Triple modular redundancy
Encyclopedia
In computing
, triple modular redundancy (TMR) is a fault tolerant form of N-modular redundancy, in which three systems perform a process and that result is processed by a voting system to produce a single output. If any one of the three systems fails, the other two systems can correct and mask the fault. If the voter fails then the complete system will fail. However, in a good TMR system the voter is much more reliable
than the other TMR components. Alternatively, if there is another stage of TMR logic following the current one (for example, in systems such as the Saturn Launch Vehicle Digital Computer), then three voters are used – one for each copy of the next stage of logic.
The TMR concept can be applied to many forms of redundancy
, such as software redundancy in the form of N-version programming
.
Some ECC memory
uses triple modular redundancy hardware (rather than the more common Hamming code
), because triple modular redundancy hardware is faster than Hamming error correction hardware.
Space satellite systems often use TMR,
although satellite RAM usually uses Hamming error correction.
To utilize triple modular redundancy, a ship must have at least three chronometers
.
At one time, the cost of three sufficiently accurate chronometers was more than the cost of a smaller merchant vessel.
Some vessels carried more than three chronometers – for example, the HMS Beagle
carried 22 chronometers.
Some communication systems use N-modular redundancy as a simple form of forward error correction
.
For example, 5-modular redundancy communication systems (such as FlexRay
) use the majority of 5 samples – if any 2 of the 5 results are erroneous, the other 3 results can correct and mask the fault.
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, triple modular redundancy (TMR) is a fault tolerant form of N-modular redundancy, in which three systems perform a process and that result is processed by a voting system to produce a single output. If any one of the three systems fails, the other two systems can correct and mask the fault. If the voter fails then the complete system will fail. However, in a good TMR system the voter is much more reliable
Reliability engineering
Reliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...
than the other TMR components. Alternatively, if there is another stage of TMR logic following the current one (for example, in systems such as the Saturn Launch Vehicle Digital Computer), then three voters are used – one for each copy of the next stage of logic.
The TMR concept can be applied to many forms of redundancy
Redundancy (engineering)
In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....
, such as software redundancy in the form of N-version programming
N-version programming
N-version programming , also known as multiversion programming, is a method or process in software engineering where multiple functionally equivalent programs are independently generated from the same initial specifications...
.
Some ECC memory
ECC memory
Error-correcting code memory is a type of computer data storage that can detect and correct the more common kinds of internal data corruption...
uses triple modular redundancy hardware (rather than the more common Hamming code
Hamming code
In telecommunication, Hamming codes are a family of linear error-correcting codes that generalize the Hamming-code invented by Richard Hamming in 1950. Hamming codes can detect up to two and correct up to one bit errors. By contrast, the simple parity code cannot correct errors, and can detect only...
), because triple modular redundancy hardware is faster than Hamming error correction hardware.
Space satellite systems often use TMR,
although satellite RAM usually uses Hamming error correction.
To utilize triple modular redundancy, a ship must have at least three chronometers
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...
.
At one time, the cost of three sufficiently accurate chronometers was more than the cost of a smaller merchant vessel.
Some vessels carried more than three chronometers – for example, the HMS Beagle
HMS Beagle
HMS Beagle was a Cherokee-class 10-gun brig-sloop of the Royal Navy. She was launched on 11 May 1820 from the Woolwich Dockyard on the River Thames, at a cost of £7,803. In July of that year she took part in a fleet review celebrating the coronation of King George IV of the United Kingdom in which...
carried 22 chronometers.
Some communication systems use N-modular redundancy as a simple form of forward error correction
Forward error correction
In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
.
For example, 5-modular redundancy communication systems (such as FlexRay
FlexRay
FlexRay is an automotive network communications protocol developed by the . It is designed to be faster and more reliable than CAN and TTP, but is also more expensive...
) use the majority of 5 samples – if any 2 of the 5 results are erroneous, the other 3 results can correct and mask the fault.
Triple modular redundancy in popular culture
- The three pre-cogs in Minority Report lead to a conviction, even when one is in the minority.
- To rule out that a single win was "a fluke", some competitions use a two out of three falls match. This isn't true TMR, however, because the three falls are not independent of each other - each competitor knows who has most falls at any point in the competition, which influences their future actions.
- In Arthur C. ClarkeArthur C. ClarkeSir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...
's science fiction novel Rendezvous with RamaRendezvous with RamaRendezvous with Rama is a novel by Arthur C. Clarke first published in 1972. Set in the 22nd century, the story involves a cylindrical alien starship that enters Earth's solar system...
, the Ramans make heavy use of triple redundancy.
See also
- Fault tolerant system
- Dual modular redundantDual modular redundantA machine which is Dual Modular Redundant has duplicated elements which work in parallel to provide one form of redundancy. A typical example is a complex computer system which has duplicated nodes, so that should one node fail, another is ready to carry on its work...
- Repetition codeRepetition codeIn coding theory, the repetition code is one of the most basic error-correcting codes. In order to transmit a message over a noisy channel that may corrupt the transmission in a few places, the idea of the repetition code is to just repeat the message several times. The hope is that the channel...
- Lockstep (computing)
External links
- Article about TMR with reference to TMR usage in avionics and industry