 five firable transitions 180 Validation of Communications Systems with SDL After the 1000 steps, the following results are obtained: > print nl procl (1) ! n1 = 131 > print n2 procl (1) ! n2 = 240 > print n3 procl (1) ! n3 = 244 > print n4 procl (1) ! n4 = 256 > print n5 procl (1) ! n5 = 129 It shows that the random function used by the Simulator is not perfect as the first and last transitions have been executed half as frequently as the others. 6.3.4.2 Running very long random simulations During simulation (except in exhaustive modes), the following information is stored: the scenario, to be able to go back to any step using undo: one line per transition executed, the textual traces: several lines per transition executed, stored into the file model_name.log and stored into the Simulator window in order to scroll up. If you plan to run a very long random simulation (many million steps), you can select Edit > Configuration and uncheck the box Scenario recording. However, in such case, no undo or redo is possible. Concerning the textual traces, it is wise to turn them off; otherwise the log file will be huge and many lines will be stored in the window. 6.4 ERRORS DETECTABLE BY RANDOM SIMULATION The errors that random simulation can detect are the same as those that are detectable by interactive simulation. The only difference is that random simulation runs automatically and reports the error. Exhaustive Simulation In this chapter, after a presentation of the exhaustive simulation algorithms and of the two sim- ple examples, you will learn how to validate the V.76 SDL model using exhaustive simulation: automatically detecting bugs in a few seconds, detecting nonsimulated symbols, exploring mil- lions of states in a few minutes, and using observers (stop conditions, rules, MSCs, processes or GOAL). Then more simulation algorithms are described (supertrace, liveness etc.), and a strat- egy is given to simulate SDL models with infinite or very large states graphs. Finally, the list of errors that can be detected by e