Software Reliability It is defined as the ability of a system or component to perform its required functions under stated conditions for a specified period of time. Software may retire only if it becomes obsolete. Some contributing factors are – change in environment, infrastructure/tech, increase in complexity, slow execution speed.

It is the probability of failure free operation of a program for a specified time in a specified environment.

Software and Hardware Reliability Failure usually occurs only when a program (design) exposed to an environment that it was not developed or tested for. H/w The probability of failure due to wear and other physical causes has usually been much greater than due to un recognized problem. It is possible to keep H/w design failures low because the h/w is less complex than s/w.

Failures and faults A fault is a defect in a program that when executed in a particular conditions, causes failure. Fault can be a source for more than one failure . Fault is a property of the program rather than a property of its execution or behavior. The term failure relates to behavior of the program.

A Reliability Model This model focuses on failure intensity. It assumes that failure intensity decreases with time, that is as execution time increases , the failure intensity decreases. If the failure is observed , the fault that failure is detected and the fault is removed. Consequently the failure intensity decreases.

λ ( µ ) = λo (1- µ / Vo) λo- initial failure intensity at start of execution µ- expected no of failures at time t Vo- total no of failures that will occur in infinite time. Failure λo Intensity Total Failures Vo

R(t)= e raise to the power – λτ R= Realibility, τ= Execution time µ (τ) = Vo (1-e rtp – λo/ Vo* τ) Putting value in eq 1 λ (τ ) = λo * e rtp – λo/ Vo* τ Failure Intensity Execution time