问题 问答题

【问题1】 请指出图1-12中(1)空缺处传输的是模拟信号,还是数字信号

答案

参考答案:

解析:数字信号这是一道要求读者掌握宽带接入网络传输信号的基本常识题。本题的解答思路如下。 1)由如图1—12所示的网络拓扑图中“有线节目源”、“自办节目源”、“电话信号”和“电话机”、“电视机”设备等可知,(1)空缺处所在的网络属于光纤同轴电缆混合接入传输系统HFC。 2)完整的HFC网络系统包括①局端系统CMTS、②用户终端系统和③HFC传输网络。 3)在HFC网络中,局端系统CMTS主要完成数据到射频RF信号的转换,即将计算机网络的数字信号与有线电视网(CATV)的射频信号进行混合,并送入到HFC网络中。它也可作为业务接入设备,通过以太网挂接本地服务器,提供本地业务。 4)在HFC网络中,电缆调制解调器(Cable MODEM)就是用户终端设备,它接收从CMTS发送来的QAM调制信号,解调后转换成MPEG2或MPEG2-TS格式的数据帧,以重建以太帧。从计算机接收到的以太帧被封装在时隙中,经QPSK调制后,通过HFC网络的上行数据通路传送给CMTS。 5)由此可见,在如图1-12所示网络拓扑图中有线电视台的电视信号、公用电话网的话音信号和数据网的数据信号送入合路器并形成混合信号后,再通过光缆线路送至各个小区的光纤节点,再经过同轴分配网络送至用户本地综合服务单元,并分别将电视信号送到用户的电视机、语音信号送至电话机、数据信号经电缆调制解调器送至用户的个人计算机。 因此,图1-12中(1)空缺处传输的是数字信号。

单项选择题
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ple — their clothes, ideas, or languages—has changed as much as what they eat. The original chocolate drink was made from the seeds of the cocoa tree(可可树)by South American. Indians. The Spanish introduced it to the rest of the world during the 1500’s. And although it was very expensive, it quickly became fashionable. In London, shops where chocolate drinks were served became important meeting places. Some still exist today.

The potato is also from the New World. Around 1600, the Spanish brought it from Peru to Eu??rope, where it soon was widely grown. Ireland became so dependent on it that thousands of Irish peo??ple starved when the crop failed during the "Potato Famine (饥荒)" of 1845—1846, and thousands more were forced to leave their homeland and move to America.

There are many other foods that have traveled from South America to the Old World. But some others went in the opposite direction. Brazil is now the world’s largest grower of coffee, and coffee is an important crop in Colombia and other South American countries. But it is native to Ethiopia, a country in Africa. It was first made into a drink by Arabs during the 1400’s.

according to an. Arabic legend, coffee was discovered when a person named Kaldi noticed that his goats were attracted to the red berries on a coffee, bush. He tried one and experienced the " wide-awake" feeling that one third of the world’s population now starts the day with.

小题1:according to the passage, which of the following has changed the most in the last 500 years?

A.Food.

B.Clothing.

C.Ideology.

D.Language.小题2:The word "some" in the last sentence of the first paragraph refers to________.

A.some cocoa trees

B.some chocolate drinks.

C.some shops

D.some South American Indians小题3:Thousands of Irish people starved during the " Potato Famine" because________

A.they were so dependent on potatoes that they refused to eat anything else

B.they were forced to leave their homeland and move to America

C.the weather conditions in Ireland were not suitable for growing potatoes

D.the potato harvest was bad小题4:according to the passage, which of the following statements is NOT true?

A.One third of the world’s population drinks coffee.

B.Coffee is native to Colombia.

C.Coffee can keep one awake.

D.Coffee drinks were first made by Arabs.