In this article, we will discuss about various data center availability models such as data center Tier Ratings 1-2-3-4, EN 50600 Availability classes (1-4), TIA 942-B Ratings (1-4), 10 Syska Hennessy Criticality Levels, ANSI/BICSI Classes (F0-F4), and N, N+1, 2N, 2(N+1) Data Center availability modes define the availability of physical infrastructure in case of any failure. It’s quite