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3SL Reference

Data Structure Diagram (DSD)

DSDs are a graphical alternative to the composition specifications in data definitions. A DSD shows the decomposition of data items into lower-level items. If the DSD contains only a data item and its decomposition then it is a graphical substitute for the data definition of the data item. If the DSD also shows the decomposition of the lower-level data items into further data items, it is a graphical substitute for a set of data definition composition specifications.

DSDs are not normally used instead of data definition composition specifications. DSDs offer a picture of the structure of a system’s data items, as a supplement to the DD. Even when DSDs are used, the DD is still to be regarded as the master source of information for a system’s data and control definitions.

DSDs show the composition of a data definition as the data items below it. The DSD notation corresponds, in effect, to a graphical version of the DD composition specification BNF.

DSDs are not hierarchical.

DSDs can appear in models in the Essential and Implementation Domains.