Availability is needed in file server applications or in call-processing applications. Traditional implementations of fault-tolerant platforms often involve proprietary hardware and software. The challenge is to provide highly available platforms without resorting to extreme or expensive measures. Availability is often expressed in percentages. A system with just 5 minutes of service outage must have 99.999 percent availability.
Availability is calculated using statistical models for all the system components.
The simplest model for a component is a binary model. The component is either in or out of service. The average downtime contribution by any component is calculated by amortizing the MTTR time over the MTBF period. A more complex model for the component, which considers partial outages, requires the use of statistical methods to calculate availability. To achieve higher reliability or to compensate for less reliable components, redundancy is used.