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Data Center Tier System - a summary

Tiers and their application to Data Centers

Data Centers are classified according to a Tier System, broadly based on the resilience of the center, and the levels of redundancy built in to account for natural or untimely system failures.

Redundancy in relation to a computer system means that as well as the hardware required to run the processes, additional hardware is installed, but not generally used i.e. is redundant until such time as a failure on the primary hardware requires it to kick in. Such switchover would be seamless, with no interruption of operation or data flow.

Fault-Tolerance means precisely that, that equipment is tolerant of faults, and can recover seamlessly. Primary examples of fault-tolerance would be for instance in a UPS (Uninterrupted Power Supply) system, such that if their is a power blackout from the main energy supply (the Grid), alternative power immediately kicks in, from battery or generator, and that there is no interruption to service, and no computer equipment will lose power.

Summary of Tiers 1 through 4 as applied to Data Center Assurance

The Data Center Tier system as currently implemented can be summarised as follows;

Tier 1
Means a basic facility, offering off-line facilities, but with single uplinks and no redundancy in case of system failure. Nevertheless, it must remain up for a minimum of 99.671% of the time.
Tier 2
Means a slightly better facility, fulfilling all Tier 1 requirements, plus additional levels of redundancy. It must remain operational for 99.741% of the time.
Tier 3
Means a good facility, fulfulling all Tier 1 and Tier 2 requirements, plus additional redundancy, including dual-power sources to all I.T. Equipment, and more than one distribution path/uplink. It must also provide complete infrastructure redundancy, which usually means having more than one available site (in another city or country) for absolute security of access. A Tier 3 facility must have an availability of 99.982%.
Tier 4
Tier 4 facilities must fulfil all Tier 1, Tier 2 and Tier 3 requirements and must in addition have dual-power sources to all building equipment, such as air-conditioning, security, etc. Such a facility must guarantee its availability for 99.995% of the time.

In actual fact, most Data Centers will fall between one of the Tiers above, so we frequently see Tier-2+, Tier-3+, etc, meaning that the facility fulfils all the requirements of the stated Tier, but goes above and beyond the specific recommendations.

Further Reading:

There is quite an informative wiki on Data Centers, please see there for further information: