1. The degree to which a system System is a set of interacting or interdependent entities forming an integrated whole, subsystem System is a set of interacting or interdependent entities forming an integrated whole, 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 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.
2. The ratio of (a) the total time a functional unit In computer engineering, an execution unit is a part of a CPU that performs the operations and calculations called for by the computer 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 such as a sub-ALU or FPU, or some smaller, more specific 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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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 A(t) at time t>0 is represented by
Average availability must be defined on an interval of the real line. If we consider an arbitrary constant c > 0, then average availability is represented as
Limiting (or steady-state) availability is represented by
Limiting average availability is also defined on an interval (0,c] as,
Example
If we are using equipment which has mean time between failure (MTBF) of 81.5 years and mean time to recovery (MDT) of 1 hour:
MTBF in hours = 81.5*365*24=713940
Availability= MTBF/(MTBF+MDT) = 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 This page is concerned with the stochastic modelling as applied to the insurance industry. For other stochastic modelling applications, please see Monte Carlo method. For mathematical definition, please see Stochastic process and optimal maintenance 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 parameters to minimize. Parameters often considered are the cost of failure,. 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 The term downtime is used to refer to periods when a system is unavailable. Downtime or outage duration refers to a period of time that a system fails to provide or perform its primary function. Reliability, availability, recovery, and unavailability are related concepts. The expected unavailability is the percentage of a timespan that a system is. 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
- High availability Users want their systems, for example wrist watches, hospitals, airplanes or computers, to be ready to serve them at all times. Availability refers to the ability of the user community to access the system, whether to submit new work, update or alter existing work, or collect the results of previous work. If a user cannot access the system, it is
- List of system quality attributes Within systems engineering, quality attributes are non-functional requirements used to evaluate the performance of a system. These are sometimes named "ilities" after the suffix many of the words share. Notable quality attributes include:
- Spurious trip level
References
This article incorporates public domain material A work of the United States government, as defined by United States copyright law, is "a work prepared by an officer or employee of the U.S. government as part of that person's official duties." The term only applies to the work of the federal government, including the governments of "non-organized territorial areas" under the from the General Services Administration The General Services Administration is an independent agency of the United States government, established in 1949 to help manage and support the basic functioning of federal agencies. The GSA supplies products and communications for U.S. government offices, provides transportation and office space to federal employees, and develops government-wide document "Federal Standard 1037C" (in support of MIL-STD-188).
External links
Categories: Applied probability | Telecommunications terms Categories: Telecommunications | Technical terminology | Communication theory
Tue, 06 Jul 2010 14:21:50 GMT+00:00
provides disaster recovery for day-to-day emergencies Search Storage Plantinum's director of technology Gregg Blank said the need for high availability became more pronounced after the firm grew from supplying 16 service ...
