Dictionary - Definition of AVAILABILITIES

Availability

For the thermodynamic function availability, in the sense of available useful work, see exergy.

For availability as a form of cognitive bias, see availability heuristic.

In telecommunications and reliability theory, the term availability has the following meanings:

1. The degree to which a system, subsystem, or equipment is operable and in a committable state at the start of a mission, when the mission is called for at an unknown, i.e., a random, time. Simply put, availability is the proportion of time a system is in a functioning condition.

Note 1: The conditions determining operability and committability must be specified.

Note 2: Expressed mathematically, availability is 1 minus the unavailability.

2. The ratio of (a) the total time a functional unit is capable of being used during a given interval to (b) the length of the interval.

Note 1: An example of availability is 100/168 if the unit is capable of being used for 100 hours in a week.

Note 2: Typical availability objectives are specified either in decimal fractions, such as 0.9998, or sometimes in a logarithmic unit called nines, which corresponds roughly to a number of nines following the decimal point, such as "five nines" for 0.99999 reliability.

Source: from Federal Standard 1037C in support of MIL-STD-188

Representation

The most simple representation for availability is as a ratio of the expected value of the uptime of a system to the aggregate of the expected values of up and down time, or

$A = \frac{E[\mathrm{Uptime}]}{E[\mathrm{Uptime}]+E[\mathrm{Downtime}]}$

If we define the status function $X (t)$ as

$X(t)= \begin{cases} 1, & \mbox{sys functions at time } t\\ 0, & \mbox{otherwise} \end{cases}$

therefore, the availability is represented by

$A(t)=\Pr[X(t)=1].$

$E[X(t)]=X.\Pr[X(t)=1] \quad t > 0.$

Average availability must be defined on an interval of the real line. If we consider an arbitrary constant $c$ , then average availability is represented as

$A_c = \frac{1}{c}\int_0c A(t)\,dt,\quad c > 0.$

Limiting (or steady-state) availability is represented by

$A = \lim_{t \rightarrow \infty} A(t).$

Limiting average availability is also defined on an interval $(0, c]$ as,

$A_{\infty}=\lim_{c \rightarrow \infty} A_c = \lim_{c \rightarrow \infty}\frac{1}{c}\int_0c A(t)\,dt,\quad c > 0.$

Literature

Availability is well established in the literature of stochastic modeling and optimal maintenance. Barlow and Proschan [1975] define availability of a repairable system as "the probability that the system is operating at a specified time t." While Blanchard [1998] gives a qualitative definition of availability as "a measure of the degree of a system which is in the operable and committable state at the start of mission when the mission is called for at an unknown random point in time." This definition comes from the MIL-STD-721. Lie, Hwang, and Tillman [1977] developed a complete survey along with a systematic classification of availability.

Availability measures are classified by either the time interval of interest or the mechanisms for the system downtime. If the time interval of interest is the primary concern, we consider instantaneous, limiting, average, and limiting average availability. The aforementioned definitions are developed in Barlow and Proschan [1975], Lie, Hwang, and Tillman [1977], and Nachlas [1998]. The second primary classification for availilability is contingent on the various mechanisms for downtime such as the inherent availability, achieved availability, and operational availability. (Blanchard [1998], Lie, Hwang, and Tillman [1977]). Mi [1998] gives some comparison results of availability considering inherent availability.

Availability considered in maintenance modeling can be found in Barlow and Proschan [1975] for replacement models, Fawzi and Hawkes [1991] for an R-out-of-N system with spares and repairs, Fawzi and Hawkes [1990] for a series system with replacement and repair, Iyer [1992] for imperfect repair models, Murdock [1995] for age replacement preventive maintenance models, Nachlas [1998, 1989] for preventive maintenance models, and Wang and Pham [1996] for imperfect maintenance models.

External links

Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Availability". Image Credit.


