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Calculating availability

It is a very long time since I have done any work on availability, and I have been shown some figures for a point to point connection that don't seem right. They have calculated the availability of each element using the following method:

MTBF = 54,000 hours

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= 6 hours

Therefore availability = 54,000/(6 +54000)% = 99.989%

Unavailability = 0.011%

However we want to know the annual availability of PtP connection so as far as I can remember:

54,000 hours is equal to 6.164 years:

So, isn't the annual unavailability 0.011%/6= 0.0018%?

And therefore total availability 99.9982%?

And to get the total availability for the connection I just add up all the unavailability figures and subtract from 1?

Many thanks in advance from a frustrated engineer.

Thank you for your question.

First off, you are correct, six hours downtime out of 54,000 hours is 99.989% availability and yes, 54,000 hours is a little more than six years.

However, your assumption that you can annualize the downtime statistics as you describe is not quite correct. Six hours out of 54,000 hours yields 99.989% availability, but if you annualize those numbers (dividing by six) you get one hour of downtime out of 9000 hours (slightly more than a year) which returns exactly the same 99.989% availability average per (slightly more than a) year.

If you know that in a particular year you had less than an hour of downtime then you'll get better availability statistics for that year but for another year of the six, the statistics will be worse. If you know that yesterday the PPTP line stayed up all day that's 100% availability for the day. Daily availability statistics are rarely useful, though.

To calculate availability statistics you count up all the time you were down (six hours), divide that into the total amount of time over which you have collected the statistics (54,000 hours). You should then have a very small number (0.00011). Subtract that small number from 1 (0.99989). Multiply that by 100 to get an availability percentage (99.989%).

Hope this helps.

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This was first published in December 2002

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