Sensor-actuator networks and interactive ubiquitous environments are distributed systems in which the sensor-actuators communicate with each other by message-passing. This paper makes three contributions. First, it gives a general system and execution model for such sensor-actuator networks in pervasive environments. Second, it examines the range of time models that are useful for specifying properties, and for implementation, in such distributed networks, and places approaches and limitations in perspective. Third, it shows that although the partial order time model has not been seen to be useful as a specification tool in real applications of sensornets, yet, it is useful for real applications in pervasive sensornets because (under certain conditions) it can serve as a viable alternative to physically synchronized clocks that provide the linear order time model.
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