Part of Speech	Definition
Noun Plural	1. Plural inflection of the noun availability.[Eve - graph theoretic]
Noun Base (availability)	1. The quality of being at hand when needed.[Wordnet]. 2. The quality of being available; availableness.[Websters]. 3. That which is available.[Websters].
Sources: compiled from various sources, (under license) copyright 2008.		Top


Part of Speech	Definition
Noun Plural	1. Plural inflection of the noun availability.[Eve - graph theoretic]
Noun Base (availability)	1. The quality of being at hand when needed.[Wordnet]. 2. The quality of being available; availableness.[Websters]. 3. That which is available.[Websters].
Sources: compiled from various sources, (under license) copyright 2008.		Top

Domain	Definition
Computing	Availability The degree to which a system suffers degradation or interruption in its service to the customer as a consequence of failures of one or more of its parts. One of the components of RAS. (2000-08-13) Source: The Free On-line Dictionary of Computing.
Aerospace	A measure of equipment, system, or network performance, usually expressed in percent; the ratio of operating time to the sum of operating time plus down- time. (references)
Business	1) The degree to which a system, subsystem, or equipment is operable and in a committable state at the start of a mission, when the mission is called for at an unknown, i.e., a random, time. Note 1: The conditions determining operability and committability must be specified. Note 2: Expressed mathematically, availability is 1 minus the unavailability. 2) The ratio of (a) the total time a functional unit is capable of being used during a given interval to (b) the length of the interval. Note 1: An example of availability is 100/168 if the unit is capable of being used for 100 hours in a week. Note 2: Typical availability objectives are specified in decimal fractions, such as 0.9998. (references)
Computing	1: The degree to which a system or resource is ready when needed to process data. Source: European Union. (references)
	2: That fraction of time the system is operational. Source: European Union. (references)
	3: The prevention of the unauthorized withholding of information or resources. Source: European Union. (references)
Electrical Engineering	1: The state of an item of being able to perform its required function. Source: European Union. (references)
	2: A state in which an installation can be energised immediately any time. Source: European Union. (references)
	3: The number of trunks to which a selector has access. Source: European Union. (references)
Energy	1: In reference to coal resources, the absence of land-use or environmental restrictions and technological restrictions. (references)
	2: Describes the reliability of power plants. It refers to the number of hours that a power plant is available to produce power divided by the total hours in a set time period, usually a year. (references)
Military	1: A measure of the degree to which an item is in an operable state and can be committed at the start of a mission when the mission is called for at an unknown (random) point in time. (references)
	2: The availability of a navigation system is the percentage of time that a signal within pre-established tolerances is being broadcast throughout the coverage area. Availability is an indication of the ability of the system to provide usable service within the specified coverage area. Signal availability is the percentage of time that navigational signals transmitted from external sources are available for use. It is a function of both the physical characteristics of the environment and the technical capabilities of the transmitter facilities. For example, the measured availability of the Loran-C chain in the eastern U.S. and Canada have averaged 99.8786% availability over the last decade. (references)
Nuclear Energy & Physics	Capability of systems to function. Source: European Union. (references)
Physics	The maximum fraction of energy in a system which, in the presence of a medium at a given temperature T o, can be converted into useful work during a process at the end of which the system temperature is T o. Source: European Union. (references)
Post & Telecom	A measure of how much time a network or a connection is running. Source: European Union. (references)
Public Administration	The condition of a person, a service, an item or other resource to perform the expected or required function at a given moment. Source: European Union. (references)
Research	The state wherein information and systems are in the place needed by the user, at the proper time, and in the form that the user requests. (references)
Source: compiled by the editor from various references; see credits.		Top

Expressions	Definition
Availability error	Availability error, related to the gambler's fallacy, is the distortion of one's perceptions of reality due to the tendency to remember one alternative outcome of a situation much more easily than another. (references)
Availability heuristic	The availability heuristic is a rule of thumb, or heuristic, which occurs when people estimate the probability of an outcome based on how easy that outcome is to imagine. As such, vividly described, emotionally-charged possibilities will be perceived as being more likely than those that are harder to picture or are difficult to understand, resulting in a corresponding cognitive bias. (references)
High Availability Cluster Multiprocessing	HACMP is IBM's solution for high-availability clusters on the AIX Unix platform. (references)
Limited availability	Due to previous technological challenges, older switches can only connect some inlets to some outlets; this is known as Limited Availability [2, 3]. In Limited Availability Switches, the circuits inside the switch are arranged into Grading groups [1]. An example of a Limited Availability Switch can be seen in Figure 1. (references)
Reliability, Availability and Serviceability	Reliability, Availability and Serviceability is a computer hardware engineering term. It originated from IBM. (references)
Service Availability Forum	The Service Availability Forum(TM) is a consortium of industry-leading communications and computing companies working together to foster an ecosystem that enables the use of commercial off-the-shelf building blocks in the creation of high availability network infrastructure products, systems and services. (references)
Source: compiled by the editor from various references; see credits.		Top

