Availability

Topic appears in the following WE1 Exams:

• 2021 07 26 Q2 (5 points)
• 2021 02 05 Q3 (3 points)

Therefore it appears in 33% of the WE1 exams and it is worth 4 points on average.

Summary

• Mean Time to Repair ($MTTR$): Average time between the occurrence of a fault and service recovery, also known as the downtime
• Mean Time To Failures ($MTTF$): Mean time between the recovery from one incident and the occurrence of the next incident, also known as uptime
• Mean Time Between failure ($MTBF$): Mean time between the occurrences of two consecutive incidents

We can then define:

1. Availability: The probability that a component is working properly at time $t$: $$A = MTTF / (MTTF+MTTR)$$ A given component could have failed in the interval $(0,t)$, but it could have been repaired before t

2. Reliability: The probability that a component has always been working properly during a time interval $(0,t)$, it requires that the component never fails in the interval. $$R = e^{-λt} \space where \space λ = 1/MTTF$$

Availability is typically specified in nines notation. For example 3-nines availability corresponds to 99.9%, 5-nines availability corresponds to 99.999% availability.

Calculating availability

Availability in series

If failure of a part leads to the combination becoming inoperable.

The combined availability is the product of the availability of the component parts