J$.$ Explain why it is practically impossible to validate reliability specifications when thenc

are expressed in terms of a very small number of failures over the lotal lifetime of a

system,

$11\mathrm{.}2.$ Suggest appropriate reliability metrics for the classes of software system below. Give rea$-$

sons foryour choice of metric. Predict the usage of these systems and suggest

appropriate values for the reliability metrics.

a system that monitors patients in a hospital intensive care unit

a ward processor

an automated vending machine control system

a system to control braking in a car

a system to control a refrigeration unit

a management report generator

$11\mathrm{.}3.$ Imagine that a network operations center monitors and controls the national telecommu$-$

nications network of country. This includes controlling and monitoring the operational.

status of switching and transmission equipment and keeping track of nationwide equip$-$

ment inventories. The center needs to have redundant systems. Explain three reliability

metrics you would use to specify the needs of such systems.

$11\mathrm{.}4.$ What is the common characteristic of all architectural styles that are geared to supporting

software fault tolerance?

$11\mathrm{.}5.$ Suggest circumstances where it is appropriate to use a fault$-$tolerant architecture when

implementing a software$-$based control system and explain why this approach is required.

$11\mathrm{.}6.$ You are responsible for the design of a communications switch that has to provide $\frac{24}{7}$

availability but that is not safety$-$critical. Giving reasons for your answer. suggest an archi$-$

tectural style that might be used for this system.

$11\mathrm{.}7.$ 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.

$11\mathrm{.}8.$ Explain why all the versions in a system designed around software diversity may fail in

a similar way.

$11\mathrm{.}9.$ Explain how programming language support of exception handling can contribute to the rali

ability of software systems.

$11\mathrm{.}10.$ Software failures can cause considerable inconvenience to users of the software. Is it

ethical for companies to release software that they know includes faults that could lead

to software failures? Should they be liable for compensating users for losses that are

caused by the failure of their software? Should they be required by law to offer software

warranties in the same way that consumer goods manufacturers must guarantee