1. Summary and Safety Assessment mMIC-SFT November 2003 Anders P. Ravn Aalborg University

2. Fault Tolerance Means to isolate component faults Prevents system failures May increase system dependability

3. Dependability - attributes Availability Reliability Safety Confidentiality Integrity Maintainability BW p. 139

4. Dependability - means Fault prevention Fault tolerance Error Removal Failure Forecasting BW p. 106,...

5. Dependability - impediments Faults Errors Failures BW p. 103,... FaultErrorFailure... Fault

6. Safety Assessment Find faults that may lead to mishaps, analyze their relations, and estimate their consequences. May involve probabilistic reasoning (Reliability Engineering)

7. Fault Tree - Events Primary Events: Basic event – fault in atomic component Undeveloped Event – fault in composite component (may be analyzed later) External event – expected event from environment Intermediate event: Nodes inside a fault-tree

8. Fault Tree - Gates... condition Inhibit gate

9. Example – ”Wake too late” Wake too late Alarm clock fails Phone fails ”Inner clock” fails

10. Example ”Alarm clock fails” Beeper fails Button fails Alarm clock fails electronics fail SW fails Power fails Button read failsBeeper not set

11. Cut Set A cut set is a set of events that causes a top level event A singleton cut set is a single point of failure

12. Example – ”Wake too late” Wake too late Alarm clock fails Phone fails ”Inner clock” fails

13. Example ”Alarm clock fails” Beeper fails Button fails Alarm clock fails electronics fail SW fails Power fails Button read failsBeeper not set

14. Extensions etc. Probabilities on edges Event tree (forward analysis from initiating event) Combinations (cause-consequence diagrams) Many tools