System availability

System availability quantifies the extent to which a system is available as required, when required. The concept applies to all systems, from IT systems to production systems to communications systems.

The simplicity of this availability metric – essentially, the percentage of required time during which the system is functioning as required – is in stark contrast to the complexity of managing service and maintenance. For example, the elevator system in a hotel must be available at all times. However, when there are multiple elevators, not all elevators need to be available during periods of low usage. So a rolling program of maintenance can maintain 100% availability even though it takes some elevators out of service at certain times.