Hardware Failure Rates
The illustration below depicts failure rate as a function of time for hardware. The
relationship, often called the "bathtub curve," indicates the typical failure rate of
individual components within a large batch. It shows that in say a batch of 100
products, a relatively large number will fail early on before settling down to a steady
rate. Eventually, age and wear and tear get the better of all them and failure rates rise
again near the end of the products life. To assist in quality control, many new batches
of products are 'soak' tested for maybe 24 hours in a hostile environment
(temperature/humidity/variation etc.) to pinpoint those that are likely to fail early on
in their life, this also highlights any inherent design/production weaknesses.
These early failure rates can be attributed to two things
• Poor or unrefined initial design. Correcting this, results in much lower failure
rates for successive batches of the product.
• Manufacturing defects i.e. defects in the product brought about by poor
assembly/materials etc. during production.
Both types of failure can be corrected (either by refining the design, or by replacing
broken components out in the field), which lead to the failure rate dropping to a
steady-state level for some period of time.
As time passes, however, the failure rates rise again as hardware components suffer
from the cumulative effects of dust, vibration, abuse, temperature extremes and many
other environmental maladies. Stated simply,
"…The hardware begins to wear out."Software Engineering Topic 1 Page 10
Software Failure Rates
Software is not susceptible to the same environmental problems that cause hardware
to wear out. In theory, therefore, the failure rate curve for software should take the
form shown below.
Undiscovered defects in the first engineered version of the software will cause high
failure rates early in the life of a program. However, these are corrected (hopefully
without introducing other errors) and the curve flattens as shown. The implication is
clear. Software doesn't wear out. However, it does deteriorate with maintenance as
shown below.
During its life, software will undergo changes and it is likely that some new defects
will be introduced as a result of this, causing the failure rate curve to spike as shown
above. Before the curve can return to the original steady-state failure rate (i.e. before
the new bugs have been removed), another change is requested, causing the curve to
spike again. Slowly, the minimum failure rate level begins to rise-- the software is
deteriorating due to change.
Thanks & Regards,
Bastin Vinoth NG
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Means that software should change from its original state by maintenance that cause some defect if it si require to change or maintenance of our software then
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In software engineering, a class diagram is used for describing the structure of a system by showing classes, attributes, operations, and relationships.
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