End-to-end Real-time Guarantees in Wireless Cyber-physical Systems

In cyber-physical systems (CPS), the communication among the sensing, actuating, and computing elements is often subject to hard real-time constraints. Real-time communication among wireless network interfaces and real-time scheduling for complex, dynamic applications have been intensively studied. Despite these major efforts, there is still a significant gap to fill. In particular, the integration of several real-time components to provide end-to-end real-time guarantees between interfaces of