问题 单项选择题 共用题干题

当千兆以太网使用UTP作为传输介质时,限制单根电缆的长度不超过(1)米,其原因是千兆以太网(2)。

空白(1)为()

A.100

B.500

C.1000

D.2000

答案

参考答案:A

解析:

常用的传输介质有:双绞线、同轴电缆、光纤、无线传输媒介。双绞线可分为非屏蔽双绞线UTP和屏蔽双绞线STP,适合于短距离通信。非屏蔽双绞线价格便宜,传输速度偏低,抗干扰能力较差。屏蔽双绞线抗干扰能力较好,具有更高的传输速度,但价格相对较贵。双绞线需用RJ-45或RJ-11连接头插接。UTP传输距离不超过100米。以太网(Ethernet)指由Xerox公司创建并由Xerox、Intel和DEC公司联合开发的基带局域网规范,是当今现有局域网采用的最通用的通信协议标准。以太网络使用CSMA/CD(载波监听多路访问及冲突检测)技术,并以10Mb/s的速率运行在多种类型的电缆上。以太网与IEEE802.3系列标准相类似。包括标准的以太网(10Mbit/s)、快速以太网(100Mbit/s)和10G(10Gbit/s)以太网。它们都符合IEEE802.3。常见的802.3应用为:10M:10Base-T(铜线UTP模式);100M:100Base-TX(铜线UTP模式);100Base-FX(光纤线);1000M:1000Base-T(铜线UTP模式)。

阅读理解

阅读理解。

     Mobile phone has become a problem for middle schools. Some middle schools in Australia have banned

(禁止) students from carrying mobile phones during school hours.

     Mobile phone use among children has become a problem for the school this year. Several children have

got mobile phones as Christmas gifts, teachers said mobile phone use is a distraction (娱乐) to students

during school hours and it also gives teachers so much trouble in their classrooms. Teachers were also

saying that sometimes students might use phone messages to cheat during exams.

     She said some schools had tried to ban mobile phones. Some parents felt unhappy because they couldn't

get in touch with (与…联系) their children.

     Many teachers said students should not have mobile phones at school, but if there was a good reason,

they could leave their phones at school office. They also said there were many reasons why the students

should not have mobile phones at school: they were easy to lose and were a distraction from studies.

      Many people say that they understand why parents would want their children to have phones, but they

think schools should let the students know when they can use their mobile phones.

1. Some middle schools in Australia have banned students from carrying mobile phones _____. [ ]

A. because they are students

B. when they are free

C. when they are at school

D. because they are children

2. We know from the passage that some children get mobile phones from _____. [ ]

A. the makers and sellers

B. the passers-by and strangers

C. their parents and friends

D. some mobile phones users

3. What does the underlined word cheat mean in the passage? [ ]

A. 聊天

B. 核对

C. 查询

D. 作弊

4. Some parents felt unhappy because they couldn't _____ during school hours. [ ]

A. use their mobile phones

B. leave their mobile phones at school office

C. help the teachers with their work

D. get in touch with their children

5. The passage tells us that _____.[ ]

A. students shouldn't have mobile phones at school except for some special reasons.

B. it is impossible to ban students from using mobile phones at school.

C. some parents felt unhappy because they couldn't use their phones at school.

D. parents should teach their children how to use mobile phones during school hours.

选择题