mmendation describes a protocol to establish Data Link Connections (DLCs) between two modems and to transfer data over those connections. For a detailed step-by-step tutorial on SDL and how to create the simplified V.76 model used here, see [Doldi01]. The V.76 SDL model and associated files can be downloaded in ObjectGeode and Tau SDL Suite formats on ftp://ftp.wiley.co.uk/pub/books/Idoldi/. To illustrate the protocol and the terms used in the ITU-T V.76 Recommendation, we have depicted in Figure 3.1 two Service Users (SU), A and B, communicating through the V.76 protocol layer. Service User A Service User B primitives primitives (L-ESTABLISH...) (L-ESTABLISH...) DLC n DLC n DLC 1 DLC 1 DLC 0 frames DLC 0 (SABME, UA, disconnected DISC...) disconnected connected connected Figure 3.1 Communication between A and B through the simplified V.76 Several connections can exist in parallel: SU A may establish DLC number 0 to transmit voice to or from SU B; while DLC 0 is running, SUA may establish DLC number 1 to transmit data to or from SU B. Validation of Communications Systems with SDL: The Art of SDL Simulation and Reachability Analysis. Laurent Doldi © 2003 John Wiley & Sons, Ltd ISBN: 0-470-85286-0 26 Validation of Communications Systems with SDL An example of this scenario is provided in Figure 3.2: first A and B perform an eXchange IDentification (XID); then A establishes DLC 0; (DLC 0 on sides A and B are in state con- nected); data is transferred through DLC 0 (between SU A and B); another XID occurs; finally the DLC 0 is released. XID (optional) Establishment of DLC 0 0 is connected Data transfer through DLC 0 0 is connected XID (optional) 0 is connected Release of DLC 0 0 is disconnected Figure 3.2 Example of V.76 scenario We remind you in Figure 3.3 of the usual conventions for signal naming in protocols; the right part also shows those conventions mapped on the architecture depicted in Figure 3.1. Service User A Service User B request 1: request 4: confirm 3: response indication 2: indication response confirm