HW4: Chapters 11 & 12

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

 Ex 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 that this is a good suggestion to have the control software implemented using N-version programming. I think this because radiation therapy is a very serious matter and this software can be the reason this person lives or dies. So it would definitely be beneficial to have multiple checks of the software to make sure that the patient always gets the correct amount of radiation. 

 Ex 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 that happen in a system need to be handled right away to prevent system failure or the system behaving incorrectly or in an unwanted manor. System failure and unwanted behavior of the system will love the level of availability.

 Ex 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:
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 will not let the train go faster than the speed limit of that segment of track. If it is attempted brakes will be applied.
2. The system will not let the train go onto segments of the track that are signaled red. The system will prevent this by redirecting the train or braking then redirecting.
1. For the system to know which segment of track the train is on currently.
2. To be able to activate the brakes of the train if the train has surpassed the speed limit.
3. The system can redirect the train when it tries to go onto a track segment that is signaled red.
4. The system should execute every second.
5. The system knows the signals of track segments 5 miles in advance

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