A data center is a facility composed of networked computers, storage systems and computing infrastructure that organizations use to assemble, process, store and disseminate large amounts of data. A business typically relies heavily on the applications, services and data contained within a data center, making it a critical asset for everyday operations. Moreover, it typically includes backup systems for power supplies, data communications, environmental conditions, and security.

Data centers are designed according to their importance and sensitivity of usage, they are categorized by tier to highlight their expected uptime and reliability as follows

• Tier 1: A Tier 1 data center has a single path for power and cooling and few, if any, redundant and backup components. It has an expected uptime of 99.671% (28.8 hours of downtime annually).
• Tier 2: A Tier 2 data center has a single path for power and cooling and some redundant and backup components. It has an expected uptime of 99.741% (22 hours of downtime annually).
• Tier 3: A Tier 3 data center has multiple paths for power and cooling and systems in place to update and maintain it without taking it offline. It has an expected uptime of 99.982% (1.6 hours of downtime annually).
• Tier 4: A Tier 4 data center is built to be completely fault-tolerant and has redundancy for every component. It has an expected uptime of 99.995% (26.3 minutes of downtime annually).