计算机网络课后习题习题1、2
第一章习题
5.A factor in the delay of a store-and-forward packet-switching system is how long it takes to store and forward a packet through a switch. If switching time is 10 μc, is this likely to be a major factor in the respon of a client-rver system where the client is in New York and the rver is in California? Assume the propagation speed in copper and fiber to be 2/3 the speed of light in vacuum.（问：交换时间是否是延迟的主要因素）
Solution：
5. No. The speed of propagation is 200,000 km/c or 200 meters/c. In 10
c the signal travels 2 km. Thus, each switch adds the equivalent of 2 km
of extra cable. If the client and rver are parated by 5000 km, traversing
even 50 switches adds only 100 km to the total path, which is only 2%. Thus,
switching delay is not a major factor under the circumstances.
13. What is the principal difference between connectionless communication and
connection-oriented communication?（13．在无连接通讯和面向连接通讯两者之间，最主要的区别是什么？）
Solution：
会计述职报告13. Connection-oriented communication has three phas. In the establishment
pha a request is made to t up a connection. Only after this pha has been
successfully completed can the data transfer pha be started and data transported.
Then comes the relea pha. Connectionless communication does not have the phas. It just nds the data.
解答：
面向连接通信有三个阶段，在使用网络服务之前必须建立一个连接，只有在成功建立连接后，才可以通信。通信完成后还要进入释放连接阶段。
无连接通信在发送报文前并不建立连接，它没有面向连接通信的三个阶段。
金鞭岩
18.Which of the OSI layers handles each of the following:win10怎么卸载软件
a. Dividing the transmitted bit stream into frames.
b. Determining which route through the subnet to u. （18．OSI模型中哪一层处理以下问题：
（A）把传输的位流分成帧；
（B）在通过子网的时候决定使用哪条路由路径。）
Solution：
18. (a) Data link layer. (b) Network layer.
中秋吃什么
解答：
（A）数据链路层
（B）网络层
21.List two ways in which the OSI reference model and the TCP/IP reference model are the same. Now list two ways in which they differ.
Solution：
21. Both models are bad on layered protocols. Both have a network, transport, and application layer. In both models, the transport rvice can provide a reliable end-to-end byte stream. On the other hand, they differ in veral ways. The number of layers is different, the TCP/IP does not have ssion or prentation layers, OSI does not support internetworking, and OSI has both connection-oriented and connectionless rvice in the network layer.
22.What is the main difference between TCP and UDP?
（22．TCP 和UDP 之间最主要的区别是什么?）
Solution：碗莲种子
22. TCP is connection oriented, whereas UDP is a connectionless rvice.
解答：
最主要的区别在于它们的可靠性与是否建立连接。TCP 是一个可靠的，面向连接的协议；而UDP则是一个不可靠的，无连接的协议。
26.Why does ATM u small, fixed-length cells?
（26．ATM 为什么使用小的，固定长度的信元？）
Solution：
26. Small, fixed-length cells can be routed through switches quickly, and completely in hardware. Small, fixed-size cells also make it easier to build hardware that handles many
cells in parallel. Also, they do not block transmission lines for very long, making it easier to provide
quality-of-rvice guarantees.
解答：
因为信元是由硬件来完成的，之所以采用固定长度的信元，主要原因有：
（1）容易设计出由硬件路由器来处理的短的，固定长度的信元。
（2）最小的信元不会阻塞线路很长时间。
（3）保证不会按错序交换信元。
28. An image is 1024 x 768 pixels with 3 bytes/pixel. Assume the image is uncompresd. How long does it take to transmit it over a 56-kbps modem channel? Over a 1-Mbps cable modem? Over a 10-Mbps Ethernet? Over 100-Mbps Ethernet?
便宜行事
幼儿保健知识（28．一幅图像分辨率为1024*768 像素，每个像素用三字节表示。假设该图像没有没压缩。请问，通过56K 的调制解调器信道来传输这幅图像需要多长时间？通过1M 的电缆调制解调器？通过10M的以太网呢？通过100M 的以太网呢？）
28. The image is 1024 ? 768 ? 3 bytes or 2,359,296 bytes. This is 18,874,368优昙花
bits. At 56,000 bits/c, it takes about 337.042 c. At 1,000,000 bits/c, it
takes about 18.874 c. At 10,000,000 bits/c, it takes about 1.887 c. At
100,000,000 bits/c, it takes about 0.189 c.
解答：
该图像占1024*768*3=2359296 个字节。传输时间如下：
56,000 bits/s 337.042 s
1,000,000 bits/s 18.874 s
10,000,000 bits/s 1.887 c.
100,000,000 bits/s 0.189 c.
第二章：
6.What is the difference between a passive star and an active repeater in a fiber network? Solution：
6. A passive star has no electronics. The light from one fiber illuminates a
number of others. An active repeater converts the optical signal to an electrical
one for further processing.
9.Is the Nyquist theorem true for optical fiber or only for copper wire?
Solution：
9. The Nyquist theorem is a property of mathematics and has nothing to do with technolo
gy. It says that if you have a function who Fourier spectrum does not contain any sines or cosines above f, then by sampling the function at a frequency of 2f you capture all the information there is. Thus, the Nyquist theorem is true for all media.
