What are the PERA 4Rs ?

The PERA 4Rs are Response, Resolution, Reliability, and Resilience. These are all time-based measures, that are used to determine the requirements of devices and networks at a certain “PERA Level”. However, the nature of these measurements and even the time parameters involved, are quite different.

• Response is the time it takes to get the answer (e.g. seconds for an update from a field device). In a process plant, this would be the update rate on the DCS screen and might be a few times per second, but on a pipeline SCADA system Response time could be many seconds or even minutes.


• Resolution is the result of the sample rate (or storage rate) of the device or network (from microseconds for a high-resolution breaker log, to seconds for a temperature scanner). In the old days, this was instantaneous, and the operator would often “tap the gauge” to see it “wiggle” to be sure that it was not stuck. With modern digital transmitters, the scan rate of the A/D converter determines the “standard deviation of the noise” which can actually be remotely read from some vendors’ sensors.


• Reliability is the classical MTBF (Mean Time Between Failure) which is often measured in months or years. Relibility may include the combined MTBF for equipment failure and failure of its protection system (e.g. for an SIS/SIL safety shutdown system). Note that statistical probabilities do not combine in a linear fashion (check your old statistics text books). It should also be noted that failure curves for electronics and software do not follow normal distribuiton cureves, but instead follow a "bathtub curve" with "infant mortality" followed by an extended low failure rate during extended operation.


• Resilience is defined as resistance to losses caused by unacceptable repair times, human error, and cyber-attacks. It is usually rated in terms of MTTR (Mean Time To Repair) or in the case of an operating error or cyber attack, the Mean Time to Recover.
  This includes the time to repair a failed device, or to swing over to a backup unit. In the case of a cyber attack this might involve reloading backup copies of programs or device configuration data.

Any of these 4Rs can be the controlling “requirement” for a device, network or system. Since each is expressed in terms of MTTR and the consequences of a failure, the most critical is used as the design requirement.

See PERA Enterprise Architecture Levels for a more detailed discussion of how 4Rs are used in design of Control and Information Systems.