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Tier Standards Overview

Data center tier standards exist to evaluate the quality and reliability of a data center’s server hosting ability. The Uptime Institute uses a somewhat mysterious four-tier ranking system as a benchmark for determining the reliability of a data center. This proprietary rating system begins with Tier I data centers, which are basically warehouses with power, and ends with Tier IV data centers, which offer 2N redundant power and cooling in addition to a 99.99% uptime guarantee.

A Tier III data center is concurrently maintainable, allowing for any planned maintenance activity of power and cooling systems to take place without disrupting the operation of computer hardware located in the data center. In terms of redundancy, Tier III offers “N+1″ availability. Any unplanned activity such as operational errors or spontaneous failures of infrastructure components can still cause an outage.In other words, Tier III isn’t completely fault tolerant. A Tier IV data center is fault-tolerant, allowing for the occurrence of any unplanned activity while still maintaining operations. Tier IV facilities have no single points of failure.

The basic concept is that a Tier IV design requires double the infrastructure of a Tier III design. Note that both Tier III and Tier IV data center specifications require IT equipment to have dual power inputs to permit maintenance of power distribution components between the UPS and IT equipment.

tier standards topology

What are the features of a Tier 1 data center?

A: Tier 1 facilities are generally utilized by small businesses and feature:
  • 99.671% Uptime
  • no redundancy
  • 28.8 Hours of downtime per year.


What are the features of a Tier 2 Data Center?

The benefits of a Tier 2 facility include:
  • 99.749% Uptime
  • Partial redundancy in power and cooling
  • Experience 22 hours of downtime per year

    What are the features of a Tier 3 Data Center?

    A: Tier 3 facilities are utilized by larger businesses and feature:
    • 99.982% uptime
    • No more than 1.6 hours of downtime per year
    • N+1 fault tolerant providing at least 72 hour power outage protection

    What are the features of a Tier 4 Data Center?

    A: Tier 4 facilities typically serve enterprise corporations and provide the following:
    • 99.995% uptime per year
    • 2N+1 fully redundant infrastructure
    • 96 hour power outage protection
    • 26.3 minutes of annual downtime.
      Unfortunately, the Uptime Institute has chosen not to fully publish the evaluation criteria for these different tier levels. Few data centers have tier certifications from the Uptime Institute. Only 38 facilities or design documents for facilities have official tier certifications at this point; these are primarily enterprise data centers. The result is that the Uptime Institute’s definitions have been misused by the industry, ignorantly in many cases.

      Facility builders, designers and owners have tried to tweak the terminology slightly to give it their own unique flavor. Enterprises should question any Tier IV claims by data center providers because it is difficult to get customers to pay the rates necessary to monetize the Tier IV investment of roughly double that of a Tier III facility. Tier I Research has established a bi-level data center quality rating system, with the following criteriaData center tier standards objectify the design features of a particular facility based upon infrastructure design, capacities, functionalities and operational sustainability.

      The Uptime Institute released a revised version of the data center tier standards overview in 2012.Such revisions were made by the advisory board to reflect the type of changes that have been made to evaluate engine generators, evaporative cooling methodologies, and oil and fuel systems located onsite. Restatement of the tier standard ranking system was necessary to further rate onsite performance during system life cycle, specifically – the UPS (Uninterruptible power supply). Let’s examine how the following variables determine a facility’s tier standard rating system.

      What are the requirements of a Tier I facility?

      A Tier I data center has basic signal paths that are not redundant in nature. This means that most of the appliances providing support to servers and other types of networking equipment have only 12 hours of backup power/onsite fuel for electrical generators.

      What are the requirements of a Tier II facility?

      Tier II data centers are redundant to the point that if one were to remove the backup generator equipment from a signal path, normal computing operations would continue without interruption. Therefore, the Tier II facility is redundant in capacity yet functions with a non-redundant signal path.Benefits of a Tier II facility include:
        • 12 hours of on-site backup power
        • Redundant power supplies can be removed from facility without causing any interruption to computing equipment.
        • Tier II facilities have to be serviced only once a year resulting in less downtime for consumers. 

        What are the benefits of a Tier III facility?

        A Tier III facility provides multiple signal paths for redundancy equipment and distribution components. Subsequently, only one power distribution path can serve computer equipment at any given moment

        Benefits of a Tier III facility include the following:

        • Multiple power distribution paths and capacity components that are supplied with concurrent energy. All critical components are supplied with dual power sources and are fault tolerant.
        • 12 hours of ‘N’ redundant power supplied by on-site fuel systems.
        • Computer equipment is served by one signal path at a time.


          What are the benefits of a Tier IV facility?

          A Tier IV data center is the most fault tolerant facility with multiple power distribution paths, redundancy components and active distribution paths serving every piece of critical computing component simultaneously

          Benefits of Tier IV facility include the following:

          • Distribution paths that are physically isolated from one another, often referred to as “compartmentalized” distribution paths, preventing harm from a single event that may take place on site.
          • 12 hours of fuel storage ‘on-site’.
          • Cooling systems that are continuously available 24/7/365.
            Furthermore, a single ended failure will not impact critical computing equipment because multiple distribution paths supply electricity simultaneously. The Uptime Institute declares that each component located within the facility must not affect distribution paths if removed for maintenance or testing. Also, each redundant component must be designed to automatically respond to failure preventing any further damage to critical hardware.

            Premium Data Center: Facility must meet N+1 redundancy standards in each of the following areas: uninterruptible power supply (UPS), backup generators and HVAC systems. Preferably, the facility should be on multiple power grids as well as having 24/7 staffing and continuous video surveillance.

            Bio-metric security is preferred, but not required. VESDA (or better) fire detection and dry-pipe pre-action or gas-based fire suppression are required. Multiple fiber providers must serve the facility through two or more entrance facilities. Data center resiliency is based not only on physical power and cooling infrastructure, but also on network communications redundancy and fire detection and suppression. These are key attributes missing from the Uptime Institute’s data center tier ratings, which focus on the power and cooling infrastructure.

            Standard Data Center: Facilities with sufficient power and cooling capacity and basic fire suppression are in this category, as are data centers with non-redundant HVAC and UPS units and only a single backup generator. The bottom line is that data center customers are looking for high availability and security for IT equipment, which implies redundancy for power, cooling and network connections; fire detection and suppression; the ability for concurrent maintainability and the 24×7 staffing and security found in a premium data center.