W congestion avoiding inherits the properties in this

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w/ congestion avoiding” inherits the properties (in this case, the attribute “identification”) from its super-type “navigation device” and extends the
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76 6 Model-Based Requirements Documentation set of attributes by an attribute that specifies the threshold length of a traffic congestion, which triggers a route recalculation. Figure 6-9 Class diagram in UML notation 6.6 Requirements Modeling in the Functional Perspective The functional perspective of requirements deals with the transformation of input data received from the environment into output data released into the environment of the system. There are a number of different model- based approaches that can be used to model the functional perspective of requirements. The majority of these techniques is based on the structured system analysis approaches of the 1970s and 1980s, such as the structured analysis [DeMarco 1978, Weinberg 1978] or the essential system analysis [McMenamin and Palmer 1988.] . 6.6.1 Data Flow Diagrams At the center of attention of modeling requirements from a functional per- spective are diagrams that model the functionality of the respective system by means of processes (functions), data stores, sources, and sinks in the system environment as well as data flow. A commonly used type of functional models are data flow diagrams, as suggested in the structured analysis according to [DeMarco 1978] . Data flow models allow modeling the system on different levels of abstraction.
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6.6 Requirements Modeling in the Functional Perspective 77 Modeling Elements of Data Flow Diagrams Figure 6-10 shows the modeling elements in data flow diagrams in the notation suggested by [DeMarco 1978]. Figure 6-10 Important modeling elements of data flow diagrams according to DeMarco Data manipulation Processes depict the functions of a particular system necessary to transform the data that flows into the system (information flow). 7 A process con- sumes the input data, processes this data, and outputs the result of the processing in the form of output data. How the data is transformed is not depicted in data flow diagrams. Resting data Data stores are abstract concepts designed to depict persistent data. Processes can access data in a data store in a read and write manner so that the processes may access necessary input data or persistently store output data. Objects in the system environment Sources/sinks describe objects (like people, groups of people, depart- ments, organizations, or systems) in the environment of the system that exchange data with the system. Sources/sinks are aspects of the system environment and cannot be altered during system development (see sec- tion 2.1 ). Sources are aspects of the system environment that deliver data to the system, while sinks receive data from the system.
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