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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
A reader that reads the values from the bus.
The system fails if no message is received by the readerin 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
reader. 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
