Reality Check On Five-Nines
by Gary Audin

Published May 2002

Abstract:

While everyone in telecom has been using the term "five-nines" for decades, I'm willing to bet that few actually know what 99.999 percent really means, and even fewer know if it's really necessary. Of course, we all know that five-nines relates to reliability, and expectations are high— dialtone is expected at all times. Way back when AT&T's #1ESS switch was being developed, the development team's goal was to have less than one day of outage in 40 years. Obviously no one kept one in service long enough to find out if that target was actually met, but the 1Es cranked along pretty predictably.

Does telecom still need its gold-standard availability metric? Maybe not.

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