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12.6. Test Design Strategies for STD based Testing

Test cases derived from STD are sequence of input events. Test cases from STD can be designed using following notions:

Possible levels of coverage that can be aimed for when designing a test suite with the help of a model:
  • All states coverage is achieved when each state of the state machine is exercised at least once during testing. This is usually not a sufficient level of coverage, because behavior faults are only accidentally found. If there is a bug in a transition between a specific state pair, it can be missed even if all states coverage is reached.
  • All transitions coverage is achieved when the test executes every transition in the model at least once. This automatically entails also all states coverage. Reaching all transitions coverage doesn’t require that any specific sequence is executed, as long as all transitions are executed once. A bug that is revealed only when a specific sequence of transitions is executed, is missed even in this coverage level. The coverage can be increased by requiring All n-transition coverage, meaning that all possible transition sequences of n or more transitions are included in the test suite.
    • All-events coverage: each event of the state machine is included in the test suite (is part of at least one test case)
    • All-actions coverage: each action is executed at least once
    • All-transitions: each transition is exercised at least once
      • Implies all-events coverage, all-states coverage, and all-actions coverage
      • Minimum acceptable strategy for responsible testing of a STD
    • All n-transition sequences: every transition sequence generated by n events is exercised at least once
      • All transitions = all 1-transition sequences
      • All n-transition sequences implies (subsumes) all (n-1)-transition sequences
    • All round-trip paths: every sequence of transitions beginning and ending in the same state is exercised at least once
    • Exhaustive: every path over the state machine is exercised at least once
      • Usually totally impossible or at least unpractical
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