HW4: Chapters 11 & 12


11.4. What is the common characteristic of all architectural styles that are geared to supporting software fault tolerance?

Redundancy and diversity are the common characteristic of all architectural styles that are geared to supporting software fault tolerance.

11.7. It has been suggested that the control software for a radiation therapy machine, used to treat patients with cancer, should be implemented using N-version programming. Comment on whether or not you think this is a good suggestion.

I think the use of N-version programming would be a good idea to eliminate errors as the slightest miscalculation could cause serious damage to a patient or even kill them.

11.9. Explain why you should explicitly handle all exceptions in a system that is intended to have a high level of availability.

All exceptions in a system should be explicitly handled in a system so that it remains available. This way if there is an exception, the system will most likely not completely fail and if it does then the cause of the failure will be much easier to find and the program will be able to be fixed/debugged in a timely manner.

12.5. A train protection system automatically applies the brakes of a train if the speed limit for a segment of track is exceeded, or if the train enters a track segment that is currently signaled with a red light (i.e., the segment should not be entered). There are two critical-safety ­requirements for this train protection system:
The train shall not enter a segment of track that is signaled with a red light.
The train shall not exceed the specified speed limit for a section of track.
Assuming that the signal status and the speed limit for the track segment are transmitted to on-board software on the train before it enters the track segment, propose five possible functional system requirements for the onboard software that may be generated from the system safety requirements.

1. The system should be able to determine the train’s speed.
2. The system should compare the train’s speed to the track speed limit.
3. The system should be able to calculate the appropriate braking distance based on the train’s current speed to allow the train to stop when necessary to prevent entrance into a red light track.
4. The system should be able to compare the speed limit of an approaching track to the train’s current speed to slow or speed the train as necessary.
5. The train should not slow down or speed up if it is currently going the correct speed limit of the track it is currently on.

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