The concept of availability was originally developed for
repairable systems that are required to operate continuously, i.e.,
round-the-clock, and are at any random point in time either operating or
“down” because of failure and are being worked upon so as to restore their
operation in minimum time. In this original concept, a system is considered to
be in only two possible states - - operating or in repair -- and availability
is defined as the probability that a system is operating satisfactorily at any
random point in time, t, when subject to a sequence of “up” and “down” cycles
which constitute an alternating renewal process.
Availability theory was treated quite extensively in Section 5;
this section will concentrate on final results and illustrative examples of
the various models.