For: exergy For: availability heuristic

In telecommunications 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.

Representation

The most simple representation for availability is as a ratio of the expected value of the uptime of a system to the aggregate of the expected values of up and down time, or

$A\; =\; \backslash frac\{E1\}\{E2+E3\}$

If we define the status function $X(t)$ as

$X(t)=$

therefore, the availability is represented by

   A(t)=\Pr4.

$E5=X.\backslash Pr6\backslash quad\; t\; >\; 0.$

Average availability must be defined on an interval of the real line. If we consider an arbitrary constant $c$, then average availability is represented as

    A_c = \frac{1}{c}\int_0^c A(t)\,dt,\quad c > 0.

Limiting (or steady-state) availability is represented by

   A = \lim_{t \rightarrow \infty} A(t).

Limiting average availability is also defined on an interval $(0,c]$ as,

   A_{\infty}=\lim_{c \rightarrow \infty} A_c = \lim_{c \rightarrow \infty}\frac{1}{c}\int_0^c A(t)\,dt,\quad c > 0.

Literature

Availability is well established in the literature of stochastic modeling and optimal maintenance. Barlow and Proschan 7 define availability of a repairable system as "the probability that the system is operating at a specified time t." While Blanchard 8 gives a qualitative definition of availability as "a measure of the degree of a system which is in the operable and committable state at the start of mission when the mission is called for at an unknown random point in time." This definition comes from the MIL-STD-721. Lie, Hwang, and Tillman 9 developed a complete survey along with a systematic classification of availability.