If you include leap years in the calculation, ie, 525,960 min/yr then yes, availability of 99.999 comes out to 5 min of downtime. The techniques the telcos follow to achieve 5 9s are amusing but here's a few comments. If I had the level of availability of my home phone on my frame/atm networks, I could probably get my handicap down to around 2 or 3. As it is, I have alot of trouble breaking 80. (golf reference) Another thing, some of the comments in this thread make me understand why the availability of the voice network infrastructure is so much superior to that of the data side of things. For one, alot of network engineers that I know don't know the ins and outs of how to calculate availability. This is troubling since it means that even if they accidently design a highly available network, they are left guessing and not predicting how available it will actually be. Every person who calls himself/herself a network *engineer* should be familiar with calculating 3 sorts of availability: 1) Device availability calculated by combining the availabilities of all the significant components of that device 2) The availability of serial vs parallel systems/designs 3) End to End availability calculated by combining the availabilities of groups of devices which function together to achieve a common purpose Some networks do not merit such analysis but you can bet that the 911 emergency services network does and should have this and much more detailed analysis. Another rant if I may. An engineer who can't calculate availability also can't calculate ROI. The cost of employing a certain network design is largely a factor of how available it's going to be. If you build a $2mil network where hour long outages cost the business $400k, maybe you should have built a $4mil network....especially if you know that you can count on 20 hours of outages during normal business hours.... If you've read this far, you may be interested in a book tip on High Availability Networks: Cisco Press: "High Availability Network Fundamentals" by Chris Oggerino. Seems to cover enough detail...to have an impact on awareness... -----Original Message----- From: blitz [mailto:blitz@macronet.net] Sent: Wednesday, April 24, 2002 8:59 PM To: brucewms@pacbell.net Subject: RE: The Myth of Five 9's Reliability (fwd) But THAT was when phones had cranks on the side. ;) 5 nines is a myth, conjured up by sales cretins to have something to sell...If I remember, 5 nines translates to 6 minutes outage a YEAR..? (Correct me if I'm wrong here) It's a marketing ploy for liar sales people and CEO's, it has absolutely NOTHING to do with real-world conditions.
BTW - One of my best friends growing up ( and we took EE together ) grandfather was the V.P. in charge of AT&T's LongLines division while a lot of the "wire was pulled". From what he said of his grandfather's remarks, they didn't think about five 9's. The question was how much spare/redundant capacity did you have, both for dependability and to support the countries growth. Not exactly "this quarter's profit" thinking -sigh-.