Expressions	Domain	Definition
Availability date	Military	(DOD) The date after notification of mobilization by which forces will be marshaled at their home station or mobilization station and available for deployment. See also home station; mobilization; mobilization station. (references)
Availability Session	Environment	Informal meeting at a public location where interested citizens can talk with EPA and state officials on a one-to-one basis. (references)
Biological Availability	Health	The extent to which the active ingredient of a drug dosage form becomes available at the site of drug action or in a biological medium believed to reflect accessibility to a site of action. (references)
Biological availability	Mining	That portion of a chemical compound or element that can be taken up readily by living organisms. (references)
Calculated Availability	Banking	The assignment of credit availability based on the actual mix of cash letter deposits. (references)
Date of availability	Regulation	The date imported cargo is available/accessible for sampling by FDA. Goods may not be available for sampling as soon as they arrive in the U.S., due to the way the items were shipped/stored. (references)
Expanded Availability	Health	Policy and procedure that permits individuals who have serious or life-threatening diseases for which there are no alternative therapies to have access to investigational drugs and devices that may be beneficial to them. Examples of expanded availability mechanisms include Treatment INDs, Parallel Track, and open study protocols. (references)
Fund Availability	Military	The status of Obligation Authority (OA). (references)
Loran-C Signal Availability	Military	The design minimum availability for a Loran-C triad is 99.7%, computed on an approximately monthly basis. For purposes of computing availability, a baseline (station pair) is considered unavailable when any of the following conditions exist: (i) TD out of tolerance, (ii) ECD out of tolerance, (iii) improper phase code or GRI, or (iv) master or secondary station off-air or operating at less than 50% of specified power output. (references)
Military manpower - availability	Military	This entry gives the total numbers of males and females age 15-49 and assumes that every individual is fit to serve. (references)
Public availability session	Health	An informal, drop-by meeting at which community members can meet one-on-one with ATSDR staff members to discuss health and site-related concerns. (references)
Standard number and terms of availability	Art	In AACR2, the area of bibliographic description in which the standard number (ISBN, ISSN, etc.), list price, and any other terms under which the item is available are entered (field 020 or 022 of the MARC record). (references)
Terms of availability	Art	The conditions under which a bibliographic item is available for sale in the market place, usually the list price, entered in field 020 or 022 of the MARC record. (references)
Time availability	Business	Synonym circuit reliability. (references)
Source: compiled by the editor from various references; see credits.			Top

Definition: AVAILABILITIES

Definition: AVAILABILITIES

Specialty Definition: availability

Common Expressions: availability

Specialty Expressions: availability

Extended Definition: Availability

Availability

Representation

Literature

See also

External links

Topics by Level of Interest: availability

Topics by Level of Interest: availability
Topics sorted by level of Interest	Level (1=low, 600=high)	Topics sorted Alphabetically	Level (1=low, 600=high)
International availability of Fanta	41	Availability	9
Emergency contraceptive availability by country	28	Availability based tariff	9
Availability heuristic	12	Availability factor	4
Expedited Funds Availability Act	12	Availability heuristic	12
High availability	10	Emergency contraceptive availability by country	28
Availability based tariff	9	Expedited Funds Availability Act	12
Availability	9	High availability	10
IBM High Availability Cluster Multiprocessing	6	IBM High Availability Cluster Multiprocessing	6
Reliability, Availability and Serviceability	6	International availability of Fanta	41
Route availability	4	International Network for the Availability of Scientific Publications	3
Limited availability	4	Law Enforcement Availability Pay	3
Service Availability Forum	4	Limited availability	4
Availability factor	4	Reliability, Availability and Serviceability	6
International Network for the Availability of Scientific Publications	3	Route availability	4
Law Enforcement Availability Pay	3	Service Availability Forum	4
Source: the editor, created by/for EVE to gauge likely levels of human interest in linguistically triggered topics (compiled across various sources, such as Wikipedia and specialty expression glosses).