电子信息工程专业英语导论》PART4u.ppt

UNIT 16 Introduction to Computer Networks,New words and Technical Terms peripheral congestion topology dismally mainframe bus dart muscle Ethernet terminal circularly bidirectional token,Text During the 1950s, most computers were similar in one respect. They had a main memory, a central processing unit (CPU), and peripherals. The memory and CPU were central to the system. Since then a new generation of computing has emerged in which computation and data storage need not be centralized. A user may retrieve a program from one place, run it on any of a variety of processors, and send the result to a third location. A system connecting different devices such as PCs, printers, and disk drives is a network. Typically, each device in a network serves a specific purpose for one or more individuals. For,example, a PC may sit on your desk providing access to information or software you need. A PC may also be devoted to managing a disk drive containing shared files. We call it a file server. Often a network covers a small geographic area and connects devices in a single building or group of buildings. Such a network is a local area network (LAN). A network that covers a larger area such as a municipality, state, country, or the world is called a wide area network (WAN). Generally speaking, most networks may involve many people using many PCs, each of which can access any of many printers or servers. With all these people accessing information, their requests inevitably will conflict1. Consequently, the devices must be connected in,a way that permits an orderly transfer of information for all concerned. A good analogy is a street layout in a large city. With only one person driving it matters little where the streets are, which ones are one-way, where the traffic signals are, or how they are synchronized. But with thousands of the cars on the streets during the morning rush hour, a bad layout will create congestion that causes major delays. The same is true of computer networks. They must be connected in a way that allows data to travel among many users with little or no delay. We call the connection strategy the network topology. The best topology depends on the types of devices and user needs. What works well for one group may perform dismally for another.,Fig. 4-16. A common bus topology.,Some common network topologies are described as following. Fig. 4-16 shows a common bus topology (or simply bus topology) connecting devices such as workstations, mainframes, and file servers. They communicate through a single bus (a collection of parallel lines). A common approach gives each device an interface that listens to the bus and examines its data traffic. If an interface determines that data are destined for the device it serves, it reads the data from the bus and transfers it to the device. Similarly, if a device wants to transmit data, the interface circuit sense when the bus is empty and then transmit data. This is not unlike waiting on a freeway entrance ramp during rush hour. You sense an opening and either quickly dart to it or muscle,your way through, depending on whether youre driving a subcompact or a large truck. Sometimes, two devices try to transmit simultaneously. Each one detects an absence of traffic and begins transmitting before becoming aware of the other devices transmission. The result is a collision of signals. As the devices transmit they continue to listen to the bus and detect the noise resulting from the collisions. When a device detects a collision it stops transmitting, waits a random period of time, and tries again. This process, called Carrier Sense, Multiple Access with Collision Detection (CSMA/CD) will be discussed later.,One popular common bus network is an Ethernet. Its common bus typically is Ethernet cable, which consists of copper, optical fiber, or combinations of both. Its design allows terminals, PCs, disk storage systems, and office machines to communicate. A major advantage of an Ethernet is the ability to add new devices to the network easily. Another common connecting arrangement is the star topology, shown in Fig. 4-17. It uses a central computer that communicates with other devices in the network. Control is centralized; if device wants to communicate, it does so only through the central computer. The computer, in turn, routes the data to its destination. Centralization provides a focal point for,responsibility, an advantage of the star topology. The bus topology, however, has some advantages over a star topology. The lack of central control makes adding new devices easy because no device needs to be aware of others. In addition, the failure or removal of a device in a bus network does not cause the network to fail. In a star topology, the failure of the central computer brings down the entire network.,Fig. 4-17. The star topology.,Fig. 4-18. A ring topology.,A ring network may be either unidirectional or bidirectional. Unidirectional means that all transmissions travel in the same direction. Thus, each device can communicate with only one neighbor. Bidirectional means that data transmissions travel in either direction, that is, a device can communicate with both neighbors. Ring topologies such as IBMs token ring network often connect PCs in a single office or department. Applications from one PC thus can access data stored on others without requiring a mainframe to coordinate communications3. Instead, communications are coordinated by,passing a token among all the stations in the ring. A station can send something only when it receives the token. A disadvantage of the ring topology is that when one station sends to another, all stations in between are involved. More time is spent relaying messages meant for others than in, for example, a bus topology4. Moreover, the failure of one station causes a break in the ring that affects communications among all the stations. Many computer networks are combinations of various topologies. Fig. 4-19 shows a possible combination.,Fig. 4-19. Combinations of various topologies.,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、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。 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、科技部、教育部以及各专业学会主办，如自动化学报由中国自动化学会主办、计算机学报由 中国计算机学会主办等。,表4-1. 电工类国内主要学术期刊,表4-2. 电子、通信类国内主要学术期刊,表4-3. 自动化、控制类国内主要学术期刊,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Science与Nature外，主要以美国IEEE会 刊、ACM会刊、英国IEE会刊为主。 表4-5. 电信类国外主要学术期刊,表4-6 自动化、控制类国外主要学术期刊,表4-7 计算机类国外主要学术期刊,表4-8 电工、电子类国外主要学术期刊,Notes 1 With all these people accessing information, their requests inevitably will conflict. 在这么