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1、UNIT 16 Introduction to Computer NetworksnNew words and Technical Terms peripheral congestion topology dismally mainframe bus dart muscle Ethernet terminal circularly bidirectional token nText During the 1950s,most computers were During the 1950s,most computers were similar in one respect.They had a

2、 main memory,similar in one respect.They had a main memory,a central processing unit(CPU),and peripherals.a central processing unit(CPU),and peripherals.The memory and CPU were central to the The memory and CPU were central to the system.Since then a new generation of system.Since then a new generat

3、ion of computing has emerged in which computation computing has emerged in which computation and data storage need not be centralized.A user and data storage need not be centralized.A user may retrieve a program from one place,run it on may retrieve a program from one place,run it on any of a variet

4、y of processors,and send the any of a variety of processors,and send the result to a result to a third location.third location.A system connecting different devices such as A system connecting different devices such as PCs,printers,and disk drives is a network.PCs,printers,and disk drives is a netwo

5、rk.Typically,each device in a network serves a Typically,each device in a network serves a specific purpose for one or more individuals.For specific purpose for one or more individuals.For example,a PC may sit on your desk providing example,a PC may sit on your desk providing access to information o

6、r software you need.A PC access to information or software you need.A PC may also be devoted to managing a disk drive may also be devoted to managing a disk drive containing shared files.We call it a file server.containing shared files.We call it a file server.Often a network covers a small geograph

7、ic area Often a network covers a small geographic area and connects devices in a single building or and connects devices in a single building or group of buildings.Such a network is a local area group of buildings.Such a network is a local area network(LAN).A network that covers a larger network(LAN

8、).A network that covers a larger area such as a municipality,state,country,or the area such as a municipality,state,country,or the world is called a wide area network(WAN).world is called a wide area network(WAN).Generally speaking,most networks may Generally speaking,most networks may involve many

9、people using many PCs,each of involve many people using many PCs,each of which can access any of many printers or servers.which can access any of many printers or servers.With all these people accessing information,With all these people accessing information,their requests inevitably will conflictth

10、eir requests inevitably will conflict1 1.Consequently,the devices must be connected inConsequently,the devices must be connected in a way that permits an orderly transfer of a way that permits an orderly transfer of information for all concerned.A good analogy is information for all concerned.A good

11、 analogy is a street layout in a large city.With only one a street layout in a large city.With only one person driving it matters little where the streets person driving it matters little where the streets are,which ones are one-way,where the traffic are,which ones are one-way,where the traffic sign

12、als are,or how they are synchronized.But signals are,or how they are synchronized.But with thousands of the cars on the streets during with thousands of the cars on the streets during the morning rush hour,a bad layout will create the morning rush hour,a bad layout will create congestion that causes

13、 major delays.The same is congestion that causes major delays.The same is true of computer networks.They must be true of computer networks.They must be connected in a way that allows data to travel connected in a way that allows data to travel among many users with little or no delay.We call among m

14、any users with little or no delay.We call the connection strategy the network topology.the connection strategy the network topology.The best topology depends on the types of The best topology depends on the types of devices and user needs.What works well for one devices and user needs.What works wel

15、l for one group may perform dismally for another.group may perform dismally for another.Fig.4-16.A common bus topology.Some common network topologies are described Some common network topologies are described as following.as following.Fig.4-16 shows a common bus topology(or Fig.4-16 shows a common b

16、us topology(or simply bus topology)connecting devices such as simply bus topology)connecting devices such as workstations,mainframes,and file servers.They workstations,mainframes,and file servers.They communicate through a single bus(a collection communicate through a single bus(a collection of para

17、llel lines).A common approach gives each of parallel lines).A common approach gives each device an interface that listens to the bus and device an interface that listens to the bus and examines its data traffic.If an interface examines its data traffic.If an interface determines that data are destin

18、ed for the device determines that data are destined for the device it serves,it reads the data from the bus and it serves,it reads the data from the bus and transfers it to the device.Similarly,if a device transfers it to the device.Similarly,if a device wants to transmit data,the interface circuit

19、wants to transmit data,the interface circuit sense when the bus is empty and then transmit sense when the bus is empty and then transmit data.This is not unlike waiting on a freeway data.This is not unlike waiting on a freeway entrance ramp during rush hour.You sense an entrance ramp during rush hou

20、r.You sense an opening and either quickly dart to it or muscleopening and either quickly dart to it or muscle your way through,depending on whether youre your way through,depending on whether youre driving a subcompact or a large truck.driving a subcompact or a large truck.Sometimes,two devices try

