Information Technology Reference
In-Depth Information
If
n
systems with availability
A
1
,A,A
3
...
A
n
must operate for the system to be
up, then the combined system availability is
n
A
=
A
i
(14.2)
i
=
1
For a software of two components with availabilities A
1
and A
2
, the probability
that either system has failed is (1 - A
1
)or(1-A
2
). If either system must operate, but
not both, then the combined system has failed only if both systems have failed. This
will occur with probability (1 - A
1
)(1 - A
2
), and the combined system availability
is, therefore,
A
=
1
−
(1
−
A
1
)(1
−
A
2
)
(14.3)
Equation (14.3) is a generalization for a system of
n
components
n
A
=
1
−
(1
−
A
i
)
(14.4)
i
=
1
14.4
SOFTWARE DESIGN FOR TESTABILITY
Design for test is a name for design techniques that add certain testability features
to a hardware product design (Design for Test, 2009). Testability means having
reliable and convenient interfaces to drive the execution and verification of tests
(Pettichord, 2002). IEEE defines software testability as “the degree to which a system
or component facilitates the establishment of test criteria and the performance of tests
to determine whether those criteria have been met.”(ANSI/IEEE Std 610.12-1990).
Pettichord (2000) identified three keys to system test automation:
1. A constructive relationship between testers and developers,
2. A commitment by the whole team for successful automation, and
3. A chance for testers to engage early in the product development cycle.
Testability should be integrated in the DFSS design process instead of dealing
with it after the product has been designed. Pettichord (2002) stated that “it is more
efficient to build testability support into a product than to construct the external
scaffolding that automation would otherwise require.” To incorporate testability in a
design, the following recommendations need to be followed:
1. Cooperation from developers and customers to add testability features. This
means that testable features should be expected in the design requirements,
2. A realization that testability issues blocking automation warrant attention from
the whole team, and
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