读理解

     Visitors to museums have to pay "voluntary" admission fees, Boris Johnson, the mayor (市长) of

London, urged yesterday. He said that encouraging visitors to set a value on museums and art galleries,

which have been free since 2001, would work extremely well. Mr. Johnson held up New York's

Metropolitan Museum of Art as a model. Although New York's museums are officially free, visitors are

strongly encouraged to pay a recommended entry fee of $20. At the Met, entry is impossible without first

going to the ticket des k.

     Mr. Johnson's spokesman later admitted that free admission was a "huge draw" for London, but he

said, "Having visited the Met last week, the mayor is impressed by how they maximize voluntary

contributions and believes there are lessons to be learnt." Mark Jones, director of the Victoria and

Albert Museum, told The Times that the museum already requested a ?3 donation but was not as

forceful as New York's institutions. He said, "I'm not in favor of anything that makes people feel they

won't want to go because they feel like they will have to pay."

      When entrance fees to national museums were decided not to be charged here in 2001, there was a

70 percent increase in visitor numbers in the first year. Political parties have since been unwilling to

suggest change, despite concerns about cost. Hugo Swire, the former Shadow Culture Secretary, was

dismissed in 2007 for suggesting that "museums and galleries should have the right to charge if they wish".

Some in the art community argue, however, that free entrance has done little to increase the breadth

(广泛) of visitors and has caused shortfalls in the budget for the museums. In response to it, Colin

Tweedy, chief executive of Arts & Business, said that Britain was "leading the world" in allowing its

treasures to be freely available. He said, "To return to museum charging would be a return to the dark

ages."

     Ben Bradshaw, the Culture Secretary, said, "The mayor revealed his true intention when he suggested

those who could afford it should be encouraged to contribute to the arts. I believe it is a disaster for the

culture, arts and sport." He thinks that free museums and galleries have once been one of this Government's great successes.

1. It can be inferred from the first two paragraphs that ______.

A. museums in London are free by now

B. most museums in London have admission fees

C. Metropolitan Museum of Art is not officially free

D. free entry is impossible to New York's museums

2. We can learn from Mark Jones' words that ______. 

A. his museum shows a lack of money        

B. admission fees should be forceful

C. he is in favor of New York's action        

D. he has a consideration for the visitors

3. According to the passage, the cancellation of entrance fees to national museums in 2001 _____.

A. was changed in 2007 by Hugo Swire

B. has been opposed by political parties

C. has no influence on the number of visitors

D. has caused financial problems to the museums

4.What does the underlined word "it" in the last paragraph refer to?

A. Charging entrance fees to museums.        

B. Decrease in the number of visitors.

C. Ensuring visitors' benefits.                

D. Free access to museums.

多项选择题