Data center tiers
The Telecommunications Industry Association is a trade association accredited by ANSI (American National Standards Institute). In 2005 it published ANSI/TIA-942, Telecommunications Infrastructure Standard for Data Centers, which defined four levels (called tiers) of data centers in a thorough, quantifiable manner. TIA-942 was amended in 2008 and again in 2010. TIA-942:Data Center Standards Overview describes the requirements for the data center infrastructure. The simplest is a Tier 1 data center, which is basically aserver room, following basic guidelines for the installation of computer systems. The most stringent level is a Tier 4 data center, which is designed to host mission critical computer systems, with fully redundant subsystems and compartmentalized security zones controlled bybiometric access controls methods. Another consideration is the placement of the data center in a subterranean context, for data security as well as environmental considerations such as cooling requirements.
The German Datacenter star audit program uses an auditing process to certify 5 levels of "gratification" that affect Data Center criticality.
Independent from the ANSI/TIA-942 standard, the Uptime Institute, a think tank and professional-services organization based in Santa Fe, New Mexico, has defined its own four levels. The levels describe the availability of data from the hardware at a location. The higher the tier, the greater the availability. The levels are:  
The difference between 99.671%, 99.741%, 99.982%, and 99.995%, while seemingly nominal, could be significant depending on the application.
Whilst no down-time is ideal, the tier system allows the below durations for services to be unavailable within one year (525,600 minutes):
- Tier 1 (99.671%) status would allow 1729.224 minutes
- Tier 2 (99.741%) status would allow 1361.304 minutes
- Tier 3 (99.982%) status would allow 94.608 minutes
- Tier 4 (99.995%) status would allow 26.28 minutes