1. Fault Tolerance & Reliability CDA 5140 Spring 2006
Chapter 1
Overview & Definitions

2. Topics basic concepts of Fault Tolerance (FT)
reliability & availability of systems, both hardware & software
tools to compare & contrast FT designs

3. What is FT? Computing in presence of errors
Some techniques from analog systems of 1940’s ’s
Digital technology adds to these to be faster, better & cheaper
Investigate architecture keeping in mind tradeoff of cost, weight & volume
Becoming more important as digital systems become more & more prevalent

4. Why Have FT? Needed more in 21st century since: Harsher environments
Many novice users
Increasing repair costs
Larger systems
Digital systems more prevalent
More users dependent on digital systems from business to government to home to school

5. How is FT Obtained? Add redundancy in form of: Hardware, e.g. RAID
Software, e.g. 2 algorithms for same task
Information, e.g. coding theory
Time, e.g. on Internet if fault, then new route

6. Definitions & Terminology
Failure - departure from correct operation
Fault - flaw in hardware or software resulting in failure, e.g. physical problems, design flaws, defects in hardware; design or implementation for software
Error - incorrect response from module leading to system failure if no FT
Type - hardware or software
Cause - improper design, hardware failure, external disturbance

7. Definitions continued
Permanent Fault - always present, needs repair to remove
Intermittent fault - not always present but still needs repair to remove
Transient fault - will disappear without repair
Fault latency - fault can go undetected & does not cause error
Fault-avoidance - use of high quality components & careful design to avoid faults
Fault-tolerance - use of redundancy (hardware, software, information or time) to correct system operation after fault occurs

8. Definitions continued
Graceful degradation - system still performs but with degraded but correct performance after faults
Fail-safe - system can fail but only to safe state to avoid catastrophes
Reliability - probability of not failing within time t given operating correctly at time 0
Availability - probability system operating correctly at time t
Maintainability - probability that system can be restored to operation by time t given not operational at time 0

9. Definitions continued
Mean-time-to-failure (MTTF) - expected value of system failure time
Mean-time-to-repair (MTTR) - expected value of system repair time
Mean-time-between-failure (MTBF) - expected value between successive system failure, MTTF + MTTR
Fault detection - method used to detect presence of fault
Fault confinement - technique to confine damage of fault to as small an area as possible

10. Definitions continued
Fault diagnosis - automatic identification of faulty modules
Recovery - system put into operating state, possibly degraded
Hardware redundancy - extra hardware to detect, mask or diagnose faults
Passive hardware redundancy - fault masking to hide faults & prevent faults from resulting in errors; no action by system

11. Definitions continued
Information redundancy - use of coding theory techniques (addition of bits)
Software redundancy - use of diagnostic software or extra modules, each with distinct algorithm
Temporal redundancy - repeating bus cycles or whole programs, new route on Internet

12. Microelectronic Growth
Density of chips dramatically increased & concomitantly, use of digital systems
Obvious need for FT in space shuttle, nuclear power plants, but with increased use in homes, more faults likely so will need FT there too
Interesting observations:
1999 typical home had microprocessors
2004 expected to be 280

13. Reliability & Availability
Goal: high reliability & availability based on sound analysis & not conjecture!
Use both reliability & availability as measures

14. Air Traffic Control Example
ATC fails once/year, so MTTF = 8766 hours
Airline Reservation System (ARS) down 5 times/year, so MTTF=1753 hours
Availability (A) = uptime/(uptime + downtime)
ATC down 1 hour, so
A = 8765/( ) =
ARS down for 1 minute, 5 times, or hours
A = /(87666) =

15. Air Traffic Control Example cont’d
Unavailability U = 1-A
So, comparing the two systems for U:
( )/( ) = 12
The ARS is 12 times better than the ATC in terms of availability.
Homework 1: 1.13, 1.14, 1.17 (3 examples)