ISBN : 978-81-963532-2-3 (E-Book) with reliability. There are six reliability metrics which can be used to quantify the reliability ofsoftwareproducts. Rateofoccurrenceoffailure(ROCOF)-ROCOFmeasuresthefrequencyofoccurrence of unexpected behavior (i.e. failures). ROCOF measure of a software productcan be obtained by observing the behavior of a software product in operation over aspecifiedtimeintervalandthenrecordingthetotalnumberoffailuresoccurringduringtheinterv al. MeanTimeToFailure(MTTF)-MTTFistheaveragetimebetweentwosuccessive failures, observed over a large number of failures. To measure MTTF, we can record thefailuredatafornfailures.Letthefailuresoccuratthetimeinstantst,t,…,t.Then, 1 2 MTTFcan becalculatedas n It is important to note that only run time is considered in the time measurements, i.e. thetime for which the system is down to fix the error, the boot time, etc are not taken intoaccountin the timemeasurements and theclockis stopped at thesetimes. Mean Time To Repair (MTTR) - Once failure occurs, sometime is required to fix theerror. MTTR measures the average time it takes to track the errors causing the failure andto fixthem. Mean Time Between Failure (MTBR) - MTTF and MTTR can be combined to get theMTBR metric: MTBF = MTTF + MTTR. Thus, MTBF of 300 hours indicates that once afailureoccurs,thenextfailureisexpectedafter300hours.Inthiscase,timemeasurementsarereal time and nottheexecution time as inMTTF. Probability of Failure on Demand (POFOD) - Unlike the other metrics discussed, thismetricdoesnotexplicitlyinvolvetimemeasurements.POFODmeasuresthelikelihoodof the system failing when a service request is made. For example, a POFOD of 0.001wouldmean that 1 outofevery1000 servicerequestswould result in afailure. Availability- Availability of a system is a measure of how likely shall the system beavailableforuseoveragivenperiodoftime.Thismetricnotonlyconsidersthenumberof failures occurring during a time interval, but also takes into account the repair time(down time) of a system when a failure occurs. This metric is important for systems suchas telecommunication systems, and operating systems, which are supposed to be neverdown and where repair and restart time are significant and loss of service during that timeis important. 124 Software Engineering Keerthana P, Manasa KN, Ganga D Bengal
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