21、to transmit Sometimes,two devices try to transmit simultaneously.Each one detects an absence of simultaneously.Each one detects an absence of traffic and begins transmitting before becoming traffic and begins transmitting before becoming aware of the other devices transmission.The aware of the other

22、 devices transmission.The result is a collision of signals.As the devices result is a collision of signals.As the devices transmit they continue to listen to the bus and transmit they continue to listen to the bus and detect the noise resulting from the collisions.detect the noise resulting from the

23、 collisions.When a device detects a collision it stops When a device detects a collision it stops transmitting,waits a random period of time,and transmitting,waits a random period of time,and tries again.This process,called Carrier Sense,tries again.This process,called Carrier Sense,Multiple Access

24、with Collision Detection Multiple Access with Collision Detection (CSMA/CD)will be discussed later.(CSMA/CD)will be discussed later.One popular common bus network is an One popular common bus network is an Ethernet.Its common bus typically is Ethernet Ethernet.Its common bus typically is Ethernet ca

25、ble,which consists of copper,optical fiber,or cable,which consists of copper,optical fiber,or combinations of both.Its design allows combinations of both.Its design allows terminals,PCs,disk storage systems,and office terminals,PCs,disk storage systems,and office machines to communicate.A major adva

26、ntage of machines to communicate.A major advantage of an Ethernet is the ability to add new devices to an Ethernet is the ability to add new devices to the network easily.the network easily.Another common connecting arrangement is Another common connecting arrangement is the star topology,shown in F

27、ig.4-17.It uses a the star topology,shown in Fig.4-17.It uses a central computer that communicates with other central computer that communicates with other devices in the network.Control is centralized;if devices in the network.Control is centralized;if device wants to communicate,it does so only de

28、vice wants to communicate,it does so only through the central computer.The computer,in through the central computer.The computer,in turn,routes the data to its destination.turn,routes the data to its destination.Centralization provides a focal point forCentralization provides a focal point for respo

29、nsibility,an advantage of the star topology.responsibility,an advantage of the star topology.The bus topology,however,has some advantages The bus topology,however,has some advantages over a star topology.The lack of central control over a star topology.The lack of central control makes adding new de

30、vices easy because no makes adding new devices easy because no device needs to be aware of others.In addition,device needs to be aware of others.In addition,the failure or removal of a device in a bus the failure or removal of a device in a bus network does not cause the network to fail.In a network

31、 does not cause the network to fail.In a star topology,the failure of the central computer star topology,the failure of the central computer brings down the entire network.brings down the entire network.Fig.4-17.The star topology.Star topologies often involve a single Star topologies often involve a

32、 single mainframe computer that services many mainframe computer that services many terminals and secondary devices.With terminals and secondary devices.With appropriate terminal emulation software,PCs appropriate terminal emulation software,PCs can communicate with the mainframe.Data can communicat

33、e with the mainframe.Data transfers between terminals or between transfers between terminals or between terminals and storage devices occur only terminals and storage devices occur only through the main computer.through the main computer.In a ring topology shown in Fig.4-18,devices In a ring topolog

34、y shown in Fig.4-18,devices are connected circularly.Each one can are connected circularly.Each one can communicate directly with either or both of its communicate directly with either or both of its neighbors but nobody elseneighbors but nobody else2 2.If it wants to.If it wants to communicate with

35、 a device farther away,it sends communicate with a device farther away,it sends a message that passes through each device in a message that passes through each device in between.between.Fig.4-18.A ring topology.A ring network may be either unidirectional or A ring network may be either unidirectiona

36、l or bidirectional.Unidirectional means that all bidirectional.Unidirectional means that all transmissions travel in the same direction.Thus,transmissions travel in the same direction.Thus,each device can communicate with only one each device can communicate with only one neighbor.Bidirectional mean

37、s that data neighbor.Bidirectional means that data transmissions travel in either direction,that is,a transmissions travel in either direction,that is,a device can communicate with both neighbors.device can communicate with both neighbors.Ring topologies such as IBMs token ring Ring topologies such

38、as IBMs token ring network often connect PCs in a single office or network often connect PCs in a single office or department.Applications from one PC thus can department.Applications from one PC thus can access data stored on others without requiring a access data stored on others without requiring

39、 a mainframe to coordinate communicationsmainframe to coordinate communications3 3.Instead,communications are coordinated by Instead,communications are coordinated by passing a token among all the stations in the passing a token among all the stations in the ring.A station can send something only wh

40、en it ring.A station can send something only when it receives the token.receives the token.A disadvantage of the ring topology is that A disadvantage of the ring topology is that when one station sends to another,all stations in when one station sends to another,all stations in between are involved.

