Khronos

Abstract

Cyber Physical Systems (CPS) combine communication, computation and data storage capabilities to oversee and control physical processes in domains including manufacturing, medical monitoring and smart grids. CPS behavior can be remotely monitored by aggregating event data from various sensors, forwarded over wireless networks. One of the main challenges for CPS application developers is to manage event arrival-time boundaries and to trade off between timeliness and completeness: waiting too long until all events arrive can fail to produce a useful result, while not waiting long enough may lead to faults because the status information is incomplete. Monitoring the production lines in a factory, for example, depends on the aggregation of event data from multiple sensors in the distributed CPS, such as temperature and movement. Yet, predicting time-boundaries for individual event arrivals is difficult, if not impossible, for an application developer, because the wireless network and the sensing devices introduce latencies which vary continuously along with the load, status or environmental conditions of the network and the sensors. This paper proposes Khronos, a middleware that automatically determines timeouts for event arrivals that improve timeliness, given completeness constraint(s) specified by the CPS application developer and taking into account variations in event propagation delays. Extensive evaluations on a physical testbed show that Khronos considerably improves timeliness under varying network configurations and conditions, while satisfying application-specific completeness constraints.