type toPeer ready ready ready V76frame(V76par) ready V76frame(V76par) Figure 4.32 SDL tracking: start transition ready (a) and executed (b) Double-click26 on the same transition (atob(1):start): this executes the transition from the start symbol to the state ready which is now in bold, as depicted in Figure 4.32(a). D. Double-click on the three transitions remaining in the list to execute them. As you can see in Figure 4.33, no transition is ready to be fired: this is a deadlock. In fact, the simulation is blocked because the SDL model now expects to receive some external signals. E. Save the current simulation scenario into the file start.scn by typing the following command in the Simulator: save start . scn 4.3.1.4 Send signals to the model Figures 4.34 and 4.35 show that the V.76 SDL model requires the input of external sig- nals, such as L_EstabReq, contained in the signallist su2dlc on the channels DLCaSU and DLCbSU. We will tell the Simulator to send such signals to the SDL model, using the feed command27. 25 The Simulation Builder converts the SDL names to lower case; thus the actual names you see in the tool do not contain capitals. If your mouse has three buttons, it is easier to click once on its middle button. 27 The Simulator output command (in the Execute menu) can also be used, but it is less convenient as it sends the signal only once: to send the signal again, the output command must be repeated, which is not possible during an automatic simulation (such as random or exhaustive). 72 Validation of Communications Systems with SDL F:ADONNEES\TRANSMET\livreSimul\models0GW76Wv76 File Edit View Execute Tools Help About selection: trans dicaldispatch start Stop if Filter + START : Watch Print + NEXTSTATE ready; step = 4 Traces: trans dicb!dispatch : start + START : Defaults Off + NEXTSTATE ready; Save Restore SDL tracking Firable Transitions: deadlock Start MSC Watch... MSC-driven: Activate Now: 0 Figure 4.33 The simulation is blocked USEV76; L EstabReq etc. system V76test (dlc2su) (dlc2su) /* Simplified