Unavailability
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In telecommunication, an unavailability is an expression of the degree to which a system, subsystem, or equipment is not operable and not in a committable state at the start of a mission, when the mission is called for at an unknown, i.e., random, time.
The conditions determining operability and committability must be specified.
Expressed mathematically, unavailability is 1 minus the availability.
Unavailability may also be expressed mathematically as the ratio of the total time a functional unit is not capable of being used during a given interval to the length of the interval, e.g., if the unit is not capable of being used for 68 hours a week, the unavailability is 68/168.