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Information about Availability

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.

Source: from Federal Standard 1037C in support of MIL-STD-188

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



If we define the status function as



therefore, the availability is represented by





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



Limiting (or steady-state) availability is represented by



Limiting average availability is defined is also defined on an interval as,

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.

See also

External links

Thermodynamics (from the Greek θερμη, therme, meaning "heat" and δυναμις, dynamis, meaning "power") is a branch of physics that studies the effects of changes in temperature, pressure, and volume on
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In thermodynamics, work is the quantity of energy transferred from one system to another without an accompanying transfer of entropy. It is a generalization of the concept of mechanical work in mechanics.
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Exergy is defined differently in different fields of study. In thermodynamics, exergy is defined as a measure of the actual potential of a system to do work. In systems energetics, exergy has been defined as entropy-free energy.
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A cognitive bias is any of a wide range of observer effects identified in cognitive science and social psychology including very basic statistical, social attribution, and memory errors that are common to all human beings.
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The availability heuristic is a rule of thumb, heuristic, or cognitive bias, where people base their prediction of the frequency of an event or the proportion within a population based on how easily an example can be brought to mind.
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Telecommunication is the transmission of signals over a distance for the purpose of communication. In modern times, this process typically involves the sending of electromagnetic waves by electronic transmitters, but in earlier times telecommunication may have involved the use of
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Reliability theory developed apart from the mainstream of probability and statistics. It was originally a tool to help nineteenth century maritime insurance and life insurance companies compute profitable rates to charge their customers.
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System (from Latin systēma, in turn from Greek σύστημα systēma) is a set of entities, real or abstract, where each entity interacts with, or is related to, at least one other
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System (from Latin systēma, in turn from Greek σύστημα systēma) is a set of entities, real or abstract, where each entity interacts with, or is related to, at least one other
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Unavailability can be defined as the probability that an item will not operate correctly at a given time and under specified conditions. It opposes availability.

Numerical values associated with the calculation of availability are often awkward, consisting of a series of 9s
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In computer engineering, an execution unit is a part of a CPU that performs the operations and calculations called for by the program. It may have its own internal control sequence unit (not to be confused with the CPUs main control unit), some registers, and other internal units
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Federal Standard 1037C, entitled Telecommunications: Glossary of Telecommunication Terms is a United States Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended.
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MIL-STD-188 is a series of U.S. military standards relating to telecommunications.

Purpose

Faced with “past technical deficiencies in telecommunications systems and equipment and software…that were traced to basic inadequacies in the application of
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This page is concerned with the stochastic modelling as applied to the insurance industry. For other stochastic modelling applications, please see Monte Carlo method.

Stochastic model

"Stochastic" means being or having a random variable.
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Optimal maintenance is the discipline within operations research concerned with maintaining a system in a manner that maximizes profit or minimizes cost. Cost functions depending on the reliability and maintainability characteristics of the system of interest determine the
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Downtime refers to a period of time or a percentage of a timespan that a machine or system (usually a computer server) is offline or not functioning, usually as a result of either system failure (such as a crash) or routine maintenance. The opposite is uptime.
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High availability is a system design protocol and associated implementation that ensures a certain absolute degree of operational continuity during a given measurement period.
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This article has been tagged since September 2007.

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Within systems engineering, -ilities are aspects or non-functional requirements. They are so-named because most of them end in "-ility."

The "-ilities" often include:
  • accessibility
  • accountability
  • agility

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