Reasoning about places, times, and actions in the presence of mobility

The current trend toward portable computing systems (e.g., cellular phones, laptop computers) brings with it the need for a new paradigm to facilitate thinking about and designing distributed applications. We use the term mobile to refer to distributed systems that include moving, autonomous agents which loosely cooperate to accomplish a task. The fluid nature of the interconnections among components of a mobile system provides new challenges and opportunities for the research community. While we do not claim to have fully grasped all the issues involved in specifying and modeling such systems, we believe that the notions of place, time, and action will play a central role in any model that is developed. We show that these concepts can be expressed and reasoned about in the UNITY logic with a minimal amount of additional notation. The formal derivation of a control system for a radio-dispatched elevator is used to show how considerations involving place, time, and actions impact the design process, be it formal or semiformal.