Data Center Tiers

Park Place Hardware Maintenance

Parker Published: March 21, 2017

What are They and Why Do They Matter?


Did you know there are four “tiers” of data centers? The categorization of data centers and colocation providers into Tiers 1 through 4 can be extremely helpful in evaluating the options in the market for housing IT equipment.

Why data center tiers came about

The data center tier system was implemented in 2005. The Telecommunications Industry Association took a leading role. Their goal was to bring quantifiable standards to an increasingly important industry, that of colocation providers.

The IT industry being what it is, parallel efforts were undertaken as well. A separate tier rubric was created by the Uptime Institute. Fortunately, the differences between the two are minimal and, for most purposes, there is broad agreement on what constitutes a Tier 1 or a Tier 4 data center.

The data center tier criteria

There are a lot of elements that go into a data center, so each tier ranking reflects multiple criteria. From redundancy to fault tolerance, the tier structure is intended to rank data centers in terms of their achievement to standards, with Tier 4 data centers representing the most robust infrastructure.

The TIA and UI tiers are based on the following:

  • Delivery paths for electricity into the building, with more paths providing more reliable electric service
  • Redundancy of core components—basically a measure of how many backups are in place for generators, cooling equipment, UPSs, and so on
  • Concurrent maintainability, which points to the ability of operations to continue while individual equipment undergoes maintenance or replacement
  • Fault tolerance for natural disaster, blackout, and other catastrophes
  • Compartmentalization of different pieces of the infrastructure, enabling them to function independently from the rest of the facility
  • Availability/downtime measurements in terms of guaranteed availability in the course of a year.

Is there a simpler way to look at tiers?

Yes. Although data center tiers officially reflect these diverse elements, most clients care most about the guaranteed uptime. Uptime is usually the most critical factor in any service level agreement. The other tier criteria indicate how and why the facility will be able to deliver on its uptime promise, with greater redundancy, fault tolerant, compartmentalized, and otherwise high-quality infrastructure.

It is generally accepted that the tiers represent the following uptime guarantees:

  • Tier 1: 99.67%
  • Tier 2: 99.74%
  • Tier 3: 99.98%
  • Tier 4: 99.995%

Although set in terms of uptime, it can be helpful to consider what each tier means in terms of unavailability. For example, a Tier 1 facility is available for all but 28 hours per year. Such facilities or portions thereof may go offline for short periods for scheduled, routine maintenance. Depending on the notice requirements and the business needs, this may have limited impact.

At Tier 3, the client expects only 95 minutes of downtime per year, and Tier 4 sees less than 30 minutes. These are truly high-availability options.

Which tier is right for me?

The tier decision usually comes down to cost-benefit analysis. As the facility’s tier increases, the annual contract costs rise as well, often exponentially.

As a rule of thumb, Tier 1 and 2 data facilities are meant for smaller businesses for which cost is as important a factor as reliability. SMBs are often willing to accept a few hours of downtime here and there, and even the possibility of an unexpected outage, to make ends meet.

Tiers 3 and 4 data centers are the “go to” for larger enterprises and various institution, such as banks, that require continuous availability. The consequences of unexpected downtime for a hospital, to take one example, could be life or death. Large internet retailers lose significant revenues whenever a customer cannot complete a transaction, so they often prioritize Tier 4 availability as well.

Choose your own adventure

There are also opportunities for blended solutions. In deciding where to house IT hardware and associated functions, it is essential to evaluate business needs and the true cost of failure.

Selecting Tier 4 for everything because “it’s the best” could quickly outstrip reasonable profit margins. Even where Tier 3 and 4 facilities are required for certain IT needs, non-mission critical workloads can often be housed in Tier 1 and 2 centers to lower the associated overhead costs.

Different choices will suit different IT organizations. The tier structure, however, helps facilitate universal “apples to apples” comparisons of cost and options across providers, so each IT department can make the best decision for their particular situation.


About the Author

Parker, Park Place Assistant