The predicted Mean Time Between Failure of the 9XStream module is 700,000 hours.
This is a computed prediction, not a field tested result. The computation takes into account the life expectancy of each of the individual components and how they are being utilized. In practice the 9XStream modules (and other Digi products) have proven to be very reliable in the field with abnormally low failure rates.
Further clarification on MTBF definition can be found here Understanding MTBF (Mean Time Between Failures)
Aug 08, 2017