Motivation
There is a subtle distinction between how often a system goes down ( reliability )3and how much total time it spends being down ( availability ). This section allows you to specify realistic expectations about the amount of time that the product will beavailable for use.
Examples
The product shall be available for use 24 hours per day, 365 days per year.
The product shall be available for use between the hours of 8:00 A.M. and 5:30 P.M.
The escalator shall run from 6 A.M. until 10 P.M. or the last flight arrives.
The product shall achieve 99 percent uptime.
Considerations
Consider carefully whether the real requirement for your product is that it is available
for use or that it does not fail at any time.
Consider also the cost of reliability and availability, and whether it is justified for
your product.
The sections on reliability and availability can sometimes be combined.