Definition: reliability

The trustworthiness to do what the system is expected or designed to do. Reliability metrics include the following averages:

POFOD (probability of failure on demand)
The likelihood that the system will fail when a user requests service. A biometric authentication device that fails to correctly identify or reject users an average of once out of a hundred times has a POFOD of 1%.

ROCOF (rate of occurrence of failure)
The number of unexpected events over a particular time of operation. A firewall that crashes an average of five times every 1,000 hours has a ROCOF of 5 per 1,000 hours.

MTTF (mean time to failure)
The average time between unexpected events. If an IDS fails on average every 300 hours, its MTTF is 300 hours.

AVAIL (availability or uptime)
The percentage of time that a system is available for use, taking into account planned and unplanned downtime. If a system is down an average of four hours out of 100 hours of operation, its AVAIL is 96%.