电子信息类专业英语(西电第二版)Unit-15--Data-Communications.ppt

Unit 15 Data Communications,Unit 15 Data Communications,Passage A Data Communications,Passage B Characteristics of Communication Channels,Passage C Communication Equipment and Software,Passage A Data Communications,The rapid growth of data communications has been influenced 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,real-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,which must handle them,that one can refer to them better as teleprocessing rather than telecommunications.,1,Teleprocessing 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 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 all their branches.Basically,there are three reasons for the development 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.,Initially,both the airline and banking operations can 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 common 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 communication networks are involved as carriers of such data.,this 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 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.,At 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 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 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 just as it is in the case 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 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.,2 Several different 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。,3 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。,2.True/False.,(1)The ARPA network is a long-distance private 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,through 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.(),Passage B Characteristics of Communication Channels,Data communications is the electronic transmission of information 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 telephone 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.,The reasons for using communications system are convincing.It,is expedient and 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.,Transmission Rate,The 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.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.,The speed at which data is transmitted is usually expressed as bits per second or as a baud rate.Bits 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 transmitted 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 baud 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.,Direction of Transmission,The direction of data transmission is classified as either simplex,half-duplex,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 placed 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,In half-duplex transmission,data can flow in 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.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.,Transmission Modes:Asynchronous and Synchronous,In 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 bit.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 designed for personal computers.,In the synchronous 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 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.,NOTES,1 The modem on the transmitting computer converts the digital signals to modulated analog signal tones and transmits them over the telephone lines.发送端计算机的调制解调器把数字信号调制为模拟信号，并通过电话线发送出去。主句结构为“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.每一个传感器都通过一根单工传输线连接到计算机上，这是因为计算机只需要接收来自温度传感器的数据，而不需要向传感器发送数据。,EXERCISES,1.Fill in the blanks with the best choice.,(1)The term modem is derived from the words,.,a.modulate emphasis b.modular emulation,c.multiplex or demultiplexer d.modulate demodulate,(2)Data transmission speed is measured in,.,a.bpsbytes per second b.bpsbits per second,c.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 above,(4)A(n),converts data from pulse form to wave 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.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 above,2.Answer the questions.,(1)Describe the basic components of a communications system.,(2)List and describe the three types of data transmission(direction)that are used.,(3)Describe the transmission modes:asynchronous and synchronous.,Passage C Communication Equipment and Software,If a terminal or a personal computer is within approximately 100 feet of another computer,the two devices can usually be directly connected 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 is most likely to encounter is modems,fax/modems,fax machine,etc.,Modems,A 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 that are used by a terminal or computer.The word modem comes from a combination of the words,mo,dulate,which means to change into a sound or analog signal,and,demo,dulate,which means to convert an analog signal into a digital signal.A modem is needed at both the sending and receiving ends of a communications channel.,An 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 terminal or computer to another.,An internal modem is a circuit board that is installed inside a computer or terminal.Internal modems are generally less expensive than comparable external modems but once installed,they are not as easy as move.,Fax/Modems,Besides sending and receiving data,many modems today can answer the telephone the way an answering machine does and store the message on the hard disk.Some even have comprehensive voice-mail capabilities similar to business voice-mail systems.Furthermore,the majority of modems have facsimile(fax)capabilities;that is,they can send graphic images and text between distant locations.Such modems emulate fax machines,allowing you to receive and send fax transmissions directly via your computer.A fax/modem is different from a standard fax machine in the way it handles documents.,The fax/modem can only send documents that are already in the computer;they can display the documents on the computer screen or print them when they receive them.How does the fax/modem user fax documents not inputted into the computer?One way is to use a scanner to scan documents into the computer.Nevertheless,the traditional fax machine is more versatile than the fax/modem.It is also more expensive.,Fax Machines,The dedicated fax machine is a combination scanner,fax modem,and printer.This machine scans a piece of paper and converts its image into coded form for transmission over the telephone system.On the other end,a fax machine reconverts the transmitted code and prints out a facsimile of the original sheet of paper.,Sometimes communications equipment is preprogrammed to accomplish its designed communications tasks.Other times,the user must load a program before transmitting data.These programs,referred to as communications software,can perform a number of tasks including dialing(if a switched telephone line is used),and data encryption,etc.,Dialing software allows you to store,review,select and dial telephone numbers of computers that can be called.The software provides a variety of meaningful messages to assist you in establishing a connection before transmitting data.For example,a person who uses a personal computer at home to communicate with a computer at the office could use dialing software to establish the comm