1、Unit Three Data Communications,Passage A Data Communications Passage B Characteristics of Communication Channels Passage C Communication Equipment and Software,Passage A Data CommunicationsThe rapid growth of data communications has been influenced primarily by the increasing need to move informatio
2、n 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
3、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 te
4、leprocessing 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 d
5、istant 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: the requirement for centralized files, the need to distribute
6、 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 nee
7、d 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 nee
8、d 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.,The most applicable model for data communications requi
9、rements 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 subscribers equipment. Several different types of computer peripherals may appear as network ter
10、minals, 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 constrained by the available communications services, but many of them have variable speeds. On
11、e approach to 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
12、 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 di
13、fficult 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 netwo
14、rk 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
15、 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, follow
16、ed 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.,KEY WORDS re
17、servation 保留，预定 graphics 图示，图解 inventory 清单，报表 teleprocessing 远程(信息)处理，遥控处理 constrain 约束，强迫 multi-access 多路存取，多路进入 feedback 反馈，回授 intelligence 智能，智力，信息 intermediate 中间的 elapse (时间)经过，消失, be called upon to 用来，(被)要求 with the aid of 借助于，用，通过 appear as 作为出现 be likely to 可能，大约，像要,NOTES1 The combination o
18、f 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. 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 handl
19、e 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”。,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
20、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
21、 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。,EXERCISES 1. 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. store-and-forward, non store-and-forwardc. packet, non packe
22、t d. computer, non computer(2) As compared with circuit switching, the packet switching has the advantage whenis cheap.a. transmission medium b. computing powerc. buffer storage d. processing power,(3) The main elements in ARPA network are and .a. interface message processors, hostsb. local terminal
23、s, IMPsc. multi-access computers, peripheral equipment d. hosts, sharing computers,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 t
24、he 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-switc
25、hed network will be enhanced by providing for connection between the public telephone network and the packet network.( ),3. Answer 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?,Pass
26、age B Characteristics of Communication ChannelsData 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.
27、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 r
28、esearch 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 resear
29、ch findings.,When you connect one computer to another, you use hardware 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
30、consist of a modem and telephone lines.The modem modulates the computer 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 ove
31、r the telephone lines.1 The receiving computers modem transforms the incoming analog signals back to their digital equivalents in order to understand them.,Transmission RateThe transmission rate of a communications channel is determined by its bandwidth and its speed. The bandwidth is the range of f
32、requencies 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
33、. 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
34、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
35、 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 TransmissionThe direction of data transmission is classified as either simplex, half-duplex, or full-duplex. In s
36、implex 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
37、 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.
38、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-duple
39、x 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 SynchronousIn asynchronous transmission mode (Figure 3.1), individual characters (
40、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
41、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.,Figure3.1,In the synchronous transmission mode (Figure
42、 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 al
43、so transmitted. Synchronous transmission requires more sophisticated and expensive equipment, but it does give much higher speeds and accuracy than asynchronous transmission.,KEY WORDS bps (bits per second) 每秒钟传输的比特 baud rate 波特率 simplex transmission 单工传输 half-duplex transmission 半双工传输 full-duplex t
44、ransmission 全双工传输 asynchronous transmission 异步传输 synchronous transmission 同步传输,NOTES1 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 tran
45、smits”。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。,3 An additional bit called a
46、 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. 在每个字符的后面有时还包括一个称为奇偶校验位的附加位，它们测试数据位在传输过程中是否被意外改变。 they指的是a parity bit。,EXERCISES1. Fill in the blanks with the best choice.(1) The term modem is derived from th
47、e words .a. modulate emphasis b. modular emulationc. multiplex or demultiplexer d. modulate demodulate(2) Data transmission speed is measured in .a. bpsbytes per second b. bpsbits per secondc. bpsblocks per second d. bpsbuffers per second(3) The transmission method which uses start stop bits after e
48、ach 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
49、 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,(8) When the sen
50、ding device, such as a temperature sensor, never requires a response from the computer, we should use 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 converts its image into coded form for transmission over the telephone system.a. Scanner b. Modem c. Printer d. Fax machine,
