To detect automatically when the SDL model is wrong or when it is correct, observers can be created. Then the SDL model is simulated together with its observers, replaying the scenarios stored previously, to check that the observers work correctly. Again, when an error is found, the SDL model is corrected. To replay scenarios automatically, scripts can be written: after each model correction or evolution, all the test scenarios are automatically replayed in a few seconds and the simulator reports any error or any violation detected by the observers. Introduction 5 1. SDL model creation SDL model exists? SDL model system is complex compilable? or safety-critical? correct the can be modeled SDL model by communicating state machines? create the SDL model do not interactive simulation (2) use SDL Figure 1.4 Step 1: SDL model creation 2. Interactive simulation simulate the main scenarios correct the errors? SDL model save scenarios and MSC traces create observers simulate with observers correct the errors? SDL model create scripts for automatic scenarios replay safety or cost-critical? end of simulation exhaustive simulation (3) Figure 1.5 Step 2: interactive simulation Simulation detects SDL symbols never executed: more scenarios must be created to try to cover them. If the system is not safety- or cost-critical, this level of simulation is sufficient. Otherwise, continue with Step 3. Step 3 is shown in Figure 1.6: exhaustive (or bit-state) simulation is run. When an error or an observer violation is found, the SDL model (or an observer) is corrected. The scenarios 6 Validation of Communications Systems with SDL 3. Exhaustive simulation run exhaustive (or bit-state) simulation correct the errors? SDL model keep scenarios and MSC traces all states explored? reduce configuration max. queue can be reduced? length etc. end of end of partial formal proof validation Figure 1.6 Step 3: exhaustive simulation generated by the simulation must be kept, together with the MSC traces of the scenarios that satisfy the observers. If