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. This is a closed book, closed notes examination. . Write your answers on the examination paper in ink or legible pencil. . If your answer cannot be read or understood , or if your answer is vague or confused, it will be marked wrong. . In any question, make any assumptions that you need to but document your assumptions. . The number in parentheses after each question is the number of points allocated to that ques- tion.
NAME ( Print Legibly. All Capitals ):
PLEDGE ( Write Out In Full And Sign ):
CS 686
DEPENDABLE C OMPUTING
FINAL EXAMINATION
SPRING 2007
Final Examination - Three Hours
You have been hired by General Motors to develop an automobile braking system. The system is to provide braking under driver control, traction control in which wheels are braked individually to help counter skidding during acceleration and cornering, and accident prevention by braking the car automatically if the system detects an imminent collision. A variety of sensors monitor vehicle conditions and a computer system attempts to detect a skid or a crash situation by compar- ing sensor values with pre-defined thresholds and with sensor data patterns (how the data has changed in the recent past). A small radar attempts to locate obstacles ahead. The system per- forms numerous self checks on the hardware whenever braking is not taking place. If a self check fails, everything but basic driver braking is disabled and a light is illuminated to inform the driver. It is not dangerous for traction control and accident avoidance to fail provided the driver’s warn- ing light is illuminated when the problem is detected and remains illuminated. The following figure illustrates the system design:
In the documentation for the braking system, the sensors and actuator are described by the follow- ing Z state schema:
Three axis vehicle linear accelerometer
Forward-looking radar
Dashboard error warning light
Computer system (^1)
Computer system (^2)
Computer system (^3)
Brake actuation system
Voting logic
Sensor data distribution
Single axis vehicle rotational accelerometer
Brake pedal position sensor
Braking_sensors_and_actuators
x_acc, y_acc, z_acc : -255.. rotational_acc : -255.. current_radar_distance : 0.. previous_radar_distance : 0.. brake_pedal_pressure : 0.. brake_force_leftfrontwheel : 0.. brake_force_leftrearwheel : 0.. brake_force_rightfrontwheel : 0.. brake_force_rightrearwheel : 0.. warning_light : 0..
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- You have to develop a safety case for the braking system. You decide that your top-level goal should be: “The braking system is adequately safe .”
(a) How would you define safety in this case? (4)
(b) Identify three hazards would you anticipate for the braking system. (6)
(c) How might each of those hazards be mitigated in practice? (20)
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You have been hired as the chief dependability engineer for Wal-Mart. The company operates a world-wide computer network that automatically manages store inventories, financial accounting, retail point-of-sale terminals, interaction with suppliers to order goods to keep inventories at the correct level, scheduling of deliveries to retail stores, and scheduling of all train and truck move- ments. Records of all sales, payments, store inventories, train and truck locations and schedules, and orders sent to suppliers are maintained in a central database located in Kansas City.
- Identify three major hazards that this information system faces. (10)
- For one of the hazards that you identified in the previous question, indicate how you would go about calculating the associated cost of failure in dollars. (10)
- For the hazard that you used in the previous question, draw the top two levels of a fault tree that you might use for analysis of the hazard. (10)
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- Software rejuvenation (Trivedi) is a technique in which software is periodically restarted, e.g., an operating system is rebooted. Empirical evidence indicates that this idea is helpful in pre- venting failures.
(a) Why does this technique make a difference? (6)
(b) Which attributes of dependability can be affected by this technique and how? (6)
(c) If you were to implement software rejuvenation, how would you determine the times when the restarts should be undertaken? (6)
(d) What are the impacts of this technique on normal operation? (6)
(e) What types of computer system would be most likely to benefit from this technique? (6)
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- Draw a diagram illustrating the principles of a software reversal check. (5)
- Draw a diagram illustrating the principles of a software acceptability check. (5)
- Draw a diagram illustrating the overall architecture of a protection shell. (10)
- Briefly explain the goal of a protection shell. (5)
- As a series of bullets, briefly describe the issues that arise in the development and use of pro- tection shells. (5)
