The Uptime Institute created an industry-standard classification system for levels of site infrastructure functionality offered by different data centers. This four tier system is as follows:

• Tier 1: composed of a single path for power and cooling distribution, without redundant components, providing 99.671% availability.
• Tier II: composed of a single path for power and cooling distribution, with redundant components, providing 99.741% availability
• Tier III: composed of multiple active power and cooling distribution paths, but only one path active, has redundant components, and is concurrently maintainable, providing 99.982% availability
• Tier IV: composed of multiple active power and cooling distribution paths, has redundant components, and is fault tolerant, providing 99.995% availability.