The Telecommunications Industry Association (TIA) released the TIA-942 Telecommunications Infrastructure Standards for Data Centers standard in 2005. The standard describes various aspects of data center design, including reliability. The standard describes four levels of reliability:
- Tier I – Basic Reliability Power and cooling distribution are in a single path. There may or may not be a raised floor, UPS, or generator. All maintenance requires downtime.
- Tier II – Redundant Components Power is in a single path; there may be redundant components for cooling. Includes raised floor, UPS, and generator. Most maintenance requires downtime.
- Tier III – Concurrently Maintainable Includes multiple power and cooling paths, but with only one path active. Includes sufficient capacity to carry power and cooling load on one path while performing maintenance on the other path. Includes raised floor, UPS, and generator.
- Tier IV – Fault Tolerant Includes multiple active power and cooling distribution paths. Includes redundant components, including UPS and generator. Includes raised floor.
Excerpt from CISA All-In-One Study Guide, 2nd edition
Peter H. Gregory 