The Tier classification (I to IV, defined by the Uptime Institute) rates a data center's redundancy: Tier III, concurrently maintainable (N+1 redundancy); Tier IV, fault tolerant (2N). It does not rate availability, and attaches no percentage to a level.
The higher the Tier, the more the infrastructure (power, cooling, paths) is redundant — and the higher the cost. Most enterprise needs are met by Tier III. The level is indicative unless certified by the Uptime Institute. Be wary of the “Tier III = 99.982%” you see everywhere: that figure comes from a 2001 survey the Uptime Institute itself filed under attributes “unrelated to Tier requirements”. The only rate you can enforce is the one written in your contract.
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