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| Availability in Layman's Terms | |
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Availability of a system over its life-cycle[disambiguation needed ] is typically measured as a factor of its reliability - as reliability increases, so does availability. However, no system can guarantee 100.000% reliability; and as such, no system can assure 100.000% availability. Further, reliability engineering and maintainability involve processes designed to optimize availability under a set of constraints, such as time and cost-effectiveness. Availability is the goal of most system users, and reliability engineering and maintainability provide the means to assure that availability performance requirements are achieved.
[edit] Tags:System,Reliability Engineering,Requirements, | |
| Representation | |
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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
A(t) = Pr[X(t) = 1] = E[X(t)].
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[citation needed]
Limiting average availability is also defined on an interval (0,c] as,
[edit] Tags: | |
| Example | |
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If we are using equipment which has mean time to failure (MTTF) of 81.5 years and mean time to repair (MTTR) of 1 hour:
MTTF in hours = 81.5*365*24=713940
Availability= MTTF/(MTTF+MTTR) = 713940/713941 =99.999859%
Unavailability = 0.000141%
Outage due to equipment in hours per year
U=0.01235 hours per year.
[edit] Tags:Mean Time To Failure, | |
| Literature | |
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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.
[edit] Tags:Stochastic Modeling,Spares, | |
| References | |
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This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).
[edit] Tags:Public Domain Material,Mil-std-188, | |
| External links | |
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Reliability and Availability Basics
System Reliability and Availability
Retrieved from "http://en.wikipedia.org/w/index.php?title=Availability&oldid=470943849"
Categories: Applied probabilityTelecommunications termsHidden categories: Articles with links needing disambiguation from December 2011All articles with unsourced statementsArticles with unsourced statements from August 2010Wikipedia articles incorporating text from the Federal Standard 1037CWikipedia articles incorporating text from MIL-STD-188
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