41、More time is spent between are involved.More time is spent relaying messages meant for others than in,for relaying messages meant for others than in,for example,a bus topologyexample,a bus topology4 4.Moreover,the.Moreover,the failure of one station causes a break in the ring failure of one station

42、causes a break in the ring that affects communications among all the that affects communications among all the stations.stations.Many computer networks are combinations of Many computer networks are combinations of various topologies.Fig.4-19 shows a possible various topologies.Fig.4-19 shows a poss

43、ible bination.Fig.4-19.Combinations of various topologies.n信息類國內(nèi)、外重要學(xué)術(shù)期刊 一般認(rèn)為，信息學(xué)科主要包括電工、電機、一般認(rèn)為，信息學(xué)科主要包括電工、電機、電子、通信、計算機、自動化等學(xué)科，下面列出電子、通信、計算機、自動化等學(xué)科，下面列出 了國內(nèi)、外信息類主要的學(xué)術(shù)期刊。了國內(nèi)、外信息類主要的學(xué)術(shù)期刊。國內(nèi)信息類學(xué)術(shù)期刊 國內(nèi)重要學(xué)術(shù)期刊一般由中國科學(xué)院、科技國內(nèi)重要學(xué)術(shù)期刊一般由中國科學(xué)院、科技部、教育部以及各專業(yè)學(xué)會主辦，如部、教育部以及各專業(yè)學(xué)會主辦，如自動化學(xué)自動化學(xué)報報由中國自動化學(xué)會主辦、由中國自動化學(xué)會主辦、計算

44、機學(xué)報計算機學(xué)報由由 中國計算機學(xué)會主辦等。中國計算機學(xué)會主辦等。1 1中國電機工程學(xué)中國電機工程學(xué)報報 2 2 電工技術(shù)學(xué)報電工技術(shù)學(xué)報 3 3 電力系統(tǒng)自動化電力系統(tǒng)自動化 4 4 電網(wǎng)技術(shù)電網(wǎng)技術(shù) 5 5 電氣自動化電氣自動化 6 6 電氣傳動電氣傳動 7 7 中小型電機中小型電機 8 8 電工技術(shù)雜志電工技術(shù)雜志 9 9 微電機微電機 10 10 微特電機微特電機 11 11 高電壓技術(shù)高電壓技術(shù) 12 12 電測與儀表電測與儀表 表4-1.電工類國內(nèi)主要學(xué)術(shù)期刊1 1 電子學(xué)報電子學(xué)報 2 2 通信學(xué)報通信學(xué)報 3 3 中國激光中國激光 4 4 電子與信息學(xué)報電子與信息學(xué)報 5 5

45、半導(dǎo)體學(xué)報半導(dǎo)體學(xué)報 6 6 紅外與微米波學(xué)報紅外與微米波學(xué)報 7 7 電子技術(shù)應(yīng)用電子技術(shù)應(yīng)用 8 8 電波科學(xué)學(xué)報電波科學(xué)學(xué)報 9 9 電路與系統(tǒng)學(xué)報電路與系統(tǒng)學(xué)報 10 10 微電子學(xué)微電子學(xué) 1111微波學(xué)報微波學(xué)報 12 12 固體電子學(xué)研究與進展固體電子學(xué)研究與進展13 13 真空科學(xué)與技術(shù)學(xué)報真空科學(xué)與技術(shù)學(xué)報 14 14 量子電子學(xué)報量子電子學(xué)報 1515激光與光電子學(xué)進展激光與光電子學(xué)進展 16 16 紅外與激光工程紅外與激光工程 表4-2.電子、通信類國內(nèi)主要學(xué)術(shù)期刊 1 1 自動化學(xué)報自動化學(xué)報 2 2 控制理論與應(yīng)用控制理論與應(yīng)用 3 3 控制與決策控制與決策 4 4

46、信息與控制信息與控制 5 5 系統(tǒng)工程與電子技術(shù)系統(tǒng)工程與電子技術(shù) 6 6 機器人機器人 7 7 系統(tǒng)仿真學(xué)報系統(tǒng)仿真學(xué)報 8 8 系統(tǒng)工程學(xué)報系統(tǒng)工程學(xué)報 9 9 模式識別與人工智能模式識別與人工智能 10 10 儀器儀表學(xué)報儀器儀表學(xué)報 1111電機與控制學(xué)報電機與控制學(xué)報 12 12 電力系統(tǒng)及其自動化學(xué)報電力系統(tǒng)及其自動化學(xué)報 13 13 數(shù)據(jù)采集與處理數(shù)據(jù)采集與處理 14 14 傳感器學(xué)報傳感器學(xué)報 表4-3.自動化、控制類國內(nèi)主要學(xué)術(shù)期刊1 1 計算機學(xué)報計算機學(xué)報 2 2 軟件學(xué)報軟件學(xué)報 3 3 計算機研究與發(fā)展計算機研究與發(fā)展 4 4 小型微型計算機系統(tǒng)小型微型計算機系統(tǒng) 5

