The Myth of Five 9's Reliability (fwd)

Wed Apr 24 20:18:13 UTC 2002

>From the Canarie news mailing list.

I don't think I've ever experienced five 9's on any telco
service, I have always assumed I must be the one customer
experiencing down-time, and the aggregate was somehow five
9's. How is network reliability calculated to end up with 
five 9's?

Pete.

---------- Forwarded message ----------
Date: Wed, 24 Apr 2002 10:08:18 -0400 (EDT)
From: CAnet-3-NEWS at canarie.ca
Subject: [news] The Myth of Five 9's Reliability

For more information on this item please visit the CANARIE CA*net 3 Optical
Internet program web site at http://www.canet3.net/news/news.html
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[A good article on the truth about five 9's reliability. Some excerpts -
BSA]

http://www.bcr.com/forum

Deep Six Five-Nines?

For much of the 20th century, the U.S. enjoyed the best
network money could buy; hands-down, it was the most modern,
most ubiquitous and most reliable in the world. And one
term--five-nines--came to symbolize the network's
robustness, its high availability, its virtual
indestructibility. When the goal of five-nines was set, the
network was planned, designed and operated by a monopoly,
which was guaranteed a return on whatever it invested. It
was in the monopoly's interest to make the network as
platinum-plated as possible.

One of the key points is that "five-nines" has long been
somewhat overrated. Five-nines is NOT an inherent capability
of circuit-switched, TDM networks. It's a manmade concept,
derived from a mathematical equation, which includes some
things and leaves out others.

It's critical to remember that when you run the performance
numbers on ALL the items in a network--those that are
included in the five-nines equation and those that
aren't--you're probably going to wind up with a number less
than 99.999 percent. A well-run network actually delivers
something around 99.45 percent.

The gap between the rhetoric of five-nines and actual
network performance leads to the conclusion that five-nines
may not be a realistic or even necessary goal.