Availability

Availability of an information service, in the context of information security means that authorized users can depend on being able to use it whenever it is needed. In U.S. law, a definition is "Ensuring timely and reliable access to and use of information."

To specify availability in detail, however, can be quite complex. For example, if a service is offered only during normal business hours between 9 and 5, the service fails at 4, is repaired at 11, and is working the next morning, has it been unavailable for:

• 1 hour, the period of outage during business hours
• 7 hours, the actual period the service was unusable if someone attempted to use it
• 17 hours, the perceived period of outage if someone saw it fail and left for the day, not knowing he could respect it in the morning