计算机网络第一章作业参考答案.docx

1、计算机网络第一章作业参考答案计算机网络第一章作业参考答案2. The performance of a client-server system is strongly influenced by two major network characteristics: the bandwidth of the network (that is, how many bits/sec it can transport) and the latency (that is, how many seconds it takes for the first bit to get from the clien

2、t to the server). Give an example of a network that exhibits high bandwidth but also high latency. Then give an example of one that has both low bandwidth and low latency. Answer: ,1,、If the distance is too far if bandwidth is very big, but the delay is also very obvious, such as the transatlantic c

3、able. ,2,、On the other hand, if just transport in a building even if bandwidth is very low, we can receive message immediately. 3. 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

4、traffic? (ii) video traffic? (iii) financial transaction traffic? Answer: ,P5557, ,i,、Sound transmission requires a corresponding fixed time slot, so the arrival of information is very important。 ,ii,、Video request is smooth, therefore the packet loss rate is very important ,iii,、Financial business

5、flow needs to be real-time and high safety requirement, so need to check the error rate。 10. What are two reasons for using layered protocols? What is one possible disadvantage of using layered protocols? Answer: Through the layered protocol can make the design problem is divided into smaller manage

6、able parts. A layer protocol change has no impact on the other layer protocol. Disadvantage: Each layer will process the information, finally may cause delay or distortion of information. There are some duplicated functions in different layers, for example, DLL and TL. 11. what is the principal diff

7、erence between connectionless communication and connection-oriented communication,Give one example of a protocol that uses (i)connectionless communication; (ii)connection-oriented communication. Answer:,P57-59, First: connection oriented communication is divided into three phases, the first is to es

8、tablish a connection, in this stage, sends a request for establishing a connection. Only when the connection is successfully established, to begin transmission of data, which is the second phase .Then, when the data has been transmitted, to release the connection. Without the connection of communica

9、tion not so much stage, it direct data transmission. Second: connection-oriented communication with data of keeping order, and connectionless communications cannot be guaranteed to receive data and send data in order. Example, Video call is a connection oriented communication, Mail is a connectionle

10、ss communication。 14. In Fig. 1-19, a service is shown. Are any other services implicit in this figure? If so, where? If not, why not? Answer: Service is provided by the k layer to K + 1 layer. Services must have the lower K is provided, namely, the K service is provided by k- 1 layer. 16. which of

11、the OSI layers and TCP/IP layers handles each of the following: (a)Dividing the transmitted bit stream into frames. (b)Determining which route through the subnet to use. Answer, (a)、Data link layer in OSI model and Link Layer in TCP model (b)、Network layer in OSI model and Internet Layer in TCP mode

12、l 17. If the unit exchanged at the data link level is called a frame and the unit exchanged at the network level is called a packet, do frames encapsulate packets or do packets encapsulate frames? Explain your answer. Answer, Frames encapsulate packets. When a packet arrives at the data link layer,

13、the entire packet, including packet head, and all of the content data, are used as the frame data area. Or, the whole package put in an envelope (frame), (if can be loaded.). 18. A system has an n-layer protocol hierarchy. Applications generate messages of length M bytes. At each of the layers, an h

14、-byte header is added. What fraction of the network bandwidth is filled with headers? Answer: hn/(hn+m)*100% . 19. 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. Answer: Similarities: Both are independent of the pro

15、tocol stack concept; layers functions are similar too. Differences: OSI better distinguish between the service, interface and the concept of agreement, so than TCP/IP has better conceal, can make it easier to replace; OSI is first the concept of some model, and then to the realization of the agreeme

16、nt, and TCP/IP is the first agreement, and then establish the description of the agreement of the model. Number level difference: TCP/IP doesnt have session layer and presentation layer, OSI does not support network interconnection. OSI in network layer support connectionless and connection-oriented

17、 communication, and in the transport layer only connection-oriented communication, and TCP/IP network layer in only one kind of communication mode (connectionless), but in the transport layer support two modes. 20. What is the main difference between TCP and UDP? Answer: TCP is a connection-oriented

18、, while UDP is a datagram service. TCP is reliable , while UDP is unreliable. 25. An image is 1024768 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 Ethern

19、et? Over gigabit Ethernet? Answer: The image is 1024 768 3 bytes =2359296 bytes= 18874368 bits. According to T=C/BW In 56000 bits/sec speed, transmission requires 337.042 seconds. In 1000000 bits/sec , about 18.874 seconds. In 10000000 bits/sec , about 1.8874 seconds. In 100000000 bits/sec , about 0

20、.18874 second. 28. List two advantages and two disadvantages of having international standards for network protocols. Answer: Advantages: 1、 if everyone uses standard, so everyone can with any other people; 2、 extensive use of standards will lead to economies of scale, such as the production of larg

21、e scale integrated circuit chip. Disadvantages , 1、 in order to achieve standardization often lead to poor standards; 2、 once the standard is widely used, it will be very difficult to make a change, even if found new and better methods or technologies, also hard to replace. 29. Suppose the algorithm

22、s used to implement the operations at layer k is changed. How does this impact operations at layers k ? 1 and k + 1? Answer: No effect. The internal processing of each layer is independent. 30. Suppose there is a change in the service (set of operations) provided by layer k. How does this impact ser

23、vices at layers k-1 and k+1? Answer: The K+1 layer Will be affected, because the services are provided by the lower to upper layer, such as k layer provided services to k+1 layer,and k layers service is provided by the k-1 layer. 33. Make a list of activities that you do every day in which computer

24、networks are used. How would your life be altered if these networks were suddenly switched off ? Answer: 1、Chat on the net; 2、Query data; 3、Transaction processing; 4、Receive and send E-mail; 5、Listening to music、See a movie If these networks were suddenly switched off, I think I will have to do a lo

25、t of things face to face, and cant do many things. 37. The Internet is made up of a large number of networks. Their arrangement determines the topology of the Internet. A considerable amount of information about the Internet topology is available on line. Use a search engine to find out more about t

26、he Internet topology and write a short report summarizing your findings. Answer: 1、Computer connection way is called network Topology (Topology). Network topology is refers with the transmission media interconnection all sorts of equipment of the physical layout, especially computer distribution pos

27、ition so as to how and cable through them. Design a network, should be based on the actual conditions of choosing the correct topology mode. Each topology has its own advantages and disadvantages. 2、classification, ,1,、Star structure Star structure is to point to each workstation By way of star conn

28、ection into network. networkA central node, other node (workstation, server) and the central node directly connected, this kind of structure with the central node as the center, so also known as Centralized network. It has the following characteristics: simple structure, easy to management ; Control

29、 is simple and convenient, easy to meshing; Network delay time is lesser, transmission error is relatively low. But the Ability is poor. shortcoming is obvious: the high cost and low reliability, Resources sharingStar Topological structureThe main advantages are: 1. Easy to management maintenance; 2

30、. Reconfigure flexible; 3. Convenient fault detection and isolation ,2,、Ring structure Ring structure by network In some node Through the point-to-point link end to form a closed ring, this kind of structure make public transmission cable composition ring type joint, the data in the loop, along the

31、a direction in each node transmission, information from one node to another node. Ring structureHas the following features: information flow in the net is along the fixed direction, two nodes only a road, the simplified path Choice of control ; Loop on the node are bootstrap control, the control sof

32、tware Simple; Because information source in the loop is serial through each node, when ring node too much, will be affected Information transmission rate, and make the network response time extension; Loop is closed, not easy to expand, Reliability is low, a node failure, will cause paralysis which; Maintenance difficult, to branch node fault location More difficult. The advantages and disadvantages of ring topology is: advantage is because each node are at