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EE551 Real-Time Operating Systems

EE551 Real-Time Operating Systems. Safety Critical Systems Analysis Course originally developed by Maj Ron Smith. Safety Critical Software Systems – ilities of Systems.

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EE551 Real-Time Operating Systems

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  1. EE551 Real-Time Operating Systems Safety Critical Systems Analysis Course originally developed by Maj Ron Smith

  2. Safety Critical Software Systems – ilities of Systems • Software safety is one of the “ilities” of that is part of non-functional requirements specifies criteria that can be used to judge the operation of a system, rather than specific behaviors

  3. Safety Critical Software Systems – ilities of Systems • Execution Qualities • Usability and Operability • Security • Reliability • Safety • Fault Tolerance • Evolution Qualities • Maintainbility, Understandability and Modifiability • Supportability (Integrated Logistics Support) • Testability • Portability • Scalability and Extensibility • Integrity – often used to encompass other ilities

  4. Safety Critical Software Systems – ilities of Systems • Safety and reliability are often misinterpreted • There is a school of thought that states that safety is a subset of reliability

  5. Reliability • reliability, R(t) - the probability that, when operating under stated environmental conditions, a system will perform its intended function adequately for a specified interval of time. • a measure of the success with which a system conforms to some authoritative specification of its behavior • most frequent hardware metric - MTBF • failure rate is more universal in software Major RW Smith Software Reliability (part1) - 5

  6. Safety Critical Software Systems – Authoritative text Safeware: System Safety and ComputersNancy G. Leveson ISBN-10: 0201119722 | ISBN-13: 978-0201119725

  7. Safety Critical Software Systems • Potential of the software to lead to hazardous system states • Hazards can lead to accidents and: • Death • Serious Injuries • Damage to environment • Significant loss of material • Loss of strategic advantage

  8. Safety Critical Systems

  9. Safety Critical Systems

  10. Safety Critical Systems

  11. Safety Critical Systems

  12. Examples of failures: Medical • Therac-25 (1985-87)(extreme case) • Bloodbank software released over 1M “failed” plasma units on the market. • Pacemakers reset to unsafe parameters due to external radiation sources (antitheft devices, microwaves,…) • Infusion pumps delivering the wrong rate of medicine.

  13. Safety Critical Software Systems • Safety Critical Software cannot be verified and validated using “traditional” methods to derive test cases • Must use risk management and hazard analysis techniques • Root Cause Analysis

  14. Safety Critical Software Systems • Hazard Analysis techniques • Hazard list from similar devices • Hazard and Operability (HAZOP) Analysis • Fault Tree Analysis (FTA) • Event Tree Analysis (ETA) • Failure Modes and Effects Analysis (FMEA) • Failure Modes, Effects and Criticality Analysis (FMECA)

  15. Safety Critical Systems - Hazard Analysis – Hazard List • Known hazards lists or reports from previous similar devices • Lessons Learned DB (internal to companies) • Recall notices (general public – industry wide) • Food and Drug Administration Web Site (MAUDE) • http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfmaude/search.cfm • Federal Aviation Agency • http://www.faa.gov/data_research/accident_incident/ • Transport Canada (CADORS) • http://wwwapps.tc.gc.ca/Saf-Sec-Sur/2/CADORS-SCREAQ/m.aspx?lang=eng

  16. Safety Critical Systems - Hazard Analysis – Hazard List • Brainstorming session • Generic lists (ISO 14971 Annex D)

  17. Safety Critical Systems - Hazard Analysis – HAZOP • Hazard and Operability Study • Process oriented • is a structured and systematic examination of a planned or existing process or operation • to identify and evaluate problems that may represent risks to personnel, equipment or environment • Originates from Chemical Industry

  18. Safety Critical Systems - Hazard Analysis – HAZOP • Analyze the behavior of a system based on operating deviations from original design or intent • Decomposition of system into sub-processes or items • (systems, subsystems, components) • Parameters • (flow, temperature, pressure,…) • Systematic qualitative analysis with Guide words • (less, more, inverse, too high, too low, before…)

  19. Safety Critical Systems - Hazard Analysis - FTA

  20. Safety Critical Systems - Hazard Analysis - ETA Control measures

  21. Safety Critical Systems - Hazard Analysis – FME(C)A

  22. Safety Critical Software Systems • State Based Analysis methods • Markov Chain Models • Petri Nets • Software Cost Reduction Methods • David Parnas and Constance L. Heitmeyer • Formal mathematical approach to specifications

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