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1、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 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
2、 California? Assume the propagation speed in copper and fiber to be 2/3 the speed of light in vacuum. Solution： 5. 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 serve
3、r 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.13. What is the principal difference between connectionless communication and connection- oriented communication? Soluti
4、on： 13. Connection-oriented communication has three phases. In the establishment phase a request is made to set up a connection. Only after this phase has been successfully completed can the data transfer phase be started and data transported. Then comes the release phase. Connectionless communicati
5、on does not have these phases. It just sends the data.18. Which of the OSI layers handles each of the following: a. Dividing the transmitted bit stream into frames. b. Determining which route through the subnet to use. Solution： 18. (a) Data link layer. (b) Network layer.21. List two ways in which t
6、he 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 based on layered protocols. Both have a network, transport, and application layer. In both models, the transport service can provide a reliable end-to-end byte st
7、ream. On the other hand, they differ in several ways. The number of layers is different, the TCP/IP does not have session or presentation layers, OSI does not support internetworking, and OSI has both connection-oriented and connectionless service in the network layer.22. What is the main difference
8、 between TCP and UDP? Solution： 22. TCP is connection oriented, whereas UDP is a connectionless service.26. Why does ATM use small, fixed-length cells? Solution： 26. Small, fixed-length cells can be routed through switches quickly, and completely in hardware. Small, fixed-size cells also make it eas
9、ier 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-service guarantees.6. What is the difference between a passive star and an active repeater in a fiber network? Solution： 6. A passive star has n
10、o 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 a
11、nd has nothing to do with technology. It says that if you have a function whose 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.29. Why has the
12、 PCM sampling time been set at 125 sec? Solution： 29. A sampling time of 125 sec corresponds to 8000 samples per second. According to the Nyquist theorem, this is the sampling frequency needed to capture all the information in a 4 kHz channel, such as a telephone channel.(Actually the nominal bandwi
13、dth is somewhat less, but the cutoff is not sharp.)22. An IP packet to be transmitted by Ethernet is 60 bytes long, including all its headers. If LLC is not in use, is padding needed in the Ethernet frame, and if so, how many bytes? Solution: 22. The minimum Ethernet frame is 64 bytes, including bot
14、h addresses in the Ethernet frame header, the type/length field, and the checksum. Since the header fields occupy 18 bytes and the packet is 60 bytes, the total frame size is 78 bytes, which exceeds the 64-byte minimum. Therefore, no padding is used.24. Some books quote the maximum size of an Ethern
15、et frame as 1518 bytes instead of 1500 bytes. Are they wrong? Explain your answer. Solution: The payload is 1500 bytes, but when the destination address, source address, type/length, and checksum fields are counted too, the total is indeed 1518.42Briefly describe the difference between store-and-for
16、ward and cut-through switches. Solution: A store-and-forward switch stores each incoming frame in its entirety, then examines it and forwards it. A cut-through switch starts to forward incoming frames before they have arrived completely. As soon as the destination address is in, the forwarding can begin.43Store-and-forward switches have an advantage over cut-through switches with respect to damaged frames. Explain what it is. Solution: 43. Store-and-fo
