数据通信系统（一）（在线收听）

　　Data Communication Systems

　　There are five basic types of data communication system:

　　Off-line data transmission is simply the use of a telephone or similar link to transmit data without involving a computer system.The equipment used at both ends of such a link is not part of a computer, or at least does not immediately make the data available for computer process, that is, the data when sent and/or received are 'off-line'. This type of data communication is relatively cheap and simple.

　　Remote batch is the term used for the way in which data communication technology is used geographically to separate the input and /or output of data from the computer on which they are processed in batch mode.

　　On-line data collection is the method of using communications technology to provide input data to a computer as such input arises-the data are then stored in the computer（say on a magnetic disk）and processed either at predetermined intervals or as required.

　　Enquiry-response systems provide, as the term suggests, the facility for a user to extract information from a computer.The enquiry facility is passive, that is, does not modify the information stored.The interrogation may be simple, for example, 'RETRIEVE THE RECORD FOR EMPLOYEE NUMBER 1234' or complex.Such systems may use terminals producing hard copy and /or visual displays.

　　Real-time systems are those in which information is made available to and processed by a computer system in a dynamic manner so that either the computer may cause action to be taken to influence events as they occur（for example as in a process control application）or human operators may be influenced by the accurate and up-to-date information stored in the computer, for example as in reservation systems.

　　有五种基本的数据通信系统：

　　脱机数据传输是简单地利用电话或类似的链路来传输数据，不包括计算机系统。这样一条链路两端所使用的设备不是计算机的部件，或至少不是立刻把数据提供给计算机处理，即数据在发送或接收时是脱机的。这种数据通信相对来说比较便宜和简单。

　　远程批处理一词适用于这样一种方法：采用数据通信技术来使数据的输入和输出在地理上远离按批处理模式处理处理它们的计算机。

　　联机数据收集指的是用数据通信技术来向计算机即时提供刚产生的输入数据这种方法。数据于是存储在计算机里（比如磁盘上），并按预定时间间隔或者根据需要进行处理。

　　询问——应答系统，顾名思义，是为用户提供从计算机提取信息的功能。询问功能是被动的。也就是说，它不修改所存储的信息。提问可以很简单，例如："检索雇员号码为1234的记录"也可以是复杂的。这类系统可能要使用能产生硬拷贝和（或）可视显示的终端。

　　实时系统是这样一类系统，其中计算机系统是在动态情况下取得和处理信息，以便可使计算机采取动作来影响正在发生的事件（比如在过程控制应用中）或者可通过存储在计算机里的准确且不断更新的信息来影响人（操作员），比如在预售系统中。