Reliability Specification

Reliability Specification

non-functional reliability requirements

define number of failures that are acceptable during normal use of the system

functional reliability requirements

define system and software functions that avoid, detect or tolerate faults in the software

probability of failure on demand (POFOD) 1

probability that the system will fail when a request for service is made

probability of failure on demand (POFOD) 2

used when demands are intermittent and relatively infrequent

probability of failure on demand (POFOD) 3

appropriate for protection systems where services are demanded occasionally with serious consequences of failure

rate of occurrence of failure (ROCOF) 1

reflects the rate of occurrence of failure in the system

rate of occurrence of failure (ROCOF) 2

ROCOF of 0.002 means 2 failures likely in each 1000 operational time units

rate of occurrence of failure (ROCOF) 3

relevant for systems that process a large number of similar requests in a defined time period e.g., credit card processing system

mean time to failure (MTTF) 1

reciprocal of rocof

mean time to failure (MTTF) 2

length of time a system can be expected to run without failing

mean time to failure (MTTF) 3

relevant for systems where system processing takes a long time e.g., conveyor belt

mean time to failure (MTTF) 4

mttf should be longer than expected transaction length so the system doesn’t normally fail during a transaction

step 1

for each sub-system, analyse consequences of possible system failures

step 2

from system failure analysis, partition failures into appropriate classes

step 3

for each failure class identified, set out the reliability using an appropriate metric

step 4

identify functional reliability requirements to reduce the chances of critical failures