Dependability - Theory

Question 1

What are the basic steps on building reliable systems?

Answer 1

Error detection, error containment, error masking

Question 2

What dependability is?

Answer 2

A measure of how much we trust a system

The ability of a system to perform its functionality while exposing reliability, availability, maintain ability, safety, security

Question 3

What is reliability?

Answer 3

Continuity of correct service

Question 4

What is availability?

Answer 4

Readiness for correct service

Question 5

What is maintainability?

Answer 5

Ability for easy maintenance

Question 6

What is safety?

Answer 6

Absence of catastrophic consequences

Question 7

What is security?

Answer 7

Confidentiality and integrity of data

Question 8

When do we think about dependability?

Answer 8

During design time and runtime

Failures in development should be avoided, failures in operation cannot be avoided, they must be dealt with

Design should take failures into account and guarantee that control and safety are achieved when failures occur. Effects of such failures should be predictable and deterministic not catastrophic.

Question 9

How can we provide dependability?

Answer 9

Through failure avoidance, and tolerance partum

Question 10

What are some of the failures avoidance procedures we can take?

Answer 10

Conservative design

Design validation

Detailed test

Infant mortality screen

Error avoidance

Question 11

What techniques can we implement in order to increase tolerance?

Answer 11

Error detection/error masking during system operations

Online monitoring

Diagnostics

Self recovery and self repair

Question 12

Define reliability and how it is calculated

Answer 12

Ability of a system or components to perform its required functions under stated conditions for a specified period of time

It is therefore the Probably that the system will operate correctly in specified operating environment until time T

Question 13

Define availability in how it is calculated

Answer 13

The degree to which system or component is operational and accessible when required for use

Is calculated by dividing the uptime by the sum of the uptime with the downtime (total time)

It is the probability that the system will be operating at time T

Question 14

Is it possible to have systems with low reliability that have high availability? What about the opposite?

Answer 14

Yes, system failures can be repaired quickly and do not damage data, low reliability may not be a problem

The opposite is generally more difficult

Question 15

What is MTTF?

Answer 15

Meantime to failure it is the meantime before any failure will occur

Question 16

What is the MTBF?

Answer 16

Meantime between failures is the meantime between two failures

Question 17

How can we calculate MTBF?

Answer 17

By dividing the total operating time by the number of failures

Question 18

What are the types of failures according to time?

Answer 18

Infant mortal, random failures, wear out

Question 19

Define fault

Answer 19

A defect within the system

Question 20

Define error

Answer 20

A deviation from the required operation of the system or sub system

Question 21

Define failure

Answer 21

The system fails to perform its required function

Question 22

What are reliability block diagrams?

Answer 22

An inductive model where a system is divided into blocks that represent distinct elements such as components or subsystems

Every element in the RBD has its own reliability (previously calculated or modeled)

Blocks are then combined together to model all the possible success paths

Question 23

How do we calculate the reliability of components in the RBD?

Answer 23

Series components we multiply the reliabilities

Parallel components we multiply the chances of each components to fail simultaneously, and from this overall failure probability, we obtain the reliability by subtracting one by this failure probability

Question 24

What is triple modular redundancy and what is the MTTF of the system?

Answer 24

System works properly if two out of three components work properly and the voter works properly

The MTF of the system is equal to five times MTTF of simple component divided by six

Question 25

Triple modular redundancies, good or bad

Answer 25

The MTTF of the system is shorter than the MTTF of the single component. But it has higher reliability if the mission time is shorter than 70% of the mean time to failure of the component

Question 26

What is standby redundancy

Answer 26

System composed of two parallel replicas, the primary replica working all time, and the redundant replica that is activated when the primary replica fails

Question 27

What is necessary in order to have a standby redundancy?

Answer 27

A mechanism to determine whether the primary replica is working properly or not

A dynamic switching mechanism to disable the primary replica and activate the redundant one

Question 28

If nothing is sad in the exercise of the exam, what can we assume about the distribution of the failure? What is the value of the reliability?

Answer 28

This is an exponential distribution of the failure. Which means that the failure rate is constant across the period.

Elder elevators to minus lambda times time