The availability of a system, as previously discussed in Sections 7.3.17 and 8.6.4, is the probability that a system will operate when called on at random to do so. A mean-value approach to availability (see Chapter 8) defines it as


where A = availability

MTBF = mean time between failures MDT = mean down time

Availability can be viewed as the percentage of time that the system is operative, on the average, in relation to the total time.

Example. If the failure rate for a system is 0.01 failure per hour and the mean-time-to-repair distribution is uniform in the range 2 to 8 hours, what is the system availability? The MTBF, from the failure rate, is 1/0.01, or 100 hours. The mean down time is taken to be the average value of the repair-time uniform distribution, which is calculated as (2 + 8)/2 = 5 hours. From the preceding formula, the availability is 100/(100 + 5) = 0.952.

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

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