Tier One

• Tier One is also called "Basic" with 99.671 percent availability. This tier is a simple "server room," such as that found in many small businesses. This data center may be disrupted by unplanned activity and does not require raised floors or generators.
  
  

Tier Two

• This tier is characterized by "Redundant Components" and has a 99.741 percent availability rate. It is less likely to be disrupted by unplanned activity and must have precautions in place for power and cooling disruptions, such as maintaining an uninterrupted power supply or a small generator. Maintenance of power infrastructure requires the Tier 2 data center to be shut down.
  
  

Tier Three

• Known as "Concurrently Maintainable," this tier has a 99.982 percent availability rate. This tier requires a raised floor and a highly resilient power infrastructure; it has an annual downtime of about an hour and a half.
  
  

Tier Four

• This tier is known as "Fault Tolerant," with a 99.995 percent availability rate. Taking more than a year to implement, this tier can sustain a worst-case outage event (such as the loss of a regional power station or physical damage to the data center) with no impact. It has an annual downtime of less than half an hour.