The Uptime Institute has developed a four-tier ranking system for determining the quality and reliability of a data center. While Tier 1 and 2 data centers lie at the lower end of the spectrum, Tier 3 and 4 are at the upper end.
Tier 3 data centers have N+1 redundancy and offer at least 72-hour protection from a power outage. Tier 3 centers offer at least 99.982% uptime per year. Tier 4 centers, on the contrary, are completely fault-tolerant. They come with 2N+1 fully redundant infrastructure with at least 99.995% uptime per year. Tier 4 offer 96-hour power outage protection.