In communication networks and reliability theory, the term availability has the following meanings:

1. The degree to which a system, subsystem, or equipment is operable and in a committable state at the start of a mission, when the mission is called for at an unknown, i.e., a random, time. Simply put, availability is the proportion of time a system is in a functioning condition.

Note 1: The conditions determining operability and committability must be specified.
Note 2: Expressed mathematically, availability is 1 minus the unavailability.

2. The ratio of (a) the total time a functional unit is capable of being used during a given interval to (b) the length of the interval.

Note 1: An example of availability is 100/168 if the unit is capable of being used for 100 hours in a week.
Note 2: Typical availability objectives are specified either in decimal fractions, such as 0.9998, or sometimes in a logarithmic unit called nines, which corresponds roughly to a number of nines following the decimal point, such as "five nines" for 0.99999 reliability.

 

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