The <a href=”http://uptimeinstitute.org” title=”Uptime Institute”>Uptime Institute</a> has a wide range of published resource material. You might wish to visit their website for more information. Below is an description from their website and a couple of helpful charts they publish.
<i>Since 1993, the Uptime Institute, Inc. has been a respected provider of educational and consulting services for Facilities and Information Technology organizations interested in maximizing data center uptime. The Institute has pioneered numerous innovations which serve as industry standards today, including the Tier Classifications which rate data center availability.
<img src=”http://itknowledgeexchange.techtarget.com/itanswers/files/2008/03/uptime-tier-chart.jpg” alt=”Chart: Typical Tier Attributes” />
And here is another chart from Uptime Institute indicating performance standards by tier level.
<img src=”http://itknowledgeexchange.techtarget.com/itanswers/files/2008/03/uptime-tier-performance.jpg” alt=”Chart: Performance Standards by Tier Level” />