# Technical Abbreviation

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(MTBF) Mean time between failures 1. Mean time between failures (MTBF) is the predicted elapsed time between inherent failures of a system during operation. [1] MTBF can be calculated as the arithmetic mean (average) time between failures of a system. The MTBF is typically part of a model that assumes the failed system is immediately repaired (zero elapsed time), as a part of a renewal process. This is in contrast to the mean time to failure (MTTF), which measures average time between failures w
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(MTBF) Mean time between failures1. Mean time between failures (MTBF) is the predicted elapsed time betweeninherent failures of a system during operation. [1] MTBF can be calculated as thearithmetic mean(average) time betweenfailuresof a system. The MTBF is typically part of a model that assumes the failed system is immediately repaired (zero elapsedtime), as a part of arenewal process. This is in contrast to the mean time to failure(MTTF), which measures average time betweenfailureswith the modelingassumption that the failed system is not repaired.The definition of MTBF depends on the definition of what is considered a systemfailure. For complex, repairable systems, failures are considered to be those out of design conditions which place the system out of service and into a state for repair.Failures which occur, can be left or maintained in an unrepaired condition, and do notplace the system out of service, are not considered failures under this definition. [2] Inaddition, units that are taken down for routine scheduled maintenance or inventorycontrol, are not considered within the definition of failure.For each observation, downtime is the instantaneous time it went down, which isafter (i.e. greater than) the moment it went up, uptime. The difference(downtime minus uptime) is the amount of time it was operating between these twoevents. Formal definition of MTBF By referring to the figure above, the MTBF is the sum of the operational periodsdivided by the number of observed failures. If the Down time (with space) refers tothe start of downtime (without space) and up time (with space) refers to thestart of uptime (without space), the formula will be:The MTBF is often denoted by theGreek letter θ  , orThe MTBF can be defined in terms of theexpected valueof the density function  ƒ ( t  )where  ƒ is the density function of time until failure – satisfying the standardrequirement of density functions – Common MTBF misconceptions MTBF is commonly confused with a component's useful life, even though the twoconcepts are not directly related. For example a battery may have a useful life of four hours, and an MTBF of 100,000 hours. These figures indicate that in apopulation of 1,000,000 batteries, there will be approximately ten battery failuresevery hour during a single battery's four-hour life span. [3]  Another common misconception about the MTBF is that it specifies the time (onaverage) when the probability of failure equals the probability of not having a failure(i.e. an availability of 50%). This is only true for certain symmetric distributions. Inmany cases, such as the (non-symmetric)exponential distribution, this is not thecase. In particular, for an exponential failure distribution, the probability that an itemwill fail at or before the MTBF is approximately 0.63 (i.e. the availability at the MTBFis 37%). For typical distributions with some variance, MTBF only represents a top-level aggregate statistic, and thus is not suitable for predicting specific time tofailure, the uncertainty arising from the variability in the time-to-failure distribution.Another misconception is to assume that the MTBF value is higher in a system thatimplements component redundancy. Component redundancy can increase thesystem MTBF, however it will decrease the hardware MTBF, since, generally thegreater number of components in a system, the more frequent a hardwarecomponent will experience failure, leading to a reduction in the system's hardwareMTBF. [4] Variations of MTBF There are many variations of MTBF, such as mean time between system aborts (MTBSA) or mean time between critical failures (MTBCF) or mean timebetween unit replacement (MTBUR). Such nomenclature is used when it isdesirable to differentiate among types of failures, such as critical and non-criticalfailures. For example, in an automobile, the failure of the FM radio does not preventthe primary operation of the vehicle. Mean time to failure (MTTF) is sometimesused instead of MTBF in cases where a system is replaced after a failure, since MTBFdenotes time between failures in a system which is repaired. 2. The-state -of -the -art :   is the highest level of development, as of a device,technique, or scientific field, achieved at a particular time. It also applies to the levelof development (as of a device, procedure, process, technique, or science) reachedat any particular time usually as a result of modern methods. User interface server: The user interface (UI) server is a process that providescommunication services between the event consoles and the event server.The UI server communicates with the dispatch engine when it needs tocontact the event server. The UI server provides transaction locking for eventconsole status updates and prevents multiple event consoles from respondingto the same event. The UI server also automatically updates the status of events on all event consoles by forwarding the event changes from the eventconsoles to the dispatch engine, which sends the changes to the eventdatabase. For example, when an operator acknowledges an event, the UIserver automatically updates the status of the event on each event consolethat contains the event.The UI server also provides a set of commands that enable an operator to changeany event attribute, list the events in a specific event group, and display onemessage on the operator's desktop.The UI server can be installed on any managed node in the Tivoli region, but only onone managed node. The UI server does not have to be located on the same host asthe event server. The name of the UI server process is tec_ui_server. If the UIserver fails, error messages are written in the /tmp/ui_server log file. The name of this log file is defined in the \$BINDIR/TME/TEC/.ui_server_diag_config file and canbe changed.
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