Unavailability
Encyclopedia
Unavailability can be defined as the probability
that an item will not operate correctly at a given time and under specified conditions. It opposes availability
.
Numerical values associated with the calculation
of availability are often awkward, consisting of a series of 9s before reaching any significant numerical information (e.g. 0.9999999654). For this reason, it is more convenient to use the complement measure of availability, namely, unavailability. Expressed mathematically, unavailability is 1 minus the availability
. Therefore, a system
with availability 0.9999999654 is more concisely described as having an unavailability of 3.46E-8.
Unavailability may be expressed mathematically as the ratio,
where MTTR is the mean time to repair, and MTTF is the mean time to failure. Alternatively, this can be written as
where λ' is the critical failure rate (λ without the tick is the total failure rate) and μ' is the critical repair rate (rate at which critical failures are repaired).
In telecommunication
, an unavailability is an expression of the degree to which a system
, subsystem, or equipment is not operable and not in a committable state at the start of a mission, when the mission is called for at an unknown, i.e., random, time
. The conditions determining operability and committability must be specified.
Probability
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...
that an item will not operate correctly at a given time and under specified conditions. It opposes availability
Availability
In telecommunications and reliability theory, the term availability has the following meanings:* The degree to which a system, 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...
.
Numerical values associated with the calculation
Calculation
A calculation is a deliberate process for transforming one or more inputs into one or more results, with variable change.The term is used in a variety of senses, from the very definite arithmetical calculation of using an algorithm to the vague heuristics of calculating a strategy in a competition...
of availability are often awkward, consisting of a series of 9s before reaching any significant numerical information (e.g. 0.9999999654). For this reason, it is more convenient to use the complement measure of availability, namely, unavailability. Expressed mathematically, unavailability is 1 minus the availability
Availability
In telecommunications and reliability theory, the term availability has the following meanings:* The degree to which a system, 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...
. Therefore, a system
System
System is a set of interacting or interdependent components forming an integrated whole....
with availability 0.9999999654 is more concisely described as having an unavailability of 3.46E-8.
Unavailability may be expressed mathematically as the ratio,
- U = MTTR/(MTTR+MTTF)
where MTTR is the mean time to repair, and MTTF is the mean time to failure. Alternatively, this can be written as
- U = λ'/(λ'+μ')
where λ' is the critical failure rate (λ without the tick is the total failure rate) and μ' is the critical repair rate (rate at which critical failures are repaired).
In telecommunication
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
, an unavailability is an expression of the degree to which a system
System
System is a set of interacting or interdependent components forming an integrated whole....
, subsystem, or equipment is not operable and not in a committable state at the start of a mission, when the mission is called for at an unknown, i.e., random, time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
. The conditions determining operability and committability must be specified.