LC A. This scenario can only occur after the DLC establishment represented in Figure 3.5. This protocol is symmetric, each side being identical: for example, the primitive L-RELEASE request can be received by DLC A or B, and the frame UA can be both sent or received by a DLC. 30 Validation of Communications Systems with SDL data_ba1 /* Data transfer from B to A. */ L_DATAreq DATAind Figure 3.7 The MSC data_bal: data transfer from B to A disc1 /* DLC release. */ L_RELEASEreq L_RELEASEind L_RELEASEind Figure 3.8 The MSC discl: DLC release 3.4 THE SDL MODEL OF V.76 The version included in this section is the version before validation by simulation; it corresponds to Step 9 in [Doldi01] with a few minor changes. During the simulations performed in the next chapters, bugs will be detected and corrected. 3.4.1 The simulation configuration of V.76 To simulate our protocol in a realistic configuration5, we have created the SDL system rep- resented in Figure 3.9: two instances DLCa and DLCb of block type V76_DLC communicate through the block dataLink. Block dataLink, represented in Figure 3.17, simulates a simplified data link layer. Blocks DLCa and DLCb communicate with the service users, not modeled, through the channels DLCaSU and DLCbSU. 3.4.2 The package V76 The block type V76_DLC and its signal declarations are contained in the package V76, imported by the system V76test and shown in Figure 3.10. The package V76 also contains the declarations of data types used as signal parameters and the procedure CRCok. Realistic means that we simulate two V.76 peer entities instead of one alone, and we add the block dataLink to enable losing frames to test the retransmission mechanism. The V.76 Protocol Case Study 31 USEV76; system V76test (dlc2su) (dlc2su) /* Simplified V76 DLCaSU DLCbSU, model. */ (su2dlc) ((su2dlc) DLCa : V76_DLC DLCb : V76_DLC V76frame V76frame DLCaDL DLCbDL V76frame V76frame dataLink Figure 3.9 The simulation configuration of V.76 package V76 SIGNAL NEWTYPE V76paramTyp SYNTYPE XIDframe = Integer /* V.76 pri