FAULT TOLERANT DESIGN

为了攒学分，这个P我不得不选这门容错设计，学到最后还不知道能不能参加考试，哎，伤心，但愿这门课不要太难，其他的看看再说吧。

授课老师： ELENA DUBROVA 大肚子待产产妇，看上去就很凶，讲话很快，PPT颜色很难看，课程首页很乱，总体影像不是太好，

Chapter 2
FUNDAMENTALS OF DEPENDABILITY

1. Introduction

2. Dependability attributes
2.1 Reliability
Reliability R(t) of a system at time t is the probability that the system operates without failure in the interval [0,t], given that the system was performing correctly at time 0.

2.2 Availability
Relatively few systems are designed to operate continuously without interruption and without maintenance of any kind. In many cases, we are interested not only in the probability of failure, but also in the number of failures and, in particular, in the time required to make repairs.

Availability A(t)of a system at time t is the probability that the system is
functioning correctly at the instant of time t.

A(t)is also referred as point availability, or instantaneous availability. Often
it is necessary to determine the interval or mission availability. It is defined by

A(T)is the value of the point availability averaged over some interval of time T. This interval might be the life-time of a system or the time to accomplish some particular task. Finally, it is often found that after some initial transient effect, the point availability assumes a time-independent value. In this case, the steady-state availability is defined by

Steady-state availability is often specified in terms of downtime per year.
the values for the availability and the corresponding downtime is calculated by
365*(1-60%) = 146 days

2.3 Safety
Safety S$t) of a system is the probability that the system will either perform its function correctly or will discontinue its operation in a fail-safe manner.

Safety is required in safety-critical applications were a failure may result in an human injury, loss of life or environmental disaster. Examples are chemical or nuclear power plant control systems, aerospace and military applications.

3. Dependability impairments

3.1 Faults, errors and failures
Faults are reasons for errors and errors are reasons for failures

3.2 Origins of faults
As we discussed earlier, failures are caused by errors and errors are caused
by faults. Faults are, in turn, caused by numerous problems occurring at specification, implementation, fabrication stages of the design process.
1.Incorrect specification results from incorrect algorithms, architectures, or
requirements.
2.Faults due to incorrect implementation, usually referred to as design faults,
3.A source of faults in hardware are component defects.
4.The fourth cause of faults are external factors, which arise from outside the
system boundary, the environment, the user or the operator.

3.3 Common-mode faults