MIS 495 -- Dr. Reithel

Chapter 12

Data Flow Diagram (DFD)

A DFD is a graphical technique that depicts information flow and the transforms that are applied as data move from input to output. The DFD may be used to represent a system or software at any level of abstraction. In fact, DFD's may be partitioned into levels that represent increasing information flow and functional detail. Therefore, the DFD provides a mechanism for functional modeling as well as information flow modeling.

Ward and Mellor Extensions

Extend basic structured analysis notation to accommodate the following demands imposed by a real-time system:

Info. flow gathered or produced on a time-continuous basis
Control info. passed throughout the system and associated control processing
Multiple instances of the same transformation (sometimes encountered in multitasking situations)
System states and the mechanism that causes transition between states.

One extension to the basic DFD notation is the dashed arrow which represents a control item or event. Another notation extension is the use of the double-headed arrow to represent a data object that is input or output to/from a process on a "continuous" basis.

Control Specification (CSPEC)

The CSPEC represents the behavior of the system in two different ways. It contains a state transition diagram (STD) that is a sequential specification of behavior. It can also contain a process activation table (PAT). The CSPEC describes the behavior of the system, but it does not give us any information about the inner working of the processes that are activated as a result of this behavior.

Process Specification (PSPEC)

The PSPEC is used to describe all flow model processes that appear at the final level of refinement. The content of the process specification can include narrative text, a program design language description of the process algorithm, mathematical equations, tables, diagrams, or charts.

Data Dictionary

Provides an organized approach for representing the characteristics of each data object and control item. The data dictionary has been proposed as a quasi-formal grammar for describing the content of objects defined during structured analysis. It is almost always implemented as part of a CASE tool, with the format varying from tool to tool. Most data dictionaries contain the following information regarding data objects and control items: name, alias, where-used/how-used, content description, supplementary information.