电子信息类专业英语(西电第二版)Unit15DataCommunications课件.ppt
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- 电子信息 类专业 英语 第二 Unit15DataCommunications 课件
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1、Unit 15 Data CommunicationsUnit 15 Data CommunicationsPassage A Data Communications Passage B Characteristics of Communication Channels Passage C Communication Equipment and Software Unit 15 Data CommunicationsPassage A Data CommunicationsThe rapid growth of data communications has been influenced p
2、rimarily by the increasing need to move information to and from computers.Modern time-shared computers can communicate with many data stations simultaneously.Other forms of data transmission which the telephone and telegraph lines are called upon to handle are batch data transmission,real-time trans
3、mission(as in the case of airline reservation systems),banking and credit data,man-computer conversation with the aid of graphics,data collection systems and automatic meter reading.The combination of computers and data communication places such new requirements on the systems,which must handle them
4、,that one can refer to them better as teleprocessing rather than telecommunications.1 Unit 15 Data CommunicationsTeleprocessing started with the airline reservation systems,where the distribution of computer data is not as important as the maintenance of an inventory of seats for the whole system an
5、d the ability to access such an inventory rapidly at a number of distant points.Another example is that of the banks,who desired to centralize their accounting and provide access to a central file for all their branches.Basically,there are three reasons for the development of teleprocessing systems:
6、the requirement for centralized files,the need to distribute computer services and the advantage gained by having flexibility in location of the operating staff which utilizes the system.Unit 15 Data CommunicationsInitially,both the airline and banking operations can be established on a private basi
7、s,which would minimize the need for switching operations.But in both cases,the need to expand beyond private operations soon arises.In the case of the airlines,the need arises to access the facilities of other airlines;and in the case of the banks,certain common data facilities may be shared with ot
8、her banks.Even though private networks may suffice for many services,the need to provide data communications between urban centers leads to regional and national networks.In the U.S.,the Bell System,the independent telephone companies and specialized data communication networks are involved as carri
9、ers of such data.Unit 15 Data CommunicationsThe most applicable model for data communications requirements is the central computer conversing with many simple terminals simultaneously.Differing from telephone communications,a data communications network must be able to interconnect a wide variety of
10、 subscribers equipment.Several different types of computer peripherals may appear as network terminals,and so also can computers functioning in several different ways.2 Terminal speeds ranging from 100 bits per sec to 10,000 bits per sec are likely to be important.Many terminals in current use are c
11、onstrained by the available communications services,but many of them have variable speeds.One approach toUnit 15 Data Communicationsthis situation is to provide a buffer store at the terminal,but this approach is expensive.At present,multi-access computers usually handle terminals of only one speed
12、or with a small number of fixed speeds.It has been proposed that feedback from the output device be used to permit one simple output program to drive terminals with different characteristics.Of course,such feedback signals would have to be handled by the communications network.Unit 15 Data Communica
13、tionsAt data technology develops,each multi-access computer is connected to an increasing number of terminals,making it expensive and difficult to provide separate lines from the computer to the network for each of the terminals.Thus,a need for multiplexed connections between the computer and the te
14、rminals arises.But this need cannot necessarily be met in a straightforward manner.One characteristic of a data communications network differing from conventional telephone networks is that the two ends of the communication link are very different.One end is a computer capable of flexible behavior,a
15、nd the other end is a relatively slow terminal with no intelligence.Unit 15 Data CommunicationsSince the two ends are so different,it would appear that the network would communicate differently with them.The flow of information in a data communication system is intermittent just as it is in the case
16、 of telephone conversations.However,instead of minutes of conversation,only a few seconds will elapse for the information flow in one direction,followed by a pause and some flow of information in the opposite direction.The access times for typical computers in use at this time are about 100 ms,but t
17、his will become faster as better memories are utilized.3 The time scale for the interchange of data messages needs to be of this order.Unit 15 Data CommunicationsNOTES1 The combination of computers and data communication places such new requirements on the systems,which must handle them,that one can
18、 refer to them better as teleprocessing rather than telecommunications.计算机和数据通信的结合向系统提出必须对这些信息进行处理的新要求,以至人们最好称它们为远程信息处理,而不是远程通信。“that onetelecommunications”为结果状态从句。“which must handle them”定语从句修饰“system”。句中“place”有“放置”的含义,这里引申为“提出”。在科技英语中,有时用“one”泛指“人们”。Unit 15 Data Communications2 Several different
19、types of computer peripherals may appear as network terminals,and so also can computers functioning in several different ways.几种不同的计算机外围设备可能作为网络终端出现,几种不同方式运行的计算机也可能作为网络终端出现。“so alsodifferent ways”为并列句,该并列句主谓倒装,用于强调。句中“so”表示“appear as network terminals”,翻译时应重复叙述,使译文表达更明确。Unit 15 Data Communications3
20、The access times for typical computers in use at this time are about 100 ms,but this will become faster as better memories are utilized.目前所用的典型计算机存取时间大约为100毫秒,但当采用较好的存储器时,存取时间将会缩短。“but this willare utilized”为并列句。其中“as better memories are utilized”为时间状语从句。在科技英语中“as”运用很灵活,此句“as”作连词。Unit 15 Data Commun
21、icationsExercises 1.Translate the following sentences into English.(1)显然电流与外加电压成正比。(2)人们发现的金属都是良导体。(3)该机器的效率取决于它能运转得多快。(4)你知道电位差是什么吗?(5)正如我们将会看到的那样,速度是一个矢量。(6)这些仪器正是我们需要的。(7)导线中的电子绝不能从正流向负。(8)所有物质,不论是水、铁还是空气,都是由原子构成的。Unit 15 Data Communications2.True/False.(1)The ARPA network is a long-distance priv
22、ate data network,which can include several multi-access,time-sharing computer system for switching packets.()(2)The host can dump a complete message into the interface message processors,which can be connected by 50,000 bits per second circuits.()(3)The largest public packet-switching network,throug
23、h which 200 data bases are accessible,in the U.S.serves 250 computers,and links together computers in other countries.()(4)The public packet-switched network will be enhanced by providing for connection between the public telephone network and the packet network.()Unit 15 Data Communications3.Answer
24、 the questions.(1)What are the differences between packet-switching network and conventional store-and-forward network?(2)What is the function of the ARPA network?Unit 15 Data CommunicationsPassage B Characteristics of Communication ChannelsData communications is the electronic transmission of infor
25、mation including data,television pictures,sound and facsimiles.It usually involves a computer,a modem,software,and a printer.With this equipment,you can communicate with a friend in St.Louis,Missouri,or Paris,France,sending and receiving anything from a manuscript to a simple message over the teleph
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