ok a systematic and practical approach aimed at engineers. It will guide you through the use of tools to perform simulation, architecture or design debugging by getting coverage of your requirements expressed as test cases using MSCs, meet your quality expectations and get a faster return on investment not only for the tools but also during product development. Jamel Marzouki Distinguished Member of the Technical Staff, Motorola Labs, Schaumburg, IL Introduction 1.1 VALIDATION OF COMMUNICATIONS SYSTEMS Communications systems and software are more and more difficult to develop: they include complex features such as wireless and mobile access, under strong constraints such as low size, weight or power consumption, interworking, total interoperability, security, short time-to-market and low cost. Respecting such an array of constraints requires a high quality of the specifications or standards used for their development. This is why the specifications of many communica- tions systems are based on SDL (Specification and Description Language) or at least contain SDL parts describing complex behaviors. Examples of such systems are the GSM second- generation mobile telephony system, the UMTS third-generation mobile telephony system, the ETSI HiperLAN 2 Broadband Radio Access Network or the IEEE 802.11 wireless Ethernet local area network. Validation of such systems by simulation of SDL models is useful, for example, at the following stages: when standards are created by the organizations, to check that the behavior of the system is correct, to generate Message Sequence Charts (MSCs) (sequence diagrams) illustrating typical use cases, or to generate TTCN (Tree and Tabular Combined Notation) test cases to test the conformance of future implementations; before the implementation of a standard by a company, because standards rarely contain a finished SDL model ready to be translated into the application code; to provide nonambiguous low-error specifications to a contractor, enabling a quicker and less expensive implementati