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We defi ne the two different types of requirements as follows:
An essential requirement describes the purely domain-specifi c intention
and is independent of the technical implementation (solution). Accordingly, we
speak of a technical requirement if it is based on a solution approach.
We take the technical requirements and ask ourselves the question about the Why.
Why should the enclosure size for the central unit be such that it fits into the
DIN radio compartment of a car ( Figure 2.9 )? What ??™ s the purpose? The car driver
should be able to operate the on-board computer and read information from it as
they drive. There are ways other than building it into the radio compartment to
achieve this goal.
Fine, we found the essential requirement. It ??™ s what the principal really wants:
operability while driving. If we continue asking for the Why, we can arrive at additional
fi ndings, such as the principal ??™ s goals. In this specifi c case, we fi nd a crossrelationship
to a requirement on the same level. Why do drivers have to be able to
operate the on-board computer while driving? This is not required for picking up
and returning cars??”our system ??™ s core functions.
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