MTBF Calculation for Time Servers
How Is MTBF for the Network Time Servers Calculated?
Tempus LX and Unison

Mean Time Between Failures (MTBF) is a common way of measuring the reliability of a product.  A simple definition of MTBF is:

   Total hours of operation of the product ÷ Number of product failures

EndRun has been shipping rackmount time servers since early 2001.  If we assume that most of the units shipped have been operating most of the time, that equals the total hours of operation.  Divide that by the number of failures that have occurred over the years and it yields a number greater than 1,900,000 hours (220 years).

The reason we don't claim an MTBF of 1,900,000 hours is because we don't know for sure that most of the units have been operating most of the time.  Plus it is better to take into account the measurement time period in order to increase confidence in the claim.  In our case the measurement period has been about 8 years (since early 2001).  So, if we lower our estimated total hours of operation from 1,900,000 to 225,000 hours (25 years) then our actual measurement period (8 years) is 30% of that.  This is a conservative enough estimate to guarantee that the reliability of the product will continue to exceed the MTBF claim.

The MTBF for the Tempus LX and Unison time servers is 25 years based on demonstrated field data.

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