!!! Overview
[{$pagename}] Answers the questions of Are you there?


[{$pagename}] is a should be classified as to the [Data Security Impact]

!! [{$pagename}] of a [Personal Entity]
[{$pagename}] of a [Personal Entity] is often referred to as [Presence]

!! [{$pagename}] of a [system|non-person entity]
[{$pagename}] of a system is typically measured as a factor of its [Reliability] - as reliability increases, so does [{$pagename}] . [{$pagename}]  of a system may also be increased by the strategy on focusing on increasing testability & maintainability and not on [reliability]. 

Improving maintainability is generally easier than reliability. Maintainability estimates (Repair rates) are also generally more accurate. However, because the uncertainties in the reliability estimates are in most cases very large, it is likely to dominate the [availability] (prediction uncertainty) problem, even while maintainability levels are very high.[1]

[{$pagename}] implies there is a some level of [Fault tolerance]

!! How Reliable Are Your Systems
The technology for [Disaster Recovery] and business continuity plans differs dramatically depending on the percentage of uptime that must be delivered.

Any project planning should include some statement of acceptable availability which directly influences design considerations.

This table shows the amount of time your service would be allowed to be un-avalable for each level of Availability.

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||[{$pagename}]||Downtime/year||Downtime/month||Downtime/week||Downtime/day
|98.0000%|7.30 days|14.4 hours|3.360 hours|28.80 minutes
|99.0000%|3.65 days|7.20 hours|1.680 hours|14.40 minutes
|99.5000%|1.83 days|3.60 hours|50.40 minutes|7.200 minutes
|99.9000%|8.76 hours|43.2 minutes|10.10 minutes|1.440 minutes
|99.9900%|52.3 minutes|4.32 minutes|1.010 minutes|8.660 seconds
|99.9990%|5.26 minutes|25.9 seconds|6.050 seconds|0.864 seconds
|99.9999%|31.5 seconds|2.59 seconds|0.065 seconds|0.009 seconds
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!!Do not Over Do it
Moving up in reliability form one level to another is very costly as a percentage of the project cost.

You should balance the risk vs rewards vs the costs for each level.

!! More Information
There might be more information for this subject on one of the following:
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* [#1] - [Availability|Wikipedia:Availability|target='_blank'] - based on information obtained 2013-04-10