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UNIT 5Reliability & Clock Synchronization: Introduction, obtaining parameter values, reliability models for hardware redundancy, software error models, Taking time into account, clock synchronization, nonfault-tolerant synchronization algorithms, Roll No: 15 impact of faults, fault tolerant synchronization in hardware.1Obtaining parameter valuesThe first step in developing a model is to decide what the input parameters should be.

A model should always be based on parameters that can either be accurately measured or estimated with confidence.

Real Time and Fault ToleranceDefinition

Obtaining Device-Failure Rates There are two ways to obtain device-failure rates, collecting field data life- cycle testing in the laboratory.

The former is more realistic, since it represents the failure rate when the devices are being used in their normal operating conditions.

Real Time and Fault Tolerance3The most common accelerant is temperature. The higher the temperature. the greater the failure rate. The acceleration factor is given by the following equation

R(T) = (8.1)

where: A : Represents a constant. Ea : Represents the activation energy and depends largely on the logic family used. K : Represents the Boltzmann constant (0.8625 x I 04e V/K). T : Represents the temperature (in degrees Kelvin).

Mathematical model classifications include Real Time and Fault Tolerance

Mathematical model classifications include Real Time and Fault Tolerance

To measure how quickly an error can propagate, we use fault injection.

This is best done on a prototype. Special-purpose hardware is used to simulate a fault on a selected line.

The status of the related lines is monitored using logic analyzers to determine how far and how quickly the error propagates.

If a prototype is not available, a software simulation can be substituted.

Measure Error Propagation TimeReal Time and Fault ToleranceReliability models for hardware redundancyRedundant system elements transition to the next higher state upon occurrence of any hardware failure.

Hardware repairs transition the system element model to the next lower state The system is a closed form semi-Markov process that can be solved for the appropriate reliability measures using conventional methods.

Closed-form solutions for the reliability measures of interest for this type of model under most common repair restrictions, types of standby, etc., are available in the technical literature referenced by this notebook.Real Time and Fault ToleranceGeneral Hardware Redundancy ModelReal Time and Fault Tolerance

The concept of three types of "coverage" is introduced as a part of the model.Fault detection coverage (Cd) is the probability of detecting a fault given that a fault has occurred.

Fault Isolation coverage (Ci) is the probability that a fault will be correctly isolated to the recoverable interface (level at which redundancy is available) given that a fault has occurred and been detected. Fault recovery coverage (Cr) is the probability that the redundant structure will recover system services given that a fault has occurred, been detected, and correctly isolatedReal Time and Fault ToleranceThe model shown in Figure is a simplified model since it does not separately consider the possible impact of transient failures.

To account for transient failures would represent an uplift of failure rate by some percentage.

The model also assumes that Cd is the same for both the primary element and the backup element. In practice, there may be different levels of fault detection coverage between primary and backup equipment due to a difference in test exposure intensity.sombody@gmail.comHardware Reliability ModelReal Time and Fault Tolerance

Software error modelsSoftware reliability is the probability of the software components of producing incorrect output.

Software should not wear out and continue to operate after a bad result.

Many software models contain:AssumptionsFactorsMathematical function

Definition

Real Time and Fault ToleranceSoftware reliability can be divided into categories: Prediction modeling Estimation modeling.

These modeling techniques follow observation and analyzes with statistical inference.TypesReal Time and Fault TolerancePrediction Model: This model uses historical data. They analyze previous data and some observations. They usually made prior development and regular test phases. The model follow the concept phase and the predication from the future time.

Estimation Model : Estimation model uses the current data from the current software development effort and doesn't use the conceptual development phases and can estimate at any timesombody@gmail.comOther Models in software:Jelinski-MorandaGoel-Okumoto (exponential)RayleighTitan Reliability Modeling Software (Predictive)WeibullDelayed s-ShapedInflexion s-ShapedReal Time and Fault ToleranceTaking time into accountCheckpoints are placed at interval of T, interval between two checkpoint constitute a miniframe

NMR is used, before writing into checkpoint probability of writing information into a checkpoint is very small and it will be ignored.

Probability of checkpoint is corrupted then it will be ignore.

Checkpoint are stored in static RAM.DefinitionReal Time and Fault Tolerance3 Types of failures are considered herePermanent FailureIndependent FailureCorrelated transient FailureReal Time and Fault ToleranceClock SynchronizationClock synchronization deals with understanding the temporal ordering of events produced by concurrent processes. It is useful for synchronizing senders and receivers of messages, controlling joint activity, and the serializing concurrent access to shared objects.

The goal is that multiple unrelated processes running on different machines should be in agreement with and be able to make consistent decisions about the ordering of events in a systemDefinitionReal Time and Fault ToleranceReal Time and Fault Tolerance

For these kinds of events, we introduce the concept of a logical clock, one where the clock need not have any bearing on the time of day but rather be able to create event sequence numbers that can be used for comparing sets of events, such as a messages.

Another aspect of clock synchronization deals with synchronizing time-of-day clocks among groups of machines. In this case, we want to ensure that all machines can report the same time, regardless of how imprecise their clocks may be or what the network latencies are between the machinesReal Time and Fault ToleranceAnother aspectsremedies the situation by forcing a resequencing of timestamps to ensure that the happens before relationship is properly depicted for events related to sending and receiving messages. It works as follows:

Each process has a clock, which can be a simple counter that is incremented for each event.The sending of a message is an event and each message carries with it a timestamp obtained from the current value of the clock at that process (sequence number).Real Time and Fault ToleranceLamports algorithmThe arrival of a message at a process is also an event will also receive a timestamp by the receiving process, of course.

The process clock is incremented prior to time stamping the event, as it would be for any other event. If the clock value is less than the timestamp in the received message, the systems clock is adjusted to the (messages timestamp + 1).

Otherwise nothing is done. The event is now time stamped.Real Time and Fault ToleranceImpact Of faultLoss of synchronySynchronization is carried out by exchanging timing messages and adjusting themselves appropriately.

Two ways when synchrony can be lost as a result of some clock becoming faulty: When multiple nonoverlapping cliques are form, When clock driven too fast or too slow. IntroductionReal Time and Fault ToleranceIt can be lost when clock are being run at their upper or lower frequency limitsReal Time and Fault ToleranceFault tolerant synchronization in hardware.To synchronize in hardware we can use phase locked loops

The basic structure of phased locked loops is shown in fig belowReal Time and Fault ToleranceComperatorFilterVCOThe objectives is to align, as closely as possible, the output of the oscillator with an oscillatory signal input.

Comparator puts out signal that is proportional to the difference between the phase of input and that of oscillator.

This is passed through a filter, resultant signal is used to modify the frequency of a voltage controlled oscillatorReal Time and Fault ToleranceFig Explained..AdvantagesVery small clock skew that can be attainedNO burden on rest of systemReal Time and Fault ToleranceTHANK YOU