# Software Testing and Quality Assurance by dffhrtcv3

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```									Software Testing and Quality
Assurance

Lecture 35 – Software Quality
Assurance

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Lecture Objectives
   Reliability Block Diagrams
   Series – Parallel Systems

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Reliability Block Diagrams
   We need some way of predicting and
evaluating the reliability of our designs.
   For system there are well established
modeling methods:
   Reliability Block Diagrams; and
   Markov Models
These models are stochastic in nature.

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Reliability Block Diagrams
   Stochastic process is one in which
   There is a sequence of events in time
where each event is part of a probability
distribution.

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Series – Parallel Systems
   Reliability block diagrams seek to
decompose systems into parallel and
serial blocks
   Where each block interacts with other
block to effect the reliability of the system
as a whole.

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Series – Parallel Systems -
Example

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Series – Parallel Systems -
Example
   A redundant pair of writers put values
from the source onto the bus, and
bus.

in the given time interval.

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Series – Parallel Systems -
Example
   A message starting
at the source then it
is passed to both
writers and both
writers pass the
message to both
channels.

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Series – Parallel Systems -
Example
   In the model,
   Each writer acts in serial with the bus and
   Both channels of bus act in serial with the
   To analyze the model;

We must calculate the reliabilities for the serial
And parallel blocks in the model.
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Series – Parallel Systems -
Example

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Reliability and Failure Logic for
Serial Blocks
   For the system to model without failure at a
given time T,
   Each of the components B1, B2 and B3 must
function independently with out failure for the time
period T.
   For system to function in the Time T, either
   Block 1 must not fail in time T;
   Block 2 must not fail in time T;
   Block 3 must not fail in time T.

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Reliability and Failure Logic for
Serial Blocks
   For system to fail in the Time T,
   Only one of the components need to fail.
   Thus, the probability of failure is

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Reliability and Failure Logic for
Serial Blocks
   The law of additive law of probability
gives us;

   Where F(Bi) is the probability that the first
failure in Bi will occur before Time T.

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Reliability Logic for Parallel
Blocks
   All blocks must fail before the system
fails and
   Only one block needs to function for the
system to function.
   Parallel blocks, work almost inversely to
serial blocks.

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Reliability Logic for Parallel
Blocks
   For a parallel block to fail in a time T,
   We need all of the blocks to fail within the
time T and so

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Reliability Logic for Parallel
Blocks
   For a parallel blocks to be reliable for a
time T,
   We have must experience no failure in time
T.

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Reliability Block Diagrams from
System Architectures -
Guidelines
   Guideline 1
   Determine what constitutes a system
failure. This in turn determines which
component failures causes a system to fail.
   Guideline 2
   Determine what components need to fail in
order to cause a system failure.
   How messages, signals or data flows
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Reliability Block Diagrams from
System Architectures -
Guidelines
   Guidelines 3
   Try to ensure that each block in the
reliability block model captures one
function of the system.
   Guideline 4
   Try to ensure that you capture the
parallel/serial connections from the system
accurately .

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Reliability Block Diagrams from
System Architectures -
Guidelines
   Guidelines 5
   There may be more than one mode for the
system.
   You need to create a reliability block
diagrams for each mode of the system.

Windows can operate in a Safe Mode,
More reliable because it is limited to basic functionality.

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Key points
   Reliability Block Diagrams; and Markov
Models
   Reliability block diagrams seek to
decompose systems into parallel and
serial blocks
   Reliability and Failure Logic

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