Hardware Origins
Let us backtrack a bit. It is likely that the origin of "five-nines" availability comes
from design specifications for network elements. It also seems likely that the
percent measure came before the current MTBF (Mean Time Between Failures) and
MTTF (Mean Time To Fix) measurement, since it is a simply expressed figure and
the MTTF requirements often match the % calculation while being expressed as an
odd-ish number. However, 99.999% is not so accurate, fundamentally when you
examine it closely, because of the fuzziness of the definition of availability. So in
MIL-HDBK-217 and Bellcore/Telecordia TR332 they standardized these measures.
The basic hardware design measures became:
MTBF - the average time between failures in hardware modules
MTTR - is the time taken to repair a failed hardware module.