Thursday, August 23, 2007

How is availability calculated?

There are 2 terms which goes into the calculation of availability:

Mean Time Between Failures (MTBF)
Mean Time to Recover (MTTR)

Availability is calculated by applying the formula below:

For example, if a network switch fails after 500,000 hours of service and it takes 24 hours to repair it, then the availability becomes 99.99520

If this availability is not acceptable and the repair time cannot be reduced, the alternative is to keep spares. In the previous example, let’s say, replacing the switch with a spare switch takes just 1 hour, then the availability comes down to 99.99980.


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