问题 问答题

【说明】
非对称数字用户线(ADSL)是接入Internet的一种宽带技术,图7-7为一台带网卡的PC机采用ADSL方式接入Internet的网络结构图。现在比较成熟的ADSL标准有两种:G.DMT和G.Lite。其中,G.DMT标准支持的最高上行、下行速率分别为1.5Mb/s、8Mb/s,G.Lite标准支持的最高上行、下行速率分别为 512Kb/s、1.5Mb/s。


【问题4】
ADSL MODEM利用数字信号处理器技术将大量的数据压缩到双绞铜质电话线上,再应用转换器、分频器、模/数转换器等组件来进行处理。目前,我国采用的是离散多音频(DMT)调制技术,其频谱分布如图7-8所示。
请将“上行信道”、“下行信道”、“话音信道”等术语填入图7-8中(A)~(C)空缺处的对应位置。

答案

参考答案:(A)话音信道 (B)上行信道
(C)下行信道

解析:这是一道要求读者掌握ADSL宽带接入技术中离散多音频(DMT)调制原理的分析理解题。本题所涉及的知识点如下。
离散多音频(DMT)调制技术核心思想是:将整个传输频带分成若干子信道,每个子信道对应不同频率的载波,在不同载波上分别进行正交幅度调制(QAM),不同信道上传输的信息容量(即每个载波调制的数据信号)根据当前子信道的传输性能决定。
DMT调制技术是一种多载波调制技术,它采用频分复用的方法将0~1.104MHz的带宽分割成256个子通道(tone),每个子通道最多可以进行15bit的星位图调制。子信道0用于传输语音信号;子信道1~子信道5用于分隔语音信道和数据信道;子信道6~子信道31用于上行数据传输,子通道33~子信道255用于下行数据传输。
由以上分析可知,在图7-8的频谱分布图中,0~4kHz频段供传统电话信号使用,即(A)空缺处可填入“语音信道”。由“频率”横向坐标的箭头方向可知,(B)空缺处可填入“上行信道”,(C)空缺处可填入“下行信道”。
由于DMT技术的每个子通道可以独立进行调制和传输,因此基于该技术的ADSL可以有较强的抗噪声的能力。DMT调制技术具有较高的带宽利用率、可实现动态带宽分配、较强的抗窄带噪声能力、较强的抗脉冲噪声能力等优点。

阅读理解

A friend of mine named Paul received an expensive car from his brother as a Christmas present.On Christmas Eve when Paul came out of his office,a street urchin (顽童)was walking around the shining car. “Is this your car,Paul?”he asked.

Paul answered,“Yes,my brother gave it to me for Christmas.”The boy was surprised.“You mean your brother gave it to you and it didn’t cost you nothing?Boy,I wish…”He hesitated(犹豫).

Of course Paul knew what he was going to wish for.He was going to wish he had a brother like that. But what the boy said surprised Paul greatly.

“I wish,”the boy went on,“that I could be a brother like that.”Paul looked at the boy in surprise, then he said again, "Would you like to take a ride in my car?”

“Oh yes,I’d love that.”

After a short ride,the boy turned and with his eyes shining,said,“Paul,would you mind driving in front of my house?”

Paul smiled a little.He thought he knew what the boy wanted.He wanted to show his neighbors that he could ride home in a big car. But Paul was wrong again. "Will you stop where those two steps are? the boy asked.

He ran up to the steps. Then in a short while Paul heard him coming back, but he was not coming fast. He was carrying his little crippled (残疾的)brother. He sat him down on the step and pointed to the car.

“There she is,Buddy,just like I told you upstairs. His brother gave it to him for Christmas and it didn’t cost him a cent. And some day I’m going to give you one just like it…then you can see for yourself all the nice things in the Christmas windows that I’ve been trying to tell you about.”

Paul got out and lifted the boy to the front seat of his car. The shining-eyed older brother climbed in beside him and the three of them began an unforgettable holiday ride.

小题1:The street urchin was very surprised when ________.

A.Paul received an expensive car

B.Paul told him about the car

C.he saw the shining car

D.he was walking around the car小题2:From the story we can see the urchin ________.

A.wished to give his brother a car

B.wanted Paul’s brother to give him a car

C.wished he could have a brother like Paul’s

D.wished Paul could be a brother like that小题3:The urchin asked Paul to stop his car in front of his house ________.

A.to show his neighbors the big car

B.to show he had a rich friend

C.to let his brother ride in the car

D.to tell his brother about his wish小题4:We can infer(推断)from the story that ________.

A.Paul couldn’t understand the urchin

B.the urchin had a deep love for his brother

C.the urchin wished to have a rich brother

D.the urchin’s wish came true in the end小题5:The best name of the name story is _________.

A.A Christmas Present

B.A Street Urchin

C.A Brother Like That

D.An Unforgettable Holiday Ride

单项选择题