Uptime Institute created the standard Tier Classification System to consistently evaluate various data center colocation facilities in terms of potential site infrastructure performance, or uptime.
The Tiers (I-IV) are progressive; each Tier incorporates the requirements of all the lower Tiers.
Tier I: Basic Capacity - A Tier I data center provides dedicated site infrastructure to support information technology beyond an office setting. Tier I infrastructure includes a dedicated space for IT systems; an uninterruptible power supply (UPS) to filter power spikes, sags, and momentary outages; dedicated cooling equipment that won’t get shut down at the end of normal office hours; and an engine generator to protect IT functions from extended power outages.
Tier II: Redundant Capacity Components - Tier II facilities include redundant critical power and cooling components to provide select maintenance opportunities and an increased margin of safety against IT process disruptions that would result from site infrastructure equipment failures. The redundant components include power and cooling equipment such as UPS modules, chillers or pumps, and engine generators.
Tier III: Concurrently Maintainable - A Tier III data center requires no shutdowns for equipment replacement and maintenance. A redundant delivery path for power and cooling is added to the redundant critical components of Tier II so that each and every component needed to support the IT processing environment can be shut down and maintained without impact on the IT operation.
Tier IV: Fault Tolerance - Tier IV site infrastructure builds on Tier III, adding the concept of Fault Tolerance to the site infrastructure topology. Fault Tolerance means that when individual equipment failures or distribution path interruptions occur, the effects of the events are stopped short of the IT operations.
Data center facilities costs and operational complexities increase with each Tier Level, and it is up to the data center owner to determine the Tier Level that fits his or her business’s need. A Tier IV solution is not “better” than a Tier II solution. The data center being selected for your server colocation should match the business needs of the application; otherwise companies can overinvest or take on too much risk.