Availability Concepts for Networks and Systems
In computer hardware and software, availability refers to the overall "uptime" of the system (or specific features of the system). For example, a personal computer may be deemed "available" for use if its operating system is booted and running.
While related to availability, the concept of reliability means something different. Reliability refers to the general likelihood of a failure occurring in a running system. A perfectly reliable system will also enjoy 100% availability, but when failures do occur, availability can be affected in different ways depending on the nature of the problem.
Serviceability affects availability as well. In a serviceable system, failures can be detected and repaired more quickly than in a unserviceable system, meaning less downtime per incident on average.
Availability Levels
The standard way to define levels or "classes" of availability in a computer network system is a "scale of nines." For example, 99% uptime translates to two nines of availability, 99.9% uptime to three nines, and so on.
The following table illustrates the meaning of this scale. It expresses each level in terms of the maximum amount of downtime per (non leap) year that could be tolerated to meet the uptime requirement. It also lists a few examples of the type of systems being built that commonly meet these requirements.
| Class | Uptime | Downtime per Year (maximum) | Examples |
| 1 | 90.0% | 36 d 12 h | personal clients, experimental systems |
| 2 | 99.0% | 87 h 36 m | entry-level business systems |
| 3 | 99.9% | 8 h 46 m | top Internet Service Providers, mainstream business systems |
| 4 | 99.99% | 52 m 33 s | high-end business systems, data centers |
| 5 | 99.999% | 5 m 15 s | carrier-grade telephony; health systems; banking |
| 6 | 99.9999% | 31.5 s | military defense systems |
When talking about availability levels, note that the overall window or time frame involved (weeks, months, years, etc.) should be specified to give the strongest meaning. A product that achieves 99.9% uptime over a period of one or more years has proven itself to a much greater degree than one whose availability has only been measured for a few weeks.
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