假设数据帧出错的概率为p，应答帧不会出现错误，允许重传的次数不受限制；成功发送两个数据帧之间的最小间隔时间为tT。此时链路的最大吞吐量λmax=______。
A．p(1-p)/2tT
B．(1-p)/(2p·tT)
C．2p/((1-p)·tT)
D．(1-p)/tT
参考答案：D
解析： 在题意所描述的情况下，正确传送一帧所需的平均时间tav=tT(1+一个帧的平均重传次数)。由以上新题4和新题5考点分析可知，每个帧正确到达接收方的概率P与出错的概率p是对立事件，即P+p=1，则一个帧的平均重传次数E=1/P=1/(1-p)。进而可得，tav=tT/(1-p)。
当传输差错率P增大时，平均时间tav也增大；当无差错(p=0)时，tav=tT。
每秒成功发送的最大数据帧就是链路的最大吞吐量λmax=1/tav=(1-p)/tT。
| 阅读理解。 |
| Babies love chocolate and sometimes they also eat the paper around it. My cat enjoys a meal of good, thick paper, old letters, for example. She doesn't like newspapers very much. Of course, the best paper comes from wood. Wood comes from trees and trees are plants. Vegetables and fruit are plants too, and we eat a lot of them. So can we also eat wood and paper? Scientists say, "All food comes in some way from plants." Well, is that true? Animals eat grass and grow fat. Then we eat their meat. Little fish eat sea-plants; then bigger fish swim along and eat the... Chickens eat bits of grass and give us...Think for a minute. What food not come from plants in some way? Scientists can do wonderful things with plants. They can make food just like meat and cheese (奶酪). And they can make it without the help of animals. It is very good food too. Now they have begun to say, "We make our paper from wood. We can also make food from wood, the next thing is not very difficult." What is the next thing? Perhaps it is food from paper. Scientists say, "We can turn paper into food. It will be good, cheap food too; cheaper than meat or fish or eggs." So please keep your old books and letters. (Don't feed them to your cat) One day, soon, they will be on your plate. There is nothing like a good story for breakfast. |
| 1. What does the best paper come from? |
[ ] |
| A. Vegetables. B. Food. C. Plants. D. Wood. |
| 2. From the passage, we can infer (推断) that _______ do not come from plants in some way. |
[ ] |
| A. few kinds of food B. meat and fish C. cheese and chickens D. wood and paper |
| 3. What's the main idea of this passage? |
[ ] |
| A. All food comes from plants in some way. B. It will not be long before food can be made from paper. C. Scientists can do wonderful things with plants. D. Scientists can make food without the help of animals. |
| 4. The writer asks us to keep our old books and letters because _______. |
[ ] |
| A. they are useful for reading B. they may be used to feed cats C. we can make food from them soon D. we can read them before meals |
| 5. The best title (题目) for the passage is _________. |
[ ] |
| A. Food From Plants B. Plants, Animals and Food C. Keep Your Books and Letters D. What Can We Make with Old Paper? |