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.