ame( sabme : (.0.) ) _estabind(0) |_releasereq( 0 ) v76frame( dm : (.0.) ) v76frame( dm : (.0.) ) lestabconf(0) Figure 7.66 The first success scenario discovered by the Simulator The first error scenario discovered by observer obs1 is contained in the file: v76.obsl.el.scn. Figure 7.67 shows the MSC generated (at block levels) after loading the error scenario into the Simulator. v76obs1_e1 inst_dlca inst_datalink inst_dlcb BLOCK/ BLOCK/ BLOCK/ v76test/dlca v76test/datalink v76test/dlcb l_estabreq( 0) v76frame( sabme : (.O.) ) v76frame( sabme : (.0.) ) signal not yet __estabind( consumed by |_releasereq( 0 DLC(1) v76frame( dm : (.0.) ) v76frame( dm (.0.) ) I_estabreq( v76frame( sabme : (.1.) ) v76frame( sabme : (. 1 ) ) L_EstabConf(0) _estabind(1) was expected I_estabresp v76frame( ua : (.1.) ) v76frame( ua : (.1.) ) I_estabconf( 1 ) Figure 7.67 The error scenario discovered by the Simulator We see that the observer has detected an error because the parameter of L_EstabConf, 1, is not equal to the parameter of L_EstabReq, 0: because Reasonable environment was off, the Simulator has transmitted a new L_EstabReq to the model before the end of the internal events. 256 Validation of Communications Systems with SDL The error scenario shows that our model could be improved by transmitting an L ReleaseInd as soon as the DM is received. Then the observer could be improved to be reset by the observation of an L_ReleaseInd when an L_EstabConf is expected. 7.5 OTHER SIMULATION ALGORITHMS 7.5.1 Tau SDL Suite Besides bit-state and exhaustive modes, Tau SDL Suite Validator proposes the following explo- ration modes: tree search: this is a free exploration of the states graph where the global states are not stored; thus the Validator cannot detect if a state has already been explored. In this mode, the exploration never stops, even on a model with few global states. power walk, tree walk: these algorithms are designed for TTCN test case automatic generation for Autolink, to maximize the SDL symbols coverage; we will not describe