发布时间 : 星期日 文章Computer network questions更新完毕开始阅读f56bca0102768e9951e738dd
Computer network questions
1. Besides bandwidth and latency, what other parameter is needed to give a good
characterization of the quality of service offered by a network used for (i) digitized voice traffic? (ii) video traffic? (iii) financial transaction traffic?
2. 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 μ sec, is this likely to be a major factor in the response of a client-server system where the client is in New York and the server is in California? Assume the propagation speed in copper and fiber to be 2/3 the speed of light in vacuum.
No. The speed of propagation is 200,000 km/sec or 200 meters/sec. In 10sec the signal travels 2 km. Thus, each switch adds the equivalent of 2 km of extra cable. If the client and server are separated 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 these circumstances. 号传播速度为20万公里/秒或200公尺/秒。在10秒信号的传播有2公里。
因此，每个交换机增加了额外的电缆2公里的等效。如果客户端和服务器被分开 5000公里，甚至穿越50交换机只增加100公里的总路径，这是只有2％。因此， 切换时延是不是在这种情况下的主要因素。
3. A client-server system uses a satellite network, with the satellite at a height of 40,000 km.
What is the best-case delay in response to a request? 答：由于请求和应答都必须通过卫星，因此传输总路径长度为160,000千
4. What are two reasons for using layered protocols? What is one possible disadvantage of
using layered protocols?
答，通过协议分层可以把设计问题划分成较小的易于处理的片段。缺點：每一层都将处理这些信息，最终可能会导致延误或信息.这样的失真在不同的层中某些重复的功能。
5. A system has an n-layer protocol hierarchy. Applications generate messages of length M
bytes. At each of the layers, an h-byte header is added. What fraction of the network bandwidth is filled with headers? 答：Hn/(hn+m)*100%
6. Two networks each provide reliable connection-oriented service. One of them offers a
reliable byte stream and the other offers a reliable message stream. Are these identical? 答；消息和字节流是不同的。在消息流中，网络跟踪消息边界。
7. What is the main difference between TCP and UDP?
答；TCP是面向连接的，而UDP是无连接的服务。
8. When a file is transferred between two computers, two acknowledgement strategies are
possible. In the first one, the file is chopped up into packets, which are individually
acknowledged by the receiver, but the file transfer as a whole is not acknowledged. In the second one, the packets are not acknowledged individually, but the en-tire file is acknowledged when it arrives. Discuss these two approaches.
当一个文件在两台计算机之间传输的时候，可能会有两种不同的确认策略。在第一种策略中，该文件被分解成许多个分组，接收方会独立地确认每一个分组，但是文件传输过程作为整体并没有被确认。在第二种策略中，这些分组并没有被单独地确认，但是当整个文件到达的时候，它会被确认。请讨论这两种方案。
如果网络容易丢失分组，那么对每一个分组逐一进行确认较好，此时仅重传丢失的分组。而在另一方面，如果网络高度可靠，那么在不发差错的情况下，仅在整个文件传送的结尾发送一次确认，从而减少了确认的次数，节省了带宽；不过，即使有单个分组丢失，也需要重传整个文件。
9. An image is 1600 × 1200 pixels with 3 bytes/pixel. Assume the image is uncompressed. 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? Over gigabit Ethernet?
10. A noiseless 4-kHz channel is sampled every 1 msec. What is the maximum data rate? How
does the maximum data rate change if the channel is noisy, with a signal-to-noise ratio of 30 dB? 答；f=1/1msec=1000Hz没有超过带宽4KHz且是无噪信道。故由那奎斯特定律可知 最大传输速率=2H logV（位、秒）
11. What are the advantages of fiber optics over copper as a transmission medium? Is there any
downside of using fiber optics over copper?
12. The following data fragment occurs in the middle of a data stream for which the byte-stuffing
algorithm described in the text is used: A B ESC C ESC FLAG FLAG D. What is the output after stuffing?
13. A bit stream 10011101 is transmitted using the standard CRC method described in the text.
The generator polynomial is x3 + 1. Show the actual bit string transmitted. Suppose that the third bit from the left is inverted during transmission. Show that this error is detected at the receiver’s end. 答；该框架is10011101。发电机是1001。追加三个零之后该消息是10011101000。的上除以10011101000由1001余数是100，所以，发送的实际位串是10011101100。在从左侧的第三个比特的错误接收的比特流是10111101100由1001除以这产生了余数100，它是不同于零。因此，接收器检测到错误，并可以要求重发。
14. Give the picture of Timing of events in packet switching( Figure 2.43)
15. A 1-km-long, 10-Mbps CSMA/CD LAN (not 802.3) has a propagation speed of 200 m / μ sec.
Repeaters are not allowed in this system. Data frames are 256 bits long, including 32 bits of header, checksum, and other overhead. The first bit slot after a successful transmission is reserved for the receiver to capture the channel in order to send a 32-bit acknowledgement frame. What is the effective data rate, excluding overhead, assuming that there are no collisions?
16. Store-and-forward switches have an advantage over cut-through switches with respect to
damaged frames. Explain what it is.
解答：
存储-转发型交换机会对已经到达的帧进行检验，若发现帧有损坏，则将其丢弃；直通型交换机在一帧的目标头域进来时就开始转发，当发现传输帧有错误的时候，这个帧已经被传输出去了，这样就不能丢弃错误的帧了。
17. Consider the extended LAN connected using bridges B1and B2in below Figure. Suppose the
hash tables in the two bridges are empty. List all ports on which a packet will be forwarded for the following sequence of data transmissions: (a) A sends a packet to C . (b) E sends a packet to F . (c) F sends a packet to E . (d) G sends a packet to E . (e) D sends a packet to A . (f)B sends a packet to F
18. Give two example computer applications for which connection-oriented service is
appropriate. Now give two examples for which connectionless service is best. 答：
19. Give three examples of protocol parameters that might be negotiated when a connection is
set up. 答：The protocol parameters that be negotiated during the setup of a connection could be maximum packet size, maximum transmission/reception speed, and quality of servive standards.
答：表示连接建立过程中协商协议参数可能是最大数据包大小，最大传输/接收速度，以及活下去标准的质量。
20. Consider the network of Fig. 5-12(a). Distance vector routing is used, and the following
vectors have just come in to router C : from B : (5, 0, 8, 12, 6, 2); from D : (16, 12,6, 0, 9, 10); and from E : (7, 6, 3, 9, 0, 4). The cost of the links from C to B , D , and E , are 6, 3, and 5, respectively. What is C ’s new routing table? Give both the outgoing line to use and the cost.