47、 5 計算機輔助設(shè)計與圖形計算機輔助設(shè)計與圖形學(xué)學(xué)報學(xué)學(xué)報 6 6 中國圖像圖形學(xué)報中國圖像圖形學(xué)報 7 7 計算機集成制造系統(tǒng)計算機集成制造系統(tǒng)-CIMS CIMS 8 8 計算機科學(xué)計算機科學(xué) 9 9 遙感學(xué)報遙感學(xué)報 10 10 計算機工程計算機工程 11 11 計算機應(yīng)用表計算機應(yīng)用表12 12 計算機應(yīng)用研究計算機應(yīng)用研究 13 13 計算機工程與設(shè)計計算機工程與設(shè)計 14 14 計算機應(yīng)用與軟件計算機應(yīng)用與軟件 1515計算機工程與應(yīng)用計算機工程與應(yīng)用 16 16 微型計算機微型計算機 表4-4.計算機類國內(nèi)主要學(xué)術(shù)期刊 國外信息類學(xué)術(shù)期刊 國外信息類學(xué)術(shù)期刊眾多，除鼎鼎大名的國外

48、信息類學(xué)術(shù)期刊眾多，除鼎鼎大名的ScienceScience與與NatureNature外，主要以美國外，主要以美國IEEEIEEE會會 刊、刊、ACMACM會刊、英國會刊、英國IEEIEE會刊為主。會刊為主。表表4-5.4-5.電信類國外主要學(xué)術(shù)期刊電信類國外主要學(xué)術(shù)期刊1 IEEE Transactions on Communications 1 IEEE Transactions on Communications 2 IEEE Transactions on Wireless 2 IEEE Transactions on Wireless CommunicationsCommunica

49、tions3 IEEE Transactions on Circuits and 3 IEEE Transactions on Circuits and SystemsSystems4 IEEE Transactions on Signal Processing4 IEEE Transactions on Signal Processing5 European Transactions on Telecommunications5 European Transactions on Telecommunications6 European Transactions on Telecommunic

50、ations 6 European Transactions on Telecommunications and Related Technologiesand Related Technologies7 IEEE Transactions on Mobile Computing7 IEEE Transactions on Mobile Computing8 IEEE Wireless Communications8 IEEE Wireless Communications9 IEEE Communications Letters9 IEEE Communications Letters10

51、IEEE Communications Magazine10 IEEE Communications Magazine11 IEEE Signal Processing Magazine11 IEEE Signal Processing Magazine12 IEE Proceedings-Communications12 IEE Proceedings-Communications1 IEEE Transactions on Automatic control1 IEEE Transactions on Automatic control2 Automatic2 Automatic3 Int

52、ernational Journal of Control3 International Journal of Control4 System&Control Letters4 System&Control Letters5 IEEE Transactions on Control Systems Technology5 IEEE Transactions on Control Systems Technology6 IEEE Control Systems Magazine6 IEEE Control Systems Magazine7 IEEE Transactions on System

53、s Man and Cybernetics Part B-7 IEEE Transactions on Systems Man and Cybernetics Part B-CyberneticsCybernetics8 IEE Proceedings-Control Theory and Applications 8 IEE Proceedings-Control Theory and Applications 9 IEEE Transactions on Robotics and Automation9 IEEE Transactions on Robotics and Automatio

54、n10 IEEE Transactions on Industrial Electronics10 IEEE Transactions on Industrial Electronics11 IEEE Transactions on Fuzzy Systems11 IEEE Transactions on Fuzzy Systems12 IEEE Intelligent Systems12 IEEE Intelligent Systems表4-6 自動化、控制類國外主要學(xué)術(shù)期刊1 Computers&Structures1 Computers&Structures 2 IEEE Softwar

55、e2 IEEE Software3 Computers3 Computers4 IEEE Transactions on Computers4 IEEE Transactions on Computers5 IEEE Transactions on Software Engineering5 IEEE Transactions on Software Engineering6 ACM Computing Surveys6 ACM Computing Surveys7 ACM Transactions on Computers Systems7 ACM Transactions on Compu

