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49 Cards in this Set
- Front
- Back
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what is a real-time system
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any information processing system which has to respond to externally generated stimuli within a finite and specified preiod
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what is important in a real-time system
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the logical result and the time it was delivered. Failure to respond in time is as bad as a wrong response
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what is hard real-time and give an example
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systems where it is imperative that the response occur within the required deadline. E.g. flight control system
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what is soft real-time and give an example
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systems where deadline is important but which will still function corretly if deadlines are occasionally missed. E.g. data acquisition system
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what is firm real-time
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systems where are soft real-time but in which there is no benefit from late delivery of service
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what does the term 'time-aware' mean and give an example |
system makes explicit reference to time and system operation references absolute time values. e.g. open vault door at 9am |
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what does the term reactive mean
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system must produce output within deadline (measured from input). Uses relative time values, must keep up with the environment
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are control systems time-aware or reactive systems |
Reactive systems. They are required to constrain input and output time variability |
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what does the term time-triggered mean and give two examples
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computation is triggered by the passage of time. E.g. release activity at 9am, release activity every 25 ms (periodic activity)
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what does the term 'event-triggered' mean |
computation is triggered by an external or internal event |
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what is meant by a sporadic activity |
an event-triggered activity where there is a bound on the arrival interval of the event |
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what is meant by an aperiodic activity |
a released event-triggered activity where there is no bound on the arrival interval of the event |
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give some examples of types of real-time systems
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fluid control systems, multi-media systems (inc mobile devices), cyber-physical systems (link web-based information and the sensed physical world)
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what are the 6 characteristics of a real-time system
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guaranteed response times, concurrent control of separate system components, can interact with special purpose hardware, support for numerical computation, large and complex, extreme reliability and safety
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what are real-time programming languages
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assembly languages, sequential systems implementation languages (like RTL/2 C. normally require operating system support), high level concurrent languages (e.g. Ada, Java)
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what are the most popular choices for real-time programming languages |
Java/Real-time Java, C and Real-time POSIX, Ada 2005. |
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in the aspects of real-time systems, what are the structure options
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time-triggered or event-triggered
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in real-time systems, what are the classification options |
criticality (hard, soft or firm) and role of time (time-aware or reactive) |
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what are the 4 aspects of real-time systems
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temporal requirements, structure, classification and characteristics
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in the aspects of real-time systems, what are the temporal requirements |
deadline/latency; input/output jitter; periodic, sporadic or aperiodic |
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what are the key concepts of reliability and fault tolerance |
safety and reliability; faults, errors and failures; failure modes; fault prevention and fault tolerance; reduncancy |
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what is meant by safety in terms of real-time systems |
irrespective of intendend function, the freedom from conditions that can cause death, injury, occupational illness, damage to equipment or environmental harm |
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what is meant by reliability in terms of real-time systems
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measure of success of a system conforming to some authoritative specification of its behaviour
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why is safety and reliability a trade off
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often the intended function can have safety issues, i.e. safest plane is one that never flies.
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what is a failure
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when the behaviour of a system deviates from its specified behaviour
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what do failures result from
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unexpected problems internal to the system that manifest themselves in the system's external behaviour. i.e. errors.
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what are faults
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the mechanical or alogrithmic cause of errors
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why do we get chains of failure -> fault -> error -> failure -> fault -> error
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because systems are composed of components (which are themselves systems) and these interact with eachother
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what are the three fault types and give examples |
transient fault (hardware components affected by radioactivity), permanent faults (broken wire or software design error) and intermittent faults (heat sensitive component that stops working, cools, works, overheats..) |
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what are software faults called and what are the two subtypes
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bugs. Bohrbugs (reproducible, identifiable) and Heisenbugs (active only under rare conditions)
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what does fault prevention attempt to do and what are the two stages |
eliminate faults creeping into the system before it goes operational. Stages: fault avoidance and fault removal |
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what is fault tolerance
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fault tolerance enables a system to continue functioning even in the presence of faults
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what do fault prevention and fault tolerance attempt to produce
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systems which have well-defined failure modes
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What choices can be made for fault avoidance to limit the introduction of faults during system construction (physical construction) |
use most reliable components for cost and performance constraints, packaging hardware to screen out expected forms of interference, use refined techniques for interconnection of components and assembly subsystems |
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How can you implement fault avoidance to limit the introduction of faults during system construction (software and project design) |
rigorous design specification of requirements, proven design methodologies, languages supporting data abstraction and modularity, software engineering enrivonments to help manage complexity |
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what procedures can be used in order to carry out fault removal |
all for finding and removing causes or errors: design reviews, program verification, code inspections and system testings |
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why can system testing never be exhastive and find all faults
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a test can only show faults, not their absence. Sometimes impossible to test under realistic conditions, simulation may not be accurate, requirements errors during development may not manifest themselves until the system is operational
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what is an alternative to fault prevention |
fault tolerance |
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when will fault prevention not be an acceptable option |
when the frequency or duration of repair times are unacceptable or the system is inaccessible for maintenance or repair |
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what are the three levels of fault tolerance
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full fault tolerance, graceful degradation (fail soft), fail safe
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what does full fault tolerance mean
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system continues to operate in the presence of faults. No significant loss of functionality or performance but may only be for a limited period
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what is meant by graceful degradation (fault tolerance) |
system continues to operate in presence of errors, accepting a partial degradation of functionality or performance during recovery or repair |
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what does fail safe mean (fault tolerance)
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system maintains its integrity while accepting a temporary halt in its operation
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give the example of graceful degradation in an air traffic control system |
full functionailty required within response times, minimum functionality required to maintain basic air traffic control, emergency functionality to provide separation between aircraft only, adjacent facility backup used in case of catastrophic failure |
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what is positive redundancy
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a fault-tolerance technique that relies on extra elements to be introduced into the system to detect and recover from faults. These are redundant as not required in a perfect system
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what is the aim in redundancy
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to minimise redundancy while maximising reliability (cost and size constraints affect this). More components increase complexity and therefore can make less reliable
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in redundancy, what is advisable with fault-tolerant components
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to separate out the fault-tolerant components from the rest of the system
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what is exception handling |
an error recovery mechanism. There is no roll-back to previous state, control is passed to handler so that recovery procedures can be intiated |
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what can exception handling be used to do |
cope with abnormal conditions from the environment, enable program design faults to be tolerated, provide general-purpose error-detection and recovery facility |
