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McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


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%.
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The first type is normal states that the system runs normally, the average time to stay in normal state is MTTF, which is decided by [[lambda].sub.F]; the second type is failure state, the average time to stay in failure state is MTTR, which is decided by XR .
From (6), the product's MTTF under the normal operation stress ([S.sub.0]) can be approximated by using
* Failure rate or failure intensity; these measures are the inverse of MTTF and MTBF.
Since failure rates, often described as a mean-time-to-failure (MTTF), increase exponentially with temperature, a 10[degrees]C-increase in temperature can double the failure rate.
The device has an operating lifetime of MTTF 10,000H at 500 mW (at an operating temperature of 25[degrees]C, operating wavelength of 670 nm, optical output power of 500 mW and laser diode reverse voltage of 2 V.
This data indicates that the mean time to failure (MTTF) estimate for the new process is similar to that estimated for the standard process, and the two processes have comparable reliability.
Reliable data is not yet available to determine the junction temperature for a desired mean-time-to-failure (MTTF) for wide bandgap devices, but output power and gain will still degrade with temperature.
Recent work on metamorphic FET device reliability has resulted in excellent results with mean-time-to-failure (MTTF) equal to that of InP FET devices.