1. Fault Tolerant Systems in a Space Environment
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EE585: Fault Tolerance Computing

2. EE585: Fault Tolerance Computing
Overview
Introduction
Error Detection Technique.
*Watchdog Processor
*Control Flow Error Detection.
*Types of Signatures.
Fault Injection.
Conclusion.
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EE585: Fault Tolerance Computing

3. EE585: Fault Tolerance Computing
Introduction
Experimented by CRC on Advanced Research and Global Observations Satellite.(ARGOS)
The approach mainly focuses on Space missions involving equipment that combines the two basic approaches of Fault Avoidance and Fault Tolerance
Mainly uses Software Techniques for detecting errors.
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EE585: Fault Tolerance Computing

4. Error Detection Techniques
Watch dog Processor
It is a small processor that sits on buses , passively observes the bus transactions generated by main processor and detects errors by monitoring.
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EE585: Fault Tolerance Computing

5. EE585: Fault Tolerance Computing
Watchdog Processor
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EE585: Fault Tolerance Computing

6. Control Flow Error Detection
Main goal is to check the correct sequencing of the instructions.
Done by Signature Analysis.
It is a method in which signature is associated with a block of instructions and saved at compile time. During runtime, generated signature is compared with saved ones and errors are detected.
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EE585: Fault Tolerance Computing

7. EE585: Fault Tolerance Computing
Types of Signatures
1. Path Signature Analysis:
* Signatures are computed for sequence of nodes, i.e., paths rather than single node.
* Two bits are used to differentiate signatures
* A special tag signals the time to compare the computed signature with embedded one.
2. Signature Instruction Streams (SIS)
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EE585: Fault Tolerance Computing

8. EE585: Fault Tolerance Computing
Contd….
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EE585: Fault Tolerance Computing

9. EE585: Fault Tolerance Computing
Contd…
Paths are grouped into sets and each set has a signature, called justifying signature.
Control flow diagram of three basic blocks
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EE585: Fault Tolerance Computing

10. 2.Signature Instruction Streams (SIS)
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EE585: Fault Tolerance Computing

11. EE585: Fault Tolerance Computing
Contd…
To reduce number of signatures embedded in the code, Branch Address hashing is used.
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EE585: Fault Tolerance Computing

12. Branch Address Hashing
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EE585: Fault Tolerance Computing

13. Stutter Step Mode (SSM)
Each group of instructions is executed twice or more and the results are compared. It detects errors missed by other techniques.
Disadvantages:
* Performance level is lowered.
* Memory overhead.
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EE585: Fault Tolerance Computing

14. Application of SSM to one instruction
Overhead is 300%
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EE585: Fault Tolerance Computing

15. EE585: Fault Tolerance Computing
Contd…
Reduced overhead by extending duplication to a basic block.
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EE585: Fault Tolerance Computing

16. EE585: Fault Tolerance Computing
Error Masking in SSM
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EE585: Fault Tolerance Computing

17. EE585: Fault Tolerance Computing
Contd…
Assume, values of registers
B= C= 7
=> A= 17 D= 3
(We know the result of dividing any number between 19 and 15 by 5 is 3.)
Say if A= 18 (instead of 17), the error is not detected.
Therefore, we need to be careful in selecting the error detection technique.
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EE585: Fault Tolerance Computing

18. EE585: Fault Tolerance Computing
Fault Injection
One way to validate Fault tolerance mechanisms
Advantages:
1. Flexibility
2. Controllability
3. Predictability
Disadvantages:
1. Its questionable whether the injected faults are good representation of faults in real environment.
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EE585: Fault Tolerance Computing

19. EE585: Fault Tolerance Computing
Contd…
In ARGOS, system is tested in Space environment created.
Different approaches to fault injection in electronic systems:
1. Disturb the signals on the pins of the pins.
2. Radiation.
3. Power Supply Disturbance.
4. Logic simulation.
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EE585: Fault Tolerance Computing

20. EE585: Fault Tolerance Computing
Conclusion
Determined the tradeoffs between fault tolerance and fault avoidance techniques and finally come up with an efficient blend of technique suitable.
Hardware and Software fault tolerance techniques are studied.
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EE585: Fault Tolerance Computing

21. EE585: Fault Tolerance Computing
References
Fault Tolerant Systems in a Space Environment.
- Philip P.Shirvani and Edward J. McCluskey. (Stanford University)
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EE585: Fault Tolerance Computing

22. EE585: Fault Tolerance Computing
Queries?
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EE585: Fault Tolerance Computing