Availability

(ə′vāl·ə′bil·ə·dē)

(computer science) Of data, data channels, and input-output devices in computers, the condition of being ready for use and not immediately committed to other tasks.
(navigation) The probability that a navigational system will function and provide required levels of accuracy, integrity, and continuity.
(physics) The difference between the enthalpy per unit mass of substance and the product of entropy per unit mass multiplied by the lowest temperature available to the substance for heat discard; used in determining the ratio of actual work performed during a process by a working substance to that which theoretically should have been performed.
(systems engineering) The probability that a system is operating satisfactorily at any point in time, excluding times when the system is under repair.

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Computer Desktop Encyclopedia: availability

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The accessibility of a system resource in a timely manner; for example, the measurement of a system's uptime. Availability is one of the six fundamental components of information security (see Parkerian Hexad). See also uptime and high availability.

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Marketing Dictionary: availability

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Broadcast time period available for purchase for a commercial message. When it has been determined by the media planner (who is frequently the media buyer as well) where an advertiser will best profit from commercial placement, he or she will contact the station or the station's sales representative and ask for Avails (the time periods that are available). The station or rep will then supply the buyer with a list of these time slots together with prices and estimated ratings of the shows in or around the available times. The ratings, and thus the prices, are based on the most recent ratings of the shows, as reported by the rating services.

Banking Dictionary: Availability

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Time period, expressed in days, from the day of deposit until funds are credited to an account's Collected Balance are available for spending or investment. Funds availability is determined by average clearing time, allowing for Mail Float and Federal Reserve Float. See also Availability Schedule; Check Hold; Deferred Availability.

Food and Nutrition: availability

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Also known as bioavailability or biological availability. In some foodstuffs, nutrients that can be demonstrated to be present chemically may not be available, or only partially so, when they are eaten. This is because the nutrients are chemically bound in a form that is not susceptible to enzymic digestion, although it is susceptible to the strong acid or alkali hydrolysis used in chemical analysis. For example, the niacin in cereal grains, calcium bound to phytate, and lysine combined with sugars in the Maillard complex, are all biologically unavailable. See also lysine.

Dental Dictionary: availability

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The supply in terms of type, volume, and location of health resources and services relative to the demands of a given individual or community.

Word Tutor: availability

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IN BRIEF: Ready for use.

Before using the computer, check its availability.

Wikipedia: Availability

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For the thermodynamic function "availability", in the sense of available useful work, see exergy.

For availability as a form of cognitive bias, see 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.

1 Representation
- 1.1 Example
2 Literature
3 See also
4 References
5 External links

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

$A = \frac{E[\mathrm{Uptime}]}{E[\mathrm{Uptime}]+E[\mathrm{Downtime}]}$

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

$X(t)= \begin{cases} 1, & \mbox{sys functions at time } t\\ 0, & \mbox{otherwise} \end{cases}$

therefore, the availability is represented by

$A(t)=\Pr[X(t)=1].$

$E[X(t)]=X.\Pr[X(t)=1] \quad t > 0.$

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,

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

Example

If we are using equipment which has mean time between failure (MTBF) of 81.5 years and mean time to recover (MTTR) of 1 hour:

MTBF in hours = 81.5*365*24=713940

Availability= MTBF/(MTBF+MTTR) = 713940/713941 =99.999859%

Unavailability = 0.000141%

Outage due to equipment in hours per year

U=0.01235 hours per year.

Literature

Availability is well established in the literature of stochastic modeling and optimal maintenance. Barlow and Proschan [1975] define availability of a repairable system as "the probability that the system is operating at a specified time t." While Blanchard [1998] 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 [1977] developed a complete survey along with a systematic classification of availability.

Availability measures are classified by either the time interval of interest or the mechanisms for the system downtime. If the time interval of interest is the primary concern, we consider instantaneous, limiting, average, and limiting average availability. The aforementioned definitions are developed in Barlow and Proschan [1975], Lie, Hwang, and Tillman [1977], and Nachlas [1998]. The second primary classification for availilability is contingent on the various mechanisms for downtime such as the inherent availability, achieved availability, and operational availability. (Blanchard [1998], Lie, Hwang, and Tillman [1977]). Mi [1998] gives some comparison results of availability considering inherent availability.

Availability considered in maintenance modeling can be found in Barlow and Proschan [1975] for replacement models, Fawzi and Hawkes [1991] for an R-out-of-N system with spares and repairs, Fawzi and Hawkes [1990] for a series system with replacement and repair, Iyer [1992] for imperfect repair models, Murdock [1995] for age replacement preventive maintenance models, Nachlas [1998, 1989] for preventive maintenance models, and Wang and Pham [1996] for imperfect maintenance models.

References

This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

External links

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

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