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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 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 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.,The 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 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 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 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.,NOTES,1 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.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，
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