问题 单项选择题

在由三台交换机Switch1~Switch3两两互连,且采用“存储,转发”方式的分组交换网络中,所有链路的数据传输速率为100Mbps,分组大小为1000B,其中分组头大小为20B。若连接在交换机Switch1某个端口的主机H1向连接在交换机Switch3某个端口的主机H2发送一个大小为980000B的文件,则在不考虑分组拆装时间和传播时延的情况下,从H1发送到H2接收完为止,需要的时间至少是______ms。

A.80.00
B.80.08
C.80.16
D.80.24

答案

参考答案:C

解析: 依题意,由题干“三台交换机Switch1~Switch3两两互连”、“连接在交换机Switch1某个端口的主机H1”、“连接在交换机Switch3某个端口的主机H2”等关键信息,可以画出如图7-3所示的分组交换网络示意图。


在图7-3所示的分组交换网络中,分组大小为1000B,其中分组头大小为20B,则每个分组承载的用户数据为980B(即1000B-20B)。若主机H1向主机H2发送一个大小为980000B的文件,则需要将该报文拆分为980000B/980B=1000个分组。
在100Mbps数据传输速率的情况下,每个分组的传输时延


在不考虑分组拆装时间和传播时延的情况下,H1发送给H2的第一个分组,最短传输路径是“H1→switch1→switch2→H2”,即第一个分组需要经过3次的“存储一转发”过程,它总的传输时延为80×3μs=240μs=0.24ms。
由于在传输第一个分组的过程中,其他分组也紧随其后进行传输,即其后999个分组的每个分组传输时延为80μs,因此H2接收完这1000个分组的总时间为(80×3)+80×999μs=80+80+80×1000μs=80.16ms。

阅读理解

阅读理解。

     British paychologists have found evidence of a link between excessive (过度的) Internet use and

depression, a research has shown.

     Leeds University researchers, writing in the Psychopathology journal, said a small part of Internet

users were classed as Internet addicts and that people in this group were more likely to be depressed

than non-addicted users.

     The article on the relationship between excessive Internet use and depression is from a

questionnaire-based study of 1,319 young people and adults.

     The respondents answered questions about how much time they spent on the Internet and what they

used it for; they also complete the Beck Depression Inventory---a series of questions designed to

measure the seriousness of depression.

     The six-page report,by the university's Institute of Psychological Science, said 18 of the people who

complete the questionnaire were Internet addict."Our research indicates that excessive Internet use is

associated with depression, but what we don't know is which comes first-are depressed people drawn

to the Internet or does the Internet cause depression?" the article's lead author Dr Catriona Morrison

said."What is clear is that, for a small part of people, excessive use of the Internet could be warning signal

for depressive tendencies."

     The age range of all respondents was between 16 and 51 years, with an average age of 21.24. The

average age of the 18 Internet addicts was 18.3 years. By comparing the levels of depression within this

group to that within a group of 18 non-addicted Internet users, researchers found the Internet addicts

had a higher chance of developing depression than non-addicts. They also discovered that addicts spent

more time visiting sexually pleasing website, online gaming sites and online communities.

     "The public speculation (推测) was further proved by this study. That's to say, over-engaging in

websites which serve to replace normal social function might be linked to psychological disorders like

depression and addiction," Morrison said."We now need to consider the wider social influence of this

relationship and clearly prove the effects of excessive Internet use on mental health."

1. Internet addicts are people who ______ according to the passage.

A. use the Internet more than enough     

B. feel depressed when using the Internet

C. seldom connect to the Internet        

D. feel depressed without the Internet

2. What is confirmed by the study according to the passage?

A. Depression leads to excessive use of Internet

B. Depression results from excessive use of Internet

C. Excessive use of internet usually accompanies depression

D. Excessive use of internet is usually earlier to depression

3. It is thought by the public that online communities______.

A. can never replace normal social function

B. are intended to replace normal social function

C. are associated with psychological disorders

D. shouldn't take the blame for psychological disorders

4. According to Dr Catriona Morrison, the public speculation________.

A. lacks scientific evidence      

B. helps clarify their study

C. finds a theoretical basis      

D. has little scientific value

单项选择题