56、ters Systems8 ACM Transactions on Database Systems8 ACM Transactions on Database Systems9 ACM Transactions on Information Systems9 ACM Transactions on Information Systems10 ACM Transactions on Mathematical Software10 ACM Transactions on Mathematical Software11 ACM Transactions on Programming Languag

57、es and Systems11 ACM Transactions on Programming Languages and Systems12 ACM Transactions on Software Engineering and Methodology12 ACM Transactions on Software Engineering and Methodology表4-7 計算機類國外主要學(xué)術(shù)期刊1 IEEE Electron Device Letters1 IEEE Electron Device Letters2 IEEE Transactions on Instrumentat

58、ion and 2 IEEE Transactions on Instrumentation and MeasurementMeasurement3 IEEE Transactions on Microwave Theory and 3 IEEE Transactions on Microwave Theory and TechniquesTechniques4 IEEE Transactions on Electron Devices4 IEEE Transactions on Electron Devices5 IEEE Transactions on Power Delivery5 IE

59、EE Transactions on Power Delivery6 IEEE Transactions on Power Systems6 IEEE Transactions on Power Systems7 IEEE Microwave and Wireless Components Letters7 IEEE Microwave and Wireless Components Letters表4-8 電工、電子類國外主要學(xué)術(shù)期刊 8 IEEE Transactions on Antennas and Propagation8 IEEE Transactions on Antennas

60、and Propagation9 IEE Proceedings-Radar and Navigation9 IEE Proceedings-Radar and Navigation10 IEE Proceedings Vision Image and Signal 10 IEE Proceedings Vision Image and Signal ProcessingProcessing11 IEE Proceedings Microwaves Antennas and 11 IEE Proceedings Microwaves Antennas and PropagationPropag

61、ation12 IEE Proceedings Electric Power Applications12 IEE Proceedings Electric Power Applications13 IEE Proceedings Circuits Devices and Systems13 IEE Proceedings Circuits Devices and Systems14 IEE-Electronics Letters14 IEE-Electronics LettersnNotes 1 With all these people accessing information,1 Wi

62、th all these people accessing information,their requests inevitably will conflict.their requests inevitably will conflict.在這么多人存取信息的情況下，不可避免地會產(chǎn)在這么多人存取信息的情況下，不可避免地會產(chǎn) 生沖突。生沖突。本 句 中 的本 句 中 的With With 表 示表 示“在在的 情 況 下的 情 況 下”，而，而accessing informationaccessing information是現(xiàn)在分詞短語，修飾前面的是現(xiàn)在分詞短語，修飾前面的 these

63、these people people。backback 2 Each one can communicate directly with 2 Each one can communicate directly with either or both of its neighbors but nobody else.either or both of its neighbors but nobody else.每一個設(shè)備只能與它相鄰的一邊或兩邊之間進行每一個設(shè)備只能與它相鄰的一邊或兩邊之間進行 的通信，而不能與此外其他設(shè)備直接通信。的通信，而不能與此外其他設(shè)備直接通信。backback 3 A

64、pplications from one PC thus can access 3 Applications from one PC thus can access data stored on others without requiring a data stored on others without requiring a mainframe to coordinate communications.mainframe to coordinate communications.一個一個PCPC上的應(yīng)用程序可以訪問其他機器上的數(shù)上的應(yīng)用程序可以訪問其他機器上的數(shù) 據(jù)，而用不著大型機對通信

65、進行協(xié)調(diào)。據(jù)，而用不著大型機對通信進行協(xié)調(diào)。stored on others stored on others 是過去分詞短語，作定語修飾它前是過去分詞短語，作定語修飾它前面 的 名 詞面 的 名 詞“數(shù) 據(jù)數(shù) 據(jù)”，而，而 without requiring a without requiring a mainframe to coordinate communications mainframe to coordinate communications 表示條件。表示條件。backback 4 More time is spent relaying messages meant 4 Mor

66、e time is spent relaying messages meant for others than in,for example,a bus topology.for others than in,for example,a bus topology.這樣，比起總線拓?fù)鋪?，要花更多的時間來為其這樣，比起總線拓?fù)鋪恚ǜ嗟臅r間來為其 他的站點轉(zhuǎn)發(fā)數(shù)據(jù)。他的站點轉(zhuǎn)發(fā)數(shù)據(jù)。本句中，本句中，relaying messages relaying messages 是一個現(xiàn)在分詞短語作是一個現(xiàn)在分詞短語作狀語，表示原因；而狀語，表示原因；而 meant for others meant for others 是一個過去是一個過去 分詞短語，修飾分詞短語，修飾messagesmessages。back