Why is the system-human interface one of the most important but ...

[Jramos095]

Why is the system-human interface one of the most important but difficult areas of safety-critical systems? Do a search on the Internet and find three good sources of information relating to how to design an effective system-human interface.

Question 2

Your company is considering using N-version programming with three software development firms and three hardware devices for the navigation system of a guided missile. Briefly describe what this means, and outline several advantages and disadvantages of this approach.

[joshbk44]

Answer to Question 1

Human behavior is not nearly as predictable as the reliability of hardware and software components in a complex system. The system designer must consider what human operators might do to make a system work less safely or effectively. The challenge is to design a system that works as it should and leaves little room for erroneous judgment on the part of the operator. For instance, a self-medicating pain-relief system must allow a patient to press a button to receive more pain reliever, but must also regulate itself to prevent an overdose. Additional risk can be introduced if a designer does not anticipate the information an operator needs and how the operator will react under the daily pressures of actual operation, especially in a crisis. Some people keep their wits about them and perform admirably in an emergency, but others may panic and make a bad situation worse.

Answer to Question 2

N-version programming is a form of redundancy that involves the execution of a series of program instructions simultaneously by two or more different systems. The systems use different algorithms to execute instructions that accomplish the same result. The results from the two or more systems are then compared; if a difference is found, another algorithm is executed to determine which system yielded the correct result. In this case, instructions for the three systems will be written by three different software development firms and run on different hardware devices.

The advantages include:

• It is highly unlikely that the three systems will fail at the same time under the same conditions.


• Having three separate firms develop the systems will help to ensure the three systems do not fail in the same way, since each firm will be independent in their implementations.


• In building a safety-critical system, N-Version systems offer more reliability than just one system.

The disadvantages include:

• If the three systems do fail and the failures are correlated, the practical gain in reliability may be limited.

The costs associated with using three separate firms may be greater than if one firm developed the three systems

[Jramos095]

Excellent

[adf223]

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