Question: how high must reliability be prime business customers expect public...
How high must reliability be? Prime business customers expect public carrier-class communications data links to be available 99.999 percent of the time. The so-called five nines rule implies only 5 minutes of downtime per year. Such high reliability is needed not only in telecommunications but also for mission-critical systems such as airline reservation systems or banking fund transfers.
Suppose a certain network web server is up only 90 percent of the time (i.e. its probability of being down is 0.10). How many independent servers are needed to ensure that the system is up at least 99.999 percent of the time?
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