電子信息類專業(yè)英語第三章

1、Unit Three Data CommunicationsUnit Three Data CommunicationsPassage A Data Communications Passage B Characteristics of Communication Channels Passage C Communication Equipment and Software Unit Three Data CommunicationsPassage A Data CommunicationsThe rapid growth of data communications has been inf

2、luenced primarily 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, rea

3、l-time transmission (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, whic

4、h must handle them, that one can refer to them better as teleprocessing rather than telecommunications.1 Unit Three 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

5、for the whole system and 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

6、 of teleprocessing systems: 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 Three Data CommunicationsInitially, both the airline and banking operations can

7、 be established on a private basis, 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 comm

8、on data facilities may be shared with other 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

9、 communication networks are involved as carriers of such data.Unit Three 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 netw

10、ork must be able to interconnect a wide variety of 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

11、 to be important. Many terminals in current use are constrained by the available communications services, but many of them have variable speeds. One approach to this situation is to provide a buffer store at the terminal, but this approach is expensive. At present, multi-access computers usually han

12、dle terminals of only one speed 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 communicat

13、ions network.Unit Three Data CommunicationsAt 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 c

14、onnections between the computer and the terminals 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

15、 is a computer capable of flexible behavior, and the other end is a relatively slow terminal with no intelligence. Since 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 jus

16、t as it is in the case of telephone conversations. Unit Three Data CommunicationsHowever, 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 typ

17、ical computers in use at this time are about 100 ms, but this 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 Three Data CommunicationsKEY WORDS reservation 保留，預(yù)定graphics 圖示，圖解inventory 清單，報表teleprocessing 遠程(信息)

18、處理，遙控處理constrain 約束，強迫multi-access 多路存取，多路進入 feedback 反饋，回授intelligence 智能，智力，信息intermediate 中間的elapse (時間)經(jīng)過，消失Unit Three Data Communications be called upon to 用來，(被)要求with the aid of 借助于，用，通過appear as 作為出現(xiàn)be likely to 可能，大約，像要Unit Three Data CommunicationsNOTES1 The combination of computers and da

19、ta communication places such new requirements on the systems, which must handle them, that one can refer to them better as teleprocessing rather than telecommunications.計算機和數(shù)據(jù)通信的結(jié)合向系統(tǒng)提出必須對這些信息進行處理的新要求，以至人們最好稱它們?yōu)檫h程信息處理，而不是遠程通信?！皌hat onetelecommunications”為結(jié)果狀態(tài)從句。“which must handle them”定語從句修飾“system”

20、。 句中“place”有“放置”的含義，這里引申為“提出”。 在科技英語中，有時用“one”泛指“人們”。Unit Three Data Communications2 Several different types of computer peripherals may appear as network terminals, and so also can computers functioning in several different ways.幾種不同的計算機外圍設(shè)備可能作為網(wǎng)絡(luò)終端出現(xiàn)，幾種不同方式運行的計算機也可能作為網(wǎng)絡(luò)終端出現(xiàn)?！皊o alsodifferent ways”

21、為并列句，該并列句主謂倒裝，用于強調(diào)。 句中“so”表示“appear as network terminals”，翻譯時應(yīng)重復(fù)敘述，使譯文表達更明確。Unit Three Data Communications3 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.目前所用的典型計算機存取時間大約為100毫秒，但當(dāng)采用較好的存儲器時，存取時間將會縮短?！癰ut this w

22、illare utilized”為并列句。其中“as better memories are utilized”為時間狀語從句。 在科技英語中“as”運用很靈活，此句“as”作連詞。Unit Three Data CommunicationsEXERCISES1. Fill in the blanks with the best choice.(1) The types of data packet are mode data from digital terminals and _ mode data from other sources.a. digital, non digital b.

23、 store-and-forward, non store-and-forwardc. packet, non packet d. computer, non computer(2) As compared with circuit switching, the packet switching has the advantage when is cheap.a. transmission medium b. computing powerc. buffer storage d. processing powerUnit Three Data Communications(3) The mai

24、n elements in ARPA network are and .a. interface message processors, hostsb. local terminals, IMPsc. multi-access computers, peripheral equipment d. hosts, sharing computersUnit Three Data Communications2. True/False.(1) The ARPA network is a long-distance private data network, which can include sev

25、eral 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, through which 200 data bases are acce

26、ssible, 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 Three Data Communications3. Answer the questions.(1) Wh

