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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 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 generation 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 variety 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 network.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 or 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 geographic 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).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 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 conflicttheir 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 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 signals 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 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 many 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 well 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 bus 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 parallel 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 destined 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 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 hour.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 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 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 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 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 cable,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 advantage 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 Fig.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 device 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 responsibility,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 devices 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 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 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 communicate 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 topology 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 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 unidirectional 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 means 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 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 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 when 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.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 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 possible bination.Fig.4-19.Combinations of various topologies.n信息类国内、外重要学术期刊 一般认为，信息学科主要包括电工、电机、一般认为，信息学科主要包括电工、电机、电子、通信、计算机、自动化等学科，下面列出电子、通信、计算机、自动化等学科，下面列出 了国内、外信息类主要的学术期刊。了国内、外信息类主要的学术期刊。国内信息类学术期刊 国内重要学术期刊一般由中国科学院、科技国内重要学术期刊一般由中国科学院、科技部、教育部以及各专业学会主办，如部、教育部以及各专业学会主办，如自动化学自动化学报报由中国自动化学会主办、由中国自动化学会主办、计算机学报计算机学报由由 中国计算机学会主办等。中国计算机学会主办等。1 1中国电机工程学中国电机工程学报报 2 2 电工技术学报电工技术学报 3 3 电力系统自动化电力系统自动化 4 4 电网技术电网技术 5 5 电气自动化电气自动化 6 6 电气传动电气传动 7 7 中小型电机中小型电机 8 8 电工技术杂志电工技术杂志 9 9 微电机微电机 10 10 微特电机微特电机 11 11 高电压技术高电压技术 12 12 电测与仪表电测与仪表 表4-1.电工类国内主要学术期刊1 1 电子学报电子学报 2 2 通信学报通信学报 3 3 中国激光中国激光 4 4 电子与信息学报电子与信息学报 5 5 半导体学报半导体学报 6 6 红外与微米波学报红外与微米波学报 7 7 电子技术应用电子技术应用 8 8 电波科学学报电波科学学报 9 9 电路与系统学报电路与系统学报 10 10 微电子学微电子学 1111微波学报微波学报 12 12 固体电子学研究与进展固体电子学研究与进展13 13 真空科学与技术学报真空科学与技术学报 14 14 量子电子学报量子电子学报 1515激光与光电子学进展激光与光电子学进展 16 16 红外与激光工程红外与激光工程 表4-2.电子、通信类国内主要学术期刊 1 1 自动化学报自动化学报 2 2 控制理论与应用控制理论与应用 3 3 控制与决策控制与决策 4 4 信息与控制信息与控制 5 5 系统工程与电子技术系统工程与电子技术 6 6 机器人机器人 7 7 系统仿真学报系统仿真学报 8 8 系统工程学报系统工程学报 9 9 模式识别与人工智能模式识别与人工智能 10 10 仪器仪表学报仪器仪表学报 1111电机与控制学报电机与控制学报 12 12 电力系统及其自动化学报电力系统及其自动化学报 13 13 数据采集与处理数据采集与处理 14 14 传感器学报传感器学报 表4-3.自动化、控制类国内主要学术期刊1 1 计算机学报计算机学报 2 2 软件学报软件学报 3 3 计算机研究与发展计算机研究与发展 4 4 小型微型计算机系统小型微型计算机系统 5 5 计算机辅助设计与图形计算机辅助设计与图形学学报学学报 6 6 中国图像图形学报中国图像图形学报 7 7 计算机集成制造系统计算机集成制造系统-CIMS CIMS 8 8 计算机科学计算机科学 9 9 遥感学报遥感学报 10 10 计算机工程计算机工程 11 11 计算机应用表计算机应用表12 12 计算机应用研究计算机应用研究 13 13 计算机工程与设计计算机工程与设计 14 14 计算机应用与软件计算机应用与软件 1515计算机工程与应用计算机工程与应用 16 16 微型计算机微型计算机 表4-4.计算机类国内主要学术期刊 国外信息类学术期刊 国外信息类学术期刊众多，除鼎鼎大名的国外信息类学术期刊众多，除鼎鼎大名的ScienceScience与与NatureNature外，主要以美国外，主要以美国IEEEIEEE会会 刊、刊、ACMACM会刊、英国会刊、英国IEEIEE会刊为主。会刊为主。表表4-5.4-5.电信类国外主要学术期刊电信类国外主要学术期刊1 IEEE Transactions on Communications 1 IEEE Transactions on Communications 2 IEEE Transactions on Wireless 2 IEEE Transactions on Wireless CommunicationsCommunications3 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 Telecommunications 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 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 International 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 Systems 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 Automation10 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 自动化、控制类国外主要学术期刊1 Computers&Structures1 Computers&Structures 2 IEEE Software2 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 Computers 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 Languages 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 计算机类国外主要学术期刊1 IEEE Electron Device Letters1 IEEE Electron Device Letters2 IEEE Transactions on Instrumentation 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 IEEE 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 电工、电子类国外主要学术期刊 8 IEEE Transactions on Antennas and Propagation8 IEEE Transactions on Antennas 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 PropagationPropagation12 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 With all these people accessing information,their requests inevitably will conflict.their requests inevitably will conflict.在这么多人存取信息的情况下，不可避免地会产在这么多人存取信息的情况下，不可避免地会产 生冲突。生冲突。本 句 中 的本 句 中 的With With 表 示表 示“在在的 情 况 下的 情 况 下”，而，而accessing informationaccessing information是现在分词短语，修饰前面的是现在分词短语，修饰前面的 these 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.每一个设备只能与它相邻的一边或两边之间进行每一个设备只能与它相邻的一边或两边之间进行 的通信，而不能与此外其他设备直接通信。的通信，而不能与此外其他设备直接通信。backback 3 Applications 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.一个一个PCPC上的应用程序可以访问其他机器上的数上的应用程序可以访问其他机器上的数 据，而用不着大型机对通信进行协调。据，而用不着大型机对通信进行协调。stored on others stored on others 是过去分词短语，作定语修饰它前是过去分词短语，作定语修饰它前面 的 名 词面 的 名 词“数 据数 据”，而，而 without requiring a without requiring a mainframe to coordinate communications mainframe to coordinate communications 表示条件。表示条件。backback 4 More time is spent relaying messages meant 4 More time is spent relaying messages meant for others than in,for example,a bus topology.for others than in,for example,a bus topology.这样，比起总线拓扑来，要花更多的时间来为其这样，比起总线拓扑来，要花更多的时间来为其 他的站点转发数据。他的站点转发数据。本句中，本句中，relaying messages relaying messages 是一个现在分词短语作是一个现在分词短语作状语，表示原因；而状语，表示原因；而 meant for others meant for others 是一个过去是一个过去 分词短语，修饰分词短语，修饰messagesmessages。back
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