问题 单项选择题

设计骨干网时,应该在性能和成本之间寻找平衡。以下叙述中,()是正确的。

A.ATM在局域网的所有应用可用ELAN来实现,带宽效率高,实时性好,适宜用作园区网

B.双星树结构的主干网,虽然不能均衡负载,成本也较高,但具有更高的可用性

C.由于建筑群布线路径复杂的特殊性,一般直线距离超过300米的建筑物之间的千兆以太网线路就必须用单模光纤

D.如经费难以支持千兆以太网,可以采用100Base-FX,用双绞线建立快速以太网,是非常经济实惠的选择

答案

参考答案:C

解析:

主干网一般用来连接建筑群和服务器群,是网络的大动脉。主干网技术的选择,要根据需求分析中的地理距离、信息流量和数据负载的轻重而定。连接建筑群的主干网一般以光缆作传输介质。

FDDI基本属于过时技术,目前较少应用;ATM是面向连接的网络,实时传输效率高,但其在局域网的应用需要ELAN仿真来实现,效率低,不适宜用作局域网或园区网。千兆以太网一般采用光缆作传输介质,多种波长的单模和多模光纤分别用于不同的场合和距离,由于建筑群布线线路路径复杂的特殊性,一般直线距离超过300米的建筑物之间的千兆以太网线路就必须要用单模光纤。如经费难以支持千兆以太网,可以采用100Base-FX,即用光纤建立快速以太网,其端口价格低,对光缆的要求也不高。

阅读理解

阅读理解。

    One day a farmer went out for a walk with his daughter. The farmer put on a pair of wrong shoes-one

with a thick sole (鞋底) and the other with a thin one. So as he began to walk, he felt very uncomfortable.

When he was just out of the house, he turned to his daughter and said, "Why should one of my legs be

longer than the other today?"

    The daughter looked at his father's legs carefully as he was walking, and then laughed, "Oh, no, Daddy,

your legs are all right. You have put on the wrong shoes." The farmer was very happy to hear that and said

to himself, "What a clever daughter I have got!" Then he asked his daughter to go back and get the other

pair of shoes for him.

    The farmer had only two pairs of shoes.

    When the daughter ran back to the house, she found that the other pair was also a pair of wrong shoes.

    She had to return to his father with nothing in her hands and said out of breath, "It's no use changing

them, Daddy! The shoes at home were not a pair, either!"

1. When the farmer went out for a walk with his daughter, he put on a pair of ______ shoes. [ ]

A. wrong   

B. comfortable    

C. old    

D. new

2. A pair of wrong shoes means ______. [ ]

A. the shoes are not the farmer's

B. the shoes are too small

C. one of the shoes has a thick sole, the other has a thin one, they are not a pair

D. the shoes are too big

3. When the farmer began to walk, he felt very uncomfortable because ______. [ ]

A. one of his legs was longer than the other one

B. he was ill that day

C. he wore a pair of wrong shoes

D. the shoes were too small

4. The farmer thought his daughter was a clever girl because ______. [ ]

A. she could make shoes

B. she could find out that her father wore a pair of wrong shoes

C. she could find out a pair of right shoes

D. she found that her father's legs were sick

5. The father asked his daughter to go and get the other pair of shoes for him and the daughter ______. [ ]

A. brought back a pair of wrong shoes

B. brought back of a pair of right shoes

C. didn't bring any shoes; she knew the other pair at home was not a pair, either

D. couldn't find anything at home

判断题