In telecommunications and reliability theory, the term availability has the following meanings: BlackBerry 7100t Telecommunication refers to communication over long distances. ... Reliability theory developed apart from the mainstream of probability and statistics, and was used originally as a tool to help nineteenth century maritime insurance and life insurance companies compute profitable rates to charge their customers. ...

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. A system is an assemblage of inter-related elements comprising a unified whole. ... 8:17 am, August 6, 1945, Japanese time. ...

Note 1: The conditions determining operability and committability must be specified.

Note 2: Expressed mathematically, availability is 1 minus the unavailability. 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, random, time. ...

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. In computer engineering, an execution unit is a part of a CPU that performs the operations and calculations called for by the program. ...

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. 9 (nine) is the natural number following 8 and preceding 10. ...

Source: from Federal Standard 1037C in support of MIL-STD-188 Federal Standard 1037C entitled Telecommunications: Glossary of Telecommunication Terms is a U.S. Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended. ... MIL-STD-188 is a series of U.S. military standards relating to telecommunications. ...

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 X(t) as

therefore, the availability is represented by

. Average ability must be defined on an interval of the real line. If we consider an arbitrary constant c, 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 (0,c] 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." 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. Downtime refers to the period of time 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. ...

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

• High availability engineering
• The Myth of the Nines
• Ilities

In computer science, the Myth of the Nines refers to the idea that standard measurements of availability can be misleading. ... Within systems engineering, -ilities are aspects or non-functional requirements. ...

External links

• Reliability and Availability Basics
• System Reliability and Availability