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1、英文资料：1、Data CommunicationsThe 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
2、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 compu
3、ters 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. Teleprocessing started with the airline reservation systems, where the distribution of computer data is not as importan
4、t 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 file for all their branches. Basicall
5、y, 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.The most applicable model for data com
6、munications 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 subscribers equipment. Several different types of computer peripherals may appe
7、ar as network terminals, and so also can computers functioning in several different ways. 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 the have vari
8、able 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 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
9、 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 ex
10、pensive 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 commun
11、ications 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 a
12、ppear 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 dire
13、ction, 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 . The time scale for the interchange of data messages needs to be of this order
14、. 2、Characteristics of Communication ChannelsData communications is the electronic transmission of information including data ,television pictures, sound and facsimiles. It usually involves s computer, a modem , software, and a printer. With this equipment, you can communicate with a friend in St. L
15、ouis, 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 re
16、search 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, Promotes distant learning in which students can share information and computer research fi
17、ndings.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 consis
18、t 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 over the
19、 telephone lines. 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 frequenc
20、ies 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 p
21、er second is the number of bits that can be transmitted in one second. Using a 10-bit byte to represent a character, a 2,400bps 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 th
22、e number of times per second that signal being transmitted changes. Wing each change, one or more bits can be transmitted. At speed 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 2,4
23、00 bps, more than one bit is transmitted with each signal change and, 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 use
24、d 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
25、 because the computer only needs to receive data from the temperature sensors and does not need to send data back to the sensors. In half-duplex transmission, data can flow in both directions but in only one direction at s time. An example is a citizens band radio. The user can talk or listen but no
26、t 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. Both parties can talk at the same time. Full-duplex tr
27、ansmission is used for most interactive computer applications and for computer-to-computer data transmission.1、数据通讯数据通讯迅速增长主要被增长的需要影响了移动信息计算机。现代分时操作的计算机可能同时联络许多数据站。电话和通信机线被要求处理是批数据传输, 实时传输的其它数据传输的形式(在飞机订票系统情况下), 在图表的帮助下，与智能计算机交谈完成开户和信用卡业务, 在这种计算机中安置这样新方式的数据收集系统、自动计算机和数据通讯的组合系统, 必须处理他们的数据, 一个可能提到他们更好
28、作为远程处理数据而不是电信。远程处理开始以飞机订票系统, 计算机数据发行像电子存货维护不是一样重要使整体系统和能力迅速地访问这样存货在一定数量遥远的地方。其它例子如银行, 对于一个中央文件集中他们的会计和为所有他们的分支提供存取。基本上, 有远程处理的系统的发展的三个原因: 集中文件的处理要求, 需要计算机维护系统和由地方人员灵活操作的计算机系统。最可适用的模型为数据通讯要求是中央计算机同时交换以许多简单地方的数据终端信息。与电话通信不同, 数据通讯网络必须能够互联各种各样的用户的终端设备。几不同的类型计算机外围有不同网络终端, 也和计算机的作用及功能上用几个不同的方式。终端速度范围应在从10
29、0 位每秒到10,000 位每秒是相对比较重要。当前使用的许多数据在终端具有各种可利用的通讯服务, 但它们有各种不同的速度差。对这个情况有一种方法是终端提供缓冲单元, 但这种方法是昂贵的。当前, 多通道计算机通常处理仅仅一种速度终端或以很小数量固定范围内的速度。就出现通过反馈输出设备的可应用简单输出程序来驱动不同特征的终端设备。当然, 这样反馈信号会必须通过通讯网络处理。随着数据通讯技术的发展，每个多通道计算机被连接到终端设备数目在不断的增长数字, 使它造价昂贵和难提供把计算机和网络中每个终端分开所许的电线。因而, 出现在计算机和终端之间更加复杂的连接方式。但这并不需要采用适应的直接方式。数据
30、通讯网络的一个特征是不同与常规电话网, 这两种通信线路的两个目是非常不同的。一个末端是具有灵活方式的计算机处理能力,而另一末端是一个相对地缓慢没有智力的终端。因为两个目的是很不同的,就会出现在网络数据通信中的不同连接方式。在电话谈话情况下信息流程在数据通讯系统是断断续续的。然而, 在信息的交谈中，在几秒中信息只有在一个方向上流动, 被暂停的信息和一些跟随的信息在相反的方向上流动, 目前所用的典型计算机存取时间大约为100毫秒，但采用较好的存储器时存取时间将会缩短。时间表对于数据信息交换作为一种命令方式使用。2、通信频道的特征数据通信是电子信息传输包括数据、视频图像、声音和传真. 它通常是一些电
31、脑、调制解调器、软件和打印机. 拥有这些设备,你还可以与在圣路易斯密苏里、或法国的巴黎、英国，通过电话线路收发任何一个简单的信息. 用同样的方法,一家孩子可以与老师在课堂上上课, 上班族可以在家中工作,并有一名医生可以利用远程计算机的研究资料. 利用通信系统的理由是令人信服的是.它具有以特点： 提高工作高效，化解工作压力,减少汽车污染,节省时间和金钱 ，允许回家办公,推广远程教学使学生可以共享一个信息和在计算机上搜集资料。 当你将一台计算机连接到另一个计算机,运用的硬件和软件. 在大多数情况下, 硬件设备分为数据传送超过某种通信线路， 如一条电话线. 这个软件控制数据流向. 所需硬件包括一个调
32、制解调器和一条电话线。现代计算机输出调制信号传送到可接受的信号,然后传送到解调设备还原成原始信号输入到计算机.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. 接收段端计算机的调制解调器将模拟信号数字化使接受端能理解. 传输速率 通讯频道的传输速率是决定于它的带宽和速度. 带宽是一个频道所能成受频率范围。 自从传送数据以来可分配到不同频率,带宽更大,更大的频率， 而更多的资料可在同一时间传送. 数据传输速度一般表现为比特每秒或作为波特率. 一比特是在一秒内可传送的多少个数据. 采用10位字节代表一个字,2400比特每秒将以每秒240字传送. 按这个速度,20页,隔行报告
33、将传送大约五分钟. 波特率是每秒钟信号传送中改变的次数,随着这种改变一个或多个比特可以传送. 在加速到2400比特每秒,通常只有一比特是每传送信号的变化,因而 ，比特每秒和波特率是相同的. 实现速度超过2400比特每秒,一个多比特传送信号的变化随着每个信号改变, 因此，比特每秒将会超越波特率. 传输的方向数据传输方向可以分为是单工、半双工或全双工. 在单工传输数据，数据流量只向一个方向.当在传送设备如温度传感器、计算机从未要求回应的时候在用单工传输方式. 举例来说,如果是用电脑控制的温度,许多地方放置传感器.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受来自温度传感器的数据，而不需要向传感器发送数据. 在半双工传输，数据可以双向传输,但在同一时刻只有一个方向传输数据. 其中一个例子是市民的对讲机. 用户可以对话或聆听,但不能在同一时间内完成对话或聆听. 半双工通常被应用在终端和计算机之间. 在全双工传输方式。数据可以在同一时间在两个方向发送数据. 一般电话线就是一个全双工传输方式的例子. 双方还可以在同一时间谈话. 全双工传输用于计算机应用、电脑之间的数据传输.
