In
reliability engineering
Reliability engineering is a sub-discipline of systems engineering that emphasizes the ability of equipment to function without failure. Reliability describes the ability of a system or component to function under stated conditions for a specifie ...
, the term availability has the following meanings:
* The degree to which a
system
A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and express ...
,
subsystem or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown, ''i.e.'' a random, time.
* The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment.
Normally high availability systems might be specified as 99.98%, 99.999% or 99.9996%.
Representation
The simplest representation of availability (''A'') is a ratio of the expected value of the uptime of a system to the aggregate of the expected values of up and down time (that results in the "total amont of time" ''C'' of the observation window)
:
Another equation for availability (''A'') is a ratio of the Mean Time To Failure (MTTF) and Mean Time To Repair (MTTR), or
:
If we define the status function
as
:
therefore, the availability ''A''(''t'') at time ''t'' > 0 is represented by
:
Average availability must be defined on an interval of the real line. If we consider an arbitrary constant
, then average availability is represented as
:
Limiting (or steady-state) availability is represented by
:
Limiting average availability is also defined on an interval