2. Introduction to Redundancy Techniques Redundancy Implies the use of hardware, software, information, or time beyond what is needed for normal system operation. Has a strong impact on a system in the areas of performance, size, weight, power consumption, and reliability.

2. Introduction to Redundancy Techniques Passive  Based on the concept of fault masking to hide the occurrence of faults and prevent the faults from resulting in errors (developed around the concept of majority voting)  Do not provide for faults detection, but simply mask them Active, or Dynamic  Attempts to achieve fault tolerance by means of fault detection, fault location, reconfiguration, and recovery (property of fault masking is not obtained: there is no attempt to prevent faults from producing errors within the system)  More suitable for applications where temporary, erroneous results are acceptable, as long as the system reconfigures and regains its operational status in a satisfactory length of time Hybrid  Combines the attractive features of both the Active and the Passive approaches 2.1 Hardware Redundancy

2. Introduction to Redundancy Techniques 2.1 Hardware Redundancy Module 1 Module 2 Module 3 Voter Output Basic concept of Triple Modular Replication (TMR) Proc 1 Proc 2 Proc 3 Voter The use of tripliacted voters in a TMR configuration Voter Mem 1 Mem 2 Mem 3

Voting at Several Levels within N-Modular Redundancy (NMR) Systems 2. Introduction to Redundancy Techniques 2.1 Hardware Redundancy 3 independent temperature sensors perform a vote on the 3 sensor values. Next, calculate the amount of heat/cooling by means of 3 separate modules, and then vote on the calculations to determine a result. X 3 independent sensors sample the temperature, perform the calculations, and then provide a single vote on the final result. Difference between the two approaches  fault containment: voting at the sensors will mask and contain the effects of an eventual sensor fault.

2. Introduction to Redundancy Techniques 2.1 Hardware Redundancy Voter Task Example of SW voting Task A Task B Task A Proc 1 Proc 3 Proc 2 HW Voting x SW Voting ? 1. The availability of processor to perform the voting 2. The speed at which voting must be performed 3. The criticality of space, power, and weight limitations 4. The # of different voters that must be provided 5. The flexibility required of the voter with respect to future changes in the system

In practical applications of voting, 3 results in a TMR system may not completely agree, even in a fault-free environment: e.g., A/D converters in sensors may produce quantities that disagree in the least-significant bits. This disagreement can propagate into larger discrepancies after computation, which can significantly affect the voting process. 2. Introduction to Redundancy Techniques 2.1 Hardware Redundancy

2. Introduction to Redundancy Techniques 2.1 Hardware Redundancy Solution  Mid-Value select Technique A TMR system selects the value that lies in the middle of the others : Corrupted signal Uncorrupted signals Selected signals