3 9s Of Availability
Tom Donahou
Availability is the probability that an item will be in an operable and committable state at the start of a mission when the mission is called for at a random time, and is generally defined as uptime divided by total time (uptime plus downtime).
Availability, inherent (Ai) [2]The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment. It excludes logistics time, waiting or administrative downtime, and preventive maintenance downtime. It includes corrective maintenance downtime. Inherent availability is generally derived from analysis of an engineering design:
Availability, achieved (Aa) [3] The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment (i.e., that personnel, tools, spares, etc. are instantaneously available). It excludes logistics time and waiting or administrative downtime. It includes active preventive and corrective maintenance downtime.
Availability, operational (Ao) [4]The probability that an item will operate satisfactorily at a given point in time when used in an actual or realistic operating and support environment. It includes logistics time, ready time, and waiting or administrative downtime, and both preventive and corrective maintenance downtime. This value is equal to the mean time between failure (MTBF) divided by the mean time between failure plus the mean downtime (MDT). This measure extends the definition of availability to elements controlled by the logisticians and mission planners such as quantity and proximity of spares, tools and manpower to the hardware item.
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." 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 availability 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. A very comprehensive recent book is by Trivedi and Bobbio [2017].
The USDA, Economic Research Service (ERS) Food Availability (Per Capita) Data System (FADS) includes three distinct but related data series on food and nutrient availability for consumption: food availability data, loss-adjusted food availability data, and nutrient availability data. The data serve as proxies for actual consumption at the national level. The food availability data series serves as the foundation for the other two series. Loss-adjusted food availability data (LAFA) are derived from food availability data by adjusting for food spoilage, plate waste, and other losses to more closely approximate actual consumption. This second data series is considered preliminary because the underlying food loss assumptions and estimates require further improvement. The nutrient availability data series calculates the daily food energy and 27 nutrients/dietary components of the U.S. food supply. This third series is no longer updated by USDA's Center for Nutrition Policy and Promotion (CNPP); ERS provides archived tables of this data series through 2010. Documentation is available for each of the three data series.
The loss-adjusted food availability data include estimates for over 200 commodities. In 2017, ERS contracted with RTI International to form a panel of academic experts to review seven critical technical issues and seven data gaps in the LAFA series. In the report, Expert Panel on Technical Questions and Data Gaps for the ERS Loss-Adjusted Food Availability (LAFA) Data Series, the panel recommended approaches and methods to improve the series. Two of the recommended changes are included in the current update: (1) new loss factors for individual fresh fruits and vegetables, and (2) a new "edible weight" estimate at the consumer level for all commodities. See the Loss-Adjusted Food Availability Documentation for more information.
Wavelength Zones allow developers to build applications that deliver ultra-lowlatencies to 5G devices and end users. Wavelength deploys standard AWS compute andstorage services to the edge of telecommunication carriers' 5G networks.
AWS operates state-of-the-art, highly available data centers. Although rare, failurescan occur that affect the availability of instances that are in the same location. If youhost all of your instances in a single location that is affected by a failure, none of yourinstances would be available.
When you view your resources, you see only the resources that are tied to the Regionthat you specified. This is because Regions are isolated from each other, and we don'tautomatically replicate resources across Regions.
An AWS account provides multiple Regions so that you can launch Amazon EC2instances in locations that meet your requirements. For example, you mightwant to launch instances in Europe to be closer to your European customersor to meet legal requirements.
The following table lists the Regions provided by an AWS account. You can'tdescribe or access additional Regions from an AWS account, such as theAWS GovCloud (US) Regions or the China Regions. To use a Region introduced after March20, 2019, you must enable the Region. For more information, see Specify which AWS Regions your account can use in theAWS Account Management Reference Guide.
The number and mapping of Availability Zones per Region may vary betweenAWS accounts To list the Availability Zones that are available to your account,you can use the Amazon EC2 console or the command line interface. For more information,see Describe yourRegions.
When you work with an instance using the command line interface or API actions,you must specify its Regional endpoint. For more information about the Regions andendpoints for Amazon EC2, see Amazon EC2 endpointsand quotas in the Amazon Web Services General Reference.
You can use the Amazon EC2 console or the command line interface to determine whichRegions are available for your account. For more information about these commandline interfaces, see Access Amazon EC2.
The longName parameter contains the Region display name. Thefollowing get-parameters-by-path command returns the display name of theaf-south-1 Region. It uses the --query option to scopethe output to the name of the Region. You must enclose the query string in singlequotes on Linux. To run this command using the Windows Command Prompt, either omitthe single quotes or change them to double quotes.
When you launch an instance, you select a Region and a virtual private cloud (VPC),and then you can either select a subnet from one of the Availability Zones or let uschoose one for you. If you distribute your instances across multiple Availability Zonesand one instance fails, you can design your application so that an instance in anotherAvailability Zone can handle requests. You can also use Elastic IP addresses to mask thefailure of an instance in one Availability Zone by rapidly remapping the address to aninstance in another Availability Zone.
The following diagram illustrates multiple Availability Zones in an AWS Region.Availability Zone A and Availability Zone B each have one subnet, and each subnet hasinstances. Availability Zone C has no subnets, therefore you can't launch instances intothis Availability Zone.
As Availability Zones grow over time, our ability to expand them can becomeconstrained. If this happens, we might restrict you from launching an instance in aconstrained Availability Zone unless you already have an instance in that AvailabilityZone. Eventually, we might also remove the constrained Availability Zone from the listof Availability Zones for new accounts. Therefore, your account might have a differentnumber of available Availability Zones in a Region than another account.
To ensure that resources are distributed across the Availability Zones for aRegion, we independently map Availability Zones to codes for each AWS accountin our oldest Regions. For example, the us-east-1a foryour AWS account might not be the same physical location as theus-east-1a for another AWS account.
To coordinate Availability Zones across accounts in all Regions even thosethat map Availability Zones, use the AZ IDs, which are unique and consistent identifiers for an Availability Zone. For example,use1-az1 is an AZ ID for the us-east-1 Region,and it has the same physical location in every AWS account. You can viewthe AZ IDs for your account to determine the physical location of your resourcesrelative to the resources in another account. For example, if you share a subnet inthe Availability Zone with the AZ ID use1-az2 with another account,this subnet is available to that account in the Availability Zone whose AZ ID isalso use1-az2.
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