L model can be detected by user-defined rules during the validation. However, user-defined rules cannot detect the correct sequencing of signals: for that, you must use MSCs. 5.2.2 Simulate with a basic MSC You will simulate the V.76 SDL model, observed by a basic MSC. Basic means that the MSC just contains a single scenario, as opposed to MSCs containing inline operators or to High-Level 118 Validation of Communications Systems with SDL MSCs (HMSC). To simplify, we reuse the basic MSC testl.msc, generated by the Simulator in Chapter 4. Naturally, you could use another MSC drawn from scratch with the Editor or rework an existing MSC. In Tau SDL Suite Validator3, an MSC is not only observing but also driving the simulation. A. Select File > Restart in the Validator, and enter the command: load-msc test1. msc Check that in testl.msc, there is either a single environment instance named env_0 or two environment instances named DLCaSU and DLCbSU (the names of the two external channels in the SDL model); otherwise the simulation would not match the loaded MSC. When an MSC is loaded, the Validator sends to the SDL model the first environment signal present in the MSC. Then, after receiving the first response from the SDL model, the Validator sends the next environment signal and SO on until the MSC is verified or violated. B. Select Commands > Toggle MSC Trace. C. Press the Navigator button in the Explore group. The Navigator window appears, as illus- trated in Figure 5.8. By double-clicking in this window, you can execute transitions in the SDL model, going forward (down) but also backward (up). Navigator No up node Next 1: Transition START Pld AtoB: 1 State : start state Figure 5.8 The Navigator window D. In the Navigator window, double-click on the lower rectangle, marked Next 1, to execute the corresponding SDL transition. E. Continue double-clicking; when there are several possibilities, select the left one. F. After around 45 transitions, the Navigator window displays: No down node - MSC testl verified, as shown i