1 Software Testing and Quality Assurance Lecture 35 – Software Quality Assurance

2 Lecture Objectives Reliability Block Diagrams Series – Parallel Systems

3 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.

4 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.

5 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.

6 Series – Parallel Systems - Example

7 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 reader in the given time interval.

8 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.

9 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.

10 Series – Parallel Systems - Example

11 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.

12 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

13 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.

14 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.

15 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

16 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.

17 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

18 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.

19 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.

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