Synchronous closing of timed SDL systems for model checking

Standard model checkers cannot handle open reactive systems directly. Closing the system is commonly done by adding an environmental process. However, for model checking, the way of closing should be well-considered to alleviate the state-space explosion problem. This is especially true in the context of model checking SDL with its asynchronous message-passing communication because of a combinatorial explosion caused by all combinations of messages in the input queues. In this paper we investigate a class of environmental processes for which the asynchronous communication scheme can safely be replaced by a synchronous one. Such a replacement is possible only if the environment is constructed under rather a severe restriction on the behavior, which can be partially softened via the use of a discrete-time semantics. We employ data-flow analysis to detect instances of variables and timers influenced by the data passing between the system and the environment