Page 13 - DCAP305_PRINCIPLES_OF_SOFTWARE_ENGINEERING

Unit 1: Introduction to Software Engineering

1.4.1 Software does not Wear out Notes
There is a renowned “bath tub curve” in dependability studies for hardware products. The curve
is given in Figure 1.5. The shape of the curve is like “bath tub”; and is known as bath tub curve.

Figure 1.5: Bath Tub Curve

Burn-in
phase
Wear out
Useful life phase phase
intensity

Failure

Time

There are three phases for the life of a hardware product. First phase is burn-in phase, where
failure intensity is high. It is expected to test the product in the industry before delivery.
Outstanding to testing and fixing faults, failure strength will come down initial and may become
stable after positive time. The second phase is the useful life phase where failure strength is just
about constant and is called useful life of a product. After few years, again failure intensity will
increase due to wearing out of components. This phase is called wear out phase. We do not have
this phase for the software as it does not wear out. The curve for software is given in Figure 1.6.

Figure 1.6: Software Curve

intensity

Failure

Time

Important point is software becomes dependable overtime rather than wearing out. It becomes
outdated, if the environment for which it was urbanized, changes. Hence software may be retired
due to environmental changes, new requirements, new prospect, etc.

1.4.2 Software is not Manufactured
The life of software is from concept exploration to the retirement of the software product. It is
one time development effort and continuous maintenance effort in order to keep it operational.
However, making 1000 copies is not an issue and it does not involve any cost. In case of hardware
product, every product costs us due to raw material and other processing expenses. We do not
have assembly line in software development. Hence it is not manufactured in the classical sense.

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