Source for availability numbers (Re: SONET ring questions)

Fri Oct 23 06:11:00 UTC 1998

alan at globalcenter.NET (Alan Hannan) writes:
>> For total system uptime
>> 90.0% (one nine or less) Desktop systems.
>> 99.0% (two nines) Intermediate business systems
>> 99.9% (three nines) Most business data systems and workgroup servers
>> 99.99% (four nines) High-end business systems and your friendly
>> neighborhood telco
>> 99.999% (five nines) Bank Data Centers and Telco Data Centers, some ISPs
>> 99.9999% (six nines) Only God and Norad live here.
>> 99.99999% (seven nines) Even God doesn't have pockets this deep.
>
>  What's your source for this data?

Here are some availability parameters I understand are used for
US Government Communication Service acquisitions by the FAA and
DOD.  I don't know what, if any, study or research went into these
definitions.  There seems to be a lot of exageration in both the
stated requirement and deliverability of many availability numbers.
However, the table below are some of the sanest ones I've seen.

The most extreme availibility number I give even the slightest
credence was for the AT&T Master Clock in Hillsboro, Missouri
at 9 nines.  I've never confirmed it with AT&T though, and wouldn't
be surprised if it was less.

Service        Service                       Expected      Expected
Type           Scope                         Availability  Restoration Time

Critical       Functions or Services         0.9999987     <6 seconds
               which, if lost, would
               prevent the network/
               system from exercising
               safe operation and control
               for the end users

Essential      Functions or services         0.999         approximately
               which, if lost, would                       10 minutes
               reduce the capability
               of the network/system
               to exercise safe operation
               and control for the end users

Routine        Functions or services         0.99          approximately
               which, if lost, would                       2 hours
               not significantly degrade
               the capability of the
               network/system to exercise
               safe operation and control
               for the end users

I took this table from another publication, which gave the reference
as U.S. Federal Standard 1037B, Telecommunications Glossary.

A note was included "The expected availability values require information
as to the population and interval over which they have been calculated."
-- 
Sean Donelan, Data Research Associates, Inc, St. Louis, MO
  Affiliation given for identification not representation