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 client 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 cable. ,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 traffic? (ii) video traffic? (iii) financial transaction traffic? Answer: ,P55~57, ,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 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 manageable 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 difference 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 establish 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 communication 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 connectionless 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 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 model 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, 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-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 protocol stack concept; layer’s 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 agreement, and TCP/IP is the first agreement, and then establish the description of the agreement of the model. Number level difference: TCP/IP doesn’t have session layer and presentation layer, OSI does not support network interconnection. OSI in network layer support connectionless and connection-oriented 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, while UDP is a datagram service. TCP is reliable , while UDP is unreliable. 25. An image is 1024×768 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? 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 0.18874 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 large 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 algorithms 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 services 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 layer’s service is provided by the k-1 layer. 33. Make a list of activities that you do every day in which computer 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 lot of things face to face, and can’t 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 the 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 position 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 connection 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 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 sharing Star Topological structureThe main advantages are: 1. Easy to management maintenance; 2. 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 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 software 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 the same time with the two directions of the each a node is connected, to the wood than one, so the loop topology has natural fault tolerance.Defect is due to exist from two square Data flow, so we have to try to distinguish the two direction, or limit, in order to avoid unable to distinguish redundancy data flow of normal communication interference. Management and maintenance is more complex. ,3,、Total linear structure Bus structureRefers to the workstation and server all hang on a bus, each workstation equal status, no center node control, public bus information in baseband form serial transmission, its transfer direction is always from sending information node began to spread at both ends, as radio transmitting information, therefore also called GuangBoShi Computer network.eachnodeIn accepting information are address check to see if with their own workstation address conform to, conform to the receiving the information on the net. Total linear structure of network Characteristics as follows: simple structure, good scalability. When the need to increase the node, only need to be bus add a branch interface and can branch node is linked together, when the bus load not allowed, they can Expansion bus;Use cable less, and easy to install, The use of equipment relative simple, high reliability, Maintenance difficult, branch node fault find difficult. ,4,、Mixed topology structure Mixed Topological structureIs the structure or star Ring structureand Total linear structure Together network Structure, the topological structure more can satisfy the large network development, solve the radial network in the transmission distance limitations, and solved Total linear networkIn connection The number of users,Limit. Mixed topology advantages: application is quite widespread, it solved the star and bus Topology structure is insufficient, meet the actual demand of the company network. Extended quite flexible. Fast: because its backbone network using high-speed Coaxial cableorcable, so the whole network in speed should not be too many restrictions. Defect is: because of the GuangBoShi still use messaging way, so in the bus length and nodeThe quantity may be restricted. Also have a general linear network structure network rate will increase as the user decreases weaknesses . More difficult to maintain, this are mainly affected by the main line Network topologyRestriction, if bus fault, the entire network is paralyzed. ,5,、Distributed structure Distributed structure networkIs the distribution in different locations of the computer through the circuit interconnecting a network form, the distributed structure of the network has the following features: due to the distributed control, even if the whole network in a local failure, also won't affect the operation of the entire network, so it has very high reliability; Net of path choose shortest path algorithm, the online delay time less, transmission rate is high, but control complex. Each node all can be established between directly Data link, the information flow the shortest; Facilitate network-wide security resources sharing. Faults for connection line with cable long, the cost is high, Network management software Complex. Packet exchange, path selection, flow control complex. In general LAN do not use this structure. Bus topology the advantages and disadvantages of is: advantage is simple structure. Defect is the single point structure may affect the whole network. ,6,、Tree structure Tree structure is a hierarchical centralized control type network, compared with the star, its communication line length short, with low cost and easy to extend node, looking for path more convenient, but in addition to leaf node and its connected line outside, any node or its connected line fault can make system affected. Tree topology advantage is: easy to expand, Easy to isolation fault。 ,7,、Mesh topology structure In the network topological structure , network each device are a bit to point between the link connection, this kind of connection not economic, only each site will frequently send the message to use this method. It of the installation is complex, but system reliability High, fault tolerant ability. Sometimes referred to as distributed structure. ,8,、Honeycomb topological structure Honeycomb topological structure is wireless local area network (LAN) The structure of the commonly used in. It with wireless transmission medium (microwave, satellite, the infrared, etc.) point to point and multipoint transmission for feature, it is a kind of wireless networks, it is suitable for city network, campus network、Enterprise network。 In computer network have other types of Topological structure, such as total linear and radial mixing. General linear and ring type mixing connection network. In LAN, use most is a main line and star structure. 40. Try to explain encapsulation and decapsulation. Answer,,mainly from 陈敏诗, Encapsulation,is top-down process happened in the sender .In each layer,encapsulation add the specialized tail and head information to the application data. Its process: information->data->segment->packet->frame->bit The information is encapsulated to form the data ,then the data is transported to the transport layer and encapsulated as the segments, the segments are encapsulated to the packets in NL, in the packets, there is the IP information. Then the packets is sent to the data link layer and become the frame which has the MAC address, finally the frame is transformed into bits in PL. De-Encapsulation: De-Encapsulation is a down-top process happened in the receiver.The process is opposite to the encapsulation .It begins in the physical layer and then the bits transform to frames, frames is de-encapsulated to the packets, the packets is de-encapsulation to the segments, the segments transform to the data and finally the data is de-encapsulated to information. 41. try to explain peer-to-peer communication (virtual communication) Answer:,mainly from 周粤娴, The entities comprising the corresponding layers on different machines are called peers. It is peers that communicate by using the protocol to talk to each other. It seems that some layer can communicate with the same layer in other sides, which means there is a “real” channel between these correspoding peers. In the layered model, in reality, no data are directly transferred from layer n on one machine to layer n on another machine. Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached. Below layer 1 is the physical medium through which actual communication occurs. 42. tell me what you want to learn from the course? Answer: ,omit,