CS577B HOMEWORK ASSIGNMENT #2

Due February 1

1. (5 points) Why is it inappropriate to use reliability metrics which were developed for hardware systems in estimating software system reliability? Illustrate with an example.

2. (8 points) Suggest appropriate reliability metrics for the following classes of software systems. Give reasons for your choices, and suggest approximate acceptable values for the reliability.
a) A system that monitors patients in a hospital intensive care unit.
b) A system to control braking in a car.
c) A management report generator.

3. (5 points) Is software system development cost linear with the required reliability? Why or why not? Identify quantities discussed in class to support your answer (hint: not from the reliability/safety lecture).

4. (5 points) Estimate the risk exposure for the following situations.
a) The reliability (probability of running without failure) of a software controlled drone is 85%, and the effect of a failure is a total loss of the $400,000 unit.
b) The availability of a financial computer network is .98. For every minute of downtime, an estimated $1.5M is lost. What is the risk exposure for an eight hour day?

5. (6 points) Identify six consumer products which may contain, or which may contain in the future, safety-critical software systems. Identify a non-consumer safety-critical system and three resulting accidents of possible software failures.

6. (12 points) A system model of a microwave oven is discussed in Chapter 13 of Sommerville. As much as possible, develop a hazard analysis for the system with associated fault trees (only consider software faults). The principal hazard is microwave leakage but over-heating should also be taken into account.

7. (4 points) For the C programming language, identify at least three language constructs which should be avoided if safety-critical systems are to be developed with it.

8. (5 points) If one uses reliability modeling and statistical testing, and a software system meets its reliability specification after such testing, will it perform to the same specification when used in the field? Briefly discuss your answer.