27、at are the differences between packet-switching network and conventional store-and-forward network?(2) What is the function of the ARPA network?Unit Three Data CommunicationsPassage B Characteristics of Communication ChannelsData communications is the electronic transmission of information including

28、 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 telephone

29、 lines. Using the same method, a home-bound child can interact with a teacher in the classroom, an office worker can work at home, and a doctor can access a remote computer for research data.Unit Three Data CommunicationsThe reasons for using communications system are convincing. It is expedient and

30、 efficient, lessens job stress, decreases car pollution, saves time and money, allows the home to serve as an office, and promotes distant learning in which students can share information and computer research findings.Unit Three Data CommunicationsWhen you connect one computer to another, you use h

31、ardware and software. In the majority of cases, the hardware consists of equipment that sends the data over some type of communications line, such as a telephone line. The software controls the flow of this data. The necessary hardware consist of a modem and telephone lines.The modem modulates the c

32、omputer output to an acceptable signal for transmission and then demodulates the signal back for computer input. The modem on the transmitting computer converts the digital signals to modulated analog signal tones and transmits them over the telephone lines.1 The receiving computers modem transforms

33、 the incoming analog signals back to their digital equivalents in order to understand them.Unit Three Data CommunicationsTransmission RateThe transmission rate of a communications channel is determined by its bandwidth and its speed. The bandwidth is the range of frequencies that a channel can carry

34、. Since transmitted data can be assigned to different frequencies, the wider the bandwidth, the more frequencies, and the more data can be transmitted at the same time.Unit Three Data CommunicationsThe speed at which data is transmitted is usually expressed as bits per second or as a baud rate. Bits

35、 per second (bps) is the number of bits that can be transmitted in one second. Using a 10-bit byte to represent a character (7 data bits,1 start,1 stop, and 1 parity bit), a 2,400 bps transmission would transmit 240 characters per second. At this rate, a 20-page, single-spaced report would be transm

36、itted in approximately five minutes. The baud rate is the number of times per second that signal being transmitted changes. With each change, one or more bits can be transmitted. At speeds up to 2,400 bps, usually only one bit is transmitted per signal change and, thus, the bits per second and the b

37、aud rate are the same. To achieve speeds in excess of 2,400 bps, more than one bit is transmitted with each signal change and, thus, the bps will exceed the baud rate.Unit Three Data CommunicationsDirection of TransmissionThe direction of data transmission is classified as either simplex, half-duple

38、x, or full-duplex. In simplex transmission, data flows in one direction only. Simplex is used only when the sending device, such as a temperature sensor, never requires a response from the computer. For example, if a computer is used to control the temperature of a building, numerous sensors are pla

39、ced throughout it. Each sensor is connected to the computer with a simplex transmission line because the computer only needs to receive data from the temperature sensors and does not need to send data back to the sensors. 2 Unit Three Data CommunicationsIn half-duplex transmission, data can flow in

40、both directions but in only one direction at a time. An example is a citizens band radio. The user can talk or listen but not do both at the same time. Half-duplex is often used between terminals and a central computer.In full-duplex transmission, data can be sent in both directions at the same time

41、. A normal telephone line is an example of full-duplex transmission.3 Both parties can talk at the same time. Full-duplex transmission is used for most interactive computer applications and for computer-to-computer data transmission.Unit Three Data CommunicationsTransmission Modes: Asynchronous and

42、SynchronousIn asynchronous transmission mode (Figure 3.1), individual characters (made up of bits) are transmitted at irregular intervals, for example, when a user enters data. To distinguish where one character stops and another starts, the asynchronous communication mode uses a start and a stop bi

43、t. An additional bit called a parity bit is sometimes included at the end of each character, and they detect if one of the data bits has been changed during transmission. The asynchronous transmission mode is used for lower speed data transmission and is used with most communications equipment desig

44、ned for personal computers.Unit Three Data CommunicationsAsynchronous Transmission Mode CHARACTER IDLE CHARACTER IDLE CHARACTER IDLE CHARACTER IDLE Synchronous Transmission Mode START BYTES CHARACTER CHARACTER CHARACTER ERROR CHARACTER STOP BYTES IDLE Figure3.1Unit Three Data CommunicationsIn the sy

45、nchronous transmission mode (Figure 3.1), large blocks of data are transmitted at regular intervals. Timing signals synchronize the communications equipment at both the sending and receiving ends and eliminate the need for start and stop bits for each character. Error-checking bits and start and end

46、 indicator called sync bytes are also transmitted. Synchronous transmission requires more sophisticated and expensive equipment, but it does give much higher speeds and accuracy than asynchronous transmission.Unit Three Data CommunicationsKEY WORDS bps (bits per second) 每秒鐘傳輸?shù)谋忍豣aud rate 波特率simplex

47、transmission 單工傳輸half-duplex transmission 半雙工傳輸full-duplex transmission 全雙工傳輸asynchronous transmission 異步傳輸 synchronous transmission 同步傳輸Unit Three Data CommunicationsNOTES1 The modem on the transmitting computer converts the digital signals to modulated analog signal tones and transmits them over t

48、he telephone lines.發(fā)送端計算機的調(diào)制解調(diào)器把數(shù)字信號調(diào)制為模擬信號，并通過電話線發(fā)送出去。 主句結(jié)構(gòu)為“The modemconvertsand transmits”。2 Each sensor is connected to the computer with a simplex transission line because the computer only needs to receive data from the temperature sensors and does not need to send data back to the sensors.每一個

49、傳感器都通過一根單工傳輸線連接到計算機上，這是因為計算機只需要接收來自溫度傳感器的數(shù)據(jù)，而不需要向傳感器發(fā)送數(shù)據(jù)。Unit Three Data Communications3 An additional bit called a parity bit is sometimes included at the end of each charcter, and they detect if one of the data bits has been changed during transmissiom.在每個字符的后面有時還包括一個稱為奇偶校驗位的附加位，它們測試數(shù)據(jù)位在傳輸過程中是否被意外

50、改變。 they指的是a parity bit。Unit Three Data CommunicationsEXERCISES1. Fill in the blanks with the best choice.(1) The term modem is derived from the words .a. modulate emphasis b. modular emulationc. multiplex or demultiplexer d. modulate demodulate(2) Data transmission speed is measured in .a. bpsbytes

51、 per second b. bpsbits per secondc. bpsblocks per second d. bpsbuffers per second(3) The transmission method which uses start stop bits after each character is . a. synchronous b. asynchronous c. full duplex d. none of aboveUnit Three Data Communications(4) A(n) converts data from pulse form to wave

52、 form and back again.a. modem b. control unit c. channel d. interface(5) Devices or media that operate at different speeds can be synchronized by using a .a. buffer b. data set c. line d. modem(6) The transmission method which sends multiple characters in a block is .a. synchronous b. asynchronous c

53、. simplex d. half-duplex(7) In transmission, data can be sent in both directions at the same time.a. simplex b. half-duplex c. full-duplex d. none of aboveUnit Three Data Communications(8) When the sending device, such as a temperature sensor, never requires a response from the computer, we should u

54、se transmission.a. full-duplex b. half-duplex c. simplex d. none of above(9) Using a 10-bit byte to represent a character (7 data bits,1 start,1 stop, and 1 parity bit), a 2,400 bps transmission would transmit characters in two seconds.a. 120 b. 240 c. 480 d. 720(10) scans a piece of paper and conve

55、rts its image into coded form for transmission over the telephone system.a. Scanner b. Modem c. Printer d. Fax machineUnit Three Data Communications2. Answer the questions.(1) Describe the basic components of a communications system.(2) List and describe the three types of data transmission (directi

56、on) that are used.(3) Describe the transmission modes: asynchronous and synchronous.Unit Three Data CommunicationsPassage C Communication Equipment and SoftwareIf a terminal or a personal computer is within approximately 100 feet of another computer, the two devices can usually be directly connected

57、 by a scale. Over 1,000 feet, however, the electrical signal weakens to the point that some type of special communications equipment is required to increase or change the signal to transmit it farther. A variety of communications equipment exists to perform this task, but the equipment that a user i

58、s most likely to encounter is modems, fax/modems, fax machine, etc.Unit Three Data CommunicationsModemsA modem converts the digital signals of a terminal or computer to analog signals that are transmitted over a communications channel. It also converts analog signals it receives into digital signals

59、 that are used by a terminal or computer. The word modem comes from a combination of the words modulate, which means to change into a sound or analog signal, and demodulate, which means to convert an analog signal into a digital signal. A modem is needed at both the sending and receiving ends of a c

60、ommunications channel.Unit Three Data CommunicationsAn external modem is a separate, or stand-alone, device that is attached to the computer or terminal by a cable and to the telephone outlet by a standard telephone cord. An advantage of an external modem is that it can be easily moved from one term