A data center tier is a level in the Uptime Institute's Tier Classification System, which rates a data center's power and cooling infrastructure from Tier I to Tier IV based on how well it keeps running through maintenance and equipment failures. For datacenter proxies, the tier describes the facility hosting the proxy servers, not the proxy service itself.

Uptime Institute states that the tiers are not a ranking. Each level fits different business needs, and the owner decides which tier is appropriate.

The Four Tiers

  • Tier I (Basic Capacity): The facility must shut down completely for preventive maintenance and repairs, and an unexpected failure can disrupt operations.
  • Tier II (Redundant Capacity Components): Adds redundant power and cooling components that can be removed for maintenance without a shutdown, but the distribution path is not redundant.
  • Tier III (Concurrently Maintainable): Adds redundant distribution paths, so any part can be taken offline for maintenance without affecting IT operations.
  • Tier IV (Fault Tolerant): Uses independent, physically isolated systems, so a single equipment failure or path interruption does not affect IT operations.

Tier Certification

Uptime Institute says it is the only licensed firm that can certify facilities against its Tier standards. It offers separate certifications for a facility's design documents, for the constructed facility, and for operational sustainability. When a host cites a tier, it helps to check which of these the claim refers to.

Tiers and Redundancy Labels

Labels such as N+1, N+2 or 2N describe how much spare equipment a facility has. Uptime Institute states that these labels do not determine a tier. A tier depends on whether the facility can be maintained, and can withstand a fault, without affecting IT operations.