问题 单项选择题

在以下网络应用中,要求带宽最高的应用是

A.可视电话
B.数字电视
C.拨号上网
D.收发邮件

答案

参考答案:B

解析:

[分析]:
题目是求四种网络应用的带宽要求,下面我们来了解一下这四种应用。
可视电话是一种新型的高科技电子产品,结构轻巧、使用灵活。目前市场上主要有支持公用交换电话网(PSTN)和支持综合业务数字网(ISDN)两种可视电话。PSTN是安装在普通电话线上实现的业务,带宽为64Kb/s,由于采用模拟线路传输,模拟话路带宽的限制和压缩技术的原因,经过传输后带宽只有30Kb/s左右,静止图像还可以令人接受,活动图像会出现马赛克等现象。ISDN可视电话带宽为128Kb/s,较PSTN可视电话对活动图像传输有所改善,图像接续较快,但价格是PSTN可视话机的2倍。
数字电视就是将传统的模拟电视信号经过抽样、量化和编码转换成用二进制数代表的数字式信号,然后进行各种功能的处理、传输、存储和记录,也可以用电子计算机进行处理、监测和控制。采用数字技术不仅使各种电视设备获得比原有模拟式设备更高的技术性能,而且还具有模拟技术不能达到的新功能,使电视技术进入崭新时代。在传统的模拟电视中,模拟全电视信号通过调制在无线电射频载波上发送出去。广播信道可以是地面广播、有线电视网或卫星广播。数字电视则是将电视信号进行数字化采样,其信号的数据率是很高的,演播室质量的数字化电视信号的数据率在200Mb/s。要在原模拟电视频道带宽内传输如此高速率的数字信号是不可能的,因此,要用到数据压缩技术。但在压缩之后,带宽仍需3~40Mb/s。
拨号上网大家应该比较熟悉,一般的传输带宽为14.4Kb/s、28.8Kb/s和56Kb/s。
收发邮件在拨号上网的条件下就可进行,对网络带宽没什么要求。
从上面的分析我们可以看出,数字电视是带宽要求最高的应用。

填空题
单项选择题

Now which are the animals really to be pitied in captivity First, those clever beings whose lively urge for activity can find no outlet behind the bars of the cage. This is most conspicuous, even for the uninitiated, in the case of animals which, when living in a free state, are accustomed to roaming about widely. Owing to this frustrated desire, foxes and wolves housed, in many old-fashioned zoos, in cages which are far too small, are among the most pitiable of all caged animals.
Though pinioned swans generally seem happy, under proper care, by hatching and rearing their young without any trouble, at migration time things become different: they repeatedly swim to the lee side of the pond, in order to have the whole extent of its surface at their disposal, trying to take off. Again and again the grand preparations end in a pathetic flutter of their half wings ; a truly sorry picture !
This, however, rarely awakens the pity of the zoo visitor, least of all when such an originally highly intelligent and mentally alert animal has deteriorated, in confinement, into a crazy idiot, a very caricature of its former self. Sentimental old ladies, the fanatical sponsors of the societies for Prevention of Cruelty to Animals, have no compunction in keeping a grey parrot in a relatively small cage or even chained to a perch. Together with the large corvines, the parrots are probably the only birds which suffer from that state of mind, common to prisoners, namely, boredom.

What does "the uninitiated" mean

A.People who visit animals in urban zoos.

B.People who do not like animals of the wild.

C.People who know little about a certain topic.

D.People who do not visit zoos every year.