or performs a loopback in case of search, to detect several occurrences of the MSC behavior in the simulation. If we set the simulation attribute of the observer MSC to verify: the first output of signal sA is detected as a success: this corresponds to the execution of the repeat operator 0 times; then at the end of the sequence sA, sB, sC, another success is detected (repeat one time). if a third sA is simulated after the second, an error is detected (because sA, sA has been observed, instead of sA, sB). To simplify, we have used examples of observation with MSC using interactive simulation, but errors and successes are also detected during random or exhaustive simulation. 5.3.4.6 Unexpected signals In Figure 5.69, you see in the simulation trace that the output of signal sZ is ignored: this is because sZ is not present in the observer MSC test_seq. The only signals monitored in this case are sA, sB, sC18 18 In Tau SDL Suite, the Validator considers by default that signals not present in the MSC must also be monitored: in the present example, if an output of sZ occurs during the simulation, the Validator will report an MSC violation. 158 Validation of Communications Systems with SDL To monitor signals that are not in the observer MSC, you can add them into the field Unexpected signals shown in Figure 5.68. For example, here, if you type sZ in the field Unexpected signals and recompile the model plus the MSC, you get the behavior depicted in Figure 5.71: after sA and sB, the observer expected sC, but sZ has occurred, which is not ignored here, and provokes an error; then no other success happens. Observer MSC Simulation trace test_seq seq_sim inst_proc1 search verify inst_1 proc1 proc1 ERROR ERROR Figure 5.71 Simulation results with sZ as unexpected signal 5.3.4.7 Time local or global In MSC simulation properties shown in Figure 5.68, you see two options concerning time: global or local. If set to global, the ordering of events is global to all the instances present in the observer MSC. For example, in Figure 5.7