A data link includes the physical transmission medium, the protocol, and associated devices and programs, so it is both a physical and a logical link. Data Message - An arrangement of data characters with appropriate communication's overhead, e.g., check characters, data identifiers, function indicators. Data Requirements - Describes those data elements needed by a computer application in order to process a specified business transaction. Data Segment - A segment is the intermediate unit of information in a message set. Segments consist of logically related data elements in a defined sequence.
Segments have a unique segment identifier that compresses the first characters of the segment.
When segments are combined to form a message set, their relationship to the message set is defined by a segment requirement designator and a segment sequence. Some segments may be repeated, and groups of segments may be repeated as loops. Data Type - The characteristic of a data element that describes whether it is numeric, alphabetic, or alphanumeric. Destination Field - A field in a message header that contains the address of the station to which a message is being directed. Downtime - The period during which computer or network resources are unavailable to users because of a failure.