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.
　　有五種基本的數(shù)據(jù)通信系統(tǒng)：
　　脫機數(shù)據(jù)傳輸是簡單地利用電話或類似的鏈路來傳輸數(shù)據(jù)，不包括計算機系統(tǒng)。這樣一條鏈路兩端所使用的設備不是計算機的部件，或至少不是立刻把數(shù)據(jù)提供給計算機處理，即數(shù)據(jù)在發(fā)送或接收時是脫機的。這種數(shù)據(jù)通信相對來說比較便宜和簡單。
　　遠程批處理一詞適用于這樣一種方法：采用數(shù)據(jù)通信技術來使數(shù)據(jù)的輸入和輸出在地理上遠離按批處理模式處理處理它們的計算機。
　　聯(lián)機數(shù)據(jù)收集指的是用數(shù)據(jù)通信技術來向計算機即時提供剛產(chǎn)生的輸入數(shù)據(jù)這種方法。數(shù)據(jù)于是存儲在計算機里（比如磁盤上），并按預定時間間隔或者根據(jù)需要進行處理。
　　詢問——應答系統(tǒng)，顧名思義，是為用戶提供從計算機提取信息的功能。詢問功能是被動的。也就是說，它不修改所存儲的信息。提問可以很簡單，例如："檢索雇員號碼為1234的記錄"也可以是復雜的。這類系統(tǒng)可能要使用能產(chǎn)生硬拷貝和（或）可視顯示的終端。
　　實時系統(tǒng)是這樣一類系統(tǒng)，其中計算機系統(tǒng)是在動態(tài)情況下取得和處理信息，以便可使計算機采取動作來影響正在發(fā)生的事件（比如在過程控制應用中）或者可通過存儲在計算機里的準確且不斷更新的信息來影響人（操作員），比如在預售系統(tǒng)中。