MTBF calculations are only considered when very high Availability requirements are required (above 99.999%, i.e. 5 “nines”). This would usually only be the case for Back Haul links and for specific corporate last mile connections. The probability of power loss should also be taken into consideration.
Equipment Availability is expressed by the equation:
A = MTBF / (MTBF + MTTR)
where MTBF is mean time between failures and MTTR is mean time to repair. Both are measured in hours.
RAy units with MTBF of 100 years and expected repair time 10 hours provide Equipment availability = 99.999% (5 nines). If Equipment Availability should be increased to 99.9999% (6 nines), the average repair time must be reduced down to 1 hour. (Repair time is the responsibility of the customer.)
The equation above only treats equipment failure and its repair time; it does not reflect outage due to fading and other atmospheric issues and/or the probability of power outage. By restating the equation, we can cover the general case:
A = uptime / (uptime + downtime)
where downtime is the number of hours of the downtime of the link whether due to equipment outage or power failures or fading and all other atmospheric issues.
If a system has an uptime of 10 000 hours and a total expected downtime of 10 hours, then
A = 10 000/ (10 000 + 10) = 0.999 or 99.9%
For typical microwave links with Availability requirement below 99.99% the influence of MTBF is negligible (for modern equipment from respected manufacturers) and can be omitted from the calculation.