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the component designs may be reassigned. If not, the decomposition (archi-
tecture and contracts) may need to be redesigned; this means that rejection in
one component design may impact others.
Ideally, though, each subprocess will provide a component design as well
as testing showing that the design meets its contract. The engineer must then
integrate the components according to the architecture, and conduct his/her
own testing on the resulting system. If it meets the contract, the aggregate
design can be passed back to the customer or higher-level engineer.
If the system does not meet the contract, there is some chance that a
component is malfunctioning, though this should have been caught in lower
level testing. Otherwise, the engineer’s model that justified the original
decomposition must be deficient; now the engineer must refine the model.
Using the refined model the engineer can produce a new decomposition
and start the cycle anew, or decide that the original contract is infeasible
and reject.
We may think of this description as an elaboration on the classic “V-
model” of systems engineering, shown in Fig. 13.2. While the V-model cap-
tures the dual processes of first passing from requirements to design and then
from design to operation, it is also misleading in a number of ways.
First of all, the representation as a V is too linear, suggesting a single
design process rather than an iterative refinement into many; in reality
the legs of the V should be replaced by trees. It also eliminates the recursive
structure of the problem; system requirements may appear at the top-left leg
of the V, but component requirements show up further down. Because the
V squashes together parallel paths and different hierarchical levels in the
design process, this hides the fact that different components may be

Fig. 13.2 The V-model of systems engineering (Osborne, Brummond, Hart, Zarean, &
Conger, 2005).