9.6.3 Allocation Based on
Operational Criticality Factors
The operational criticality factors method allocates failure
rates based on the system impact of a software
failure. Criticality is a measure of the systemís ability to continue to
operate and the systemís ability to be fail-safe. For certain modes of
operation, the criticality of that mode may call for a lower failure rate to
be allocated. In order to meet very low failure rates, fault-tolerance or
other methods may be needed.
The following procedure is used to
allocate the appropriate value to the failure rate of each software CSCI in an
aggregate, provided that the criticality factor of each CSCI is known. A
CSCIís criticality refers to the degree to which the reliability and/or safety
of the system as a whole is dependent on the proper functioning of the CSCI.
Furthermore, gradations of safety hazards translate into gradations of
criticality. The greater the criticality, the lower the failure rate that
should be allocated.
(1) Determine the failure rate
goal of the software aggregate; ls
(2) Determine the number of
software CSCIs in the aggregate; N
(3) For each
ith CSCI, i = 1, 2, ...,
N, determine its criticality factor ci. The lower the ci the more
critical the CSCI.
(4) Determine ti' the total
active time of the ith CSCI, i = 1, 2, ..., N. Determine T, the
mission time of the aggregate.
(5) Compute the failure rate adjustment factor K:
(6) Compute the allocated failure
rate goal of each CSCI
= ls (ci / K)