What levels are there anyway? And how
many? The scheme is shown in Figure 2.73 .
A system consists of several subsystems and a subsystem, in turn, consists of
several blocks. This scheme is only a rough rule though. If a system is small, e.g., it
117 2.7 Realizing Use Cases
may not have a subsystem level at all. Such a system would then be composed of
blocks only. On the other hand, there can be blocks on subsystem level.
We will now start looking for the structures of our on-board computer. I ??™ m
drawing a na??ve approach here, which would probably not be used in this form in
practice. But it is suitable for showing the relations that should be present in any
arbitrary approach.
We fi rst take a use case. The services represented by use cases have to be
offered by the system. Their realization is the primary goal. In working toward this
goal, we also have to take the requirements that somehow relate to a use case
into account, e.g., a defi ned system response time. Depending on how complete
the requirements are covered by use cases (refi ne relationship), we also have to
directly select a requirement instead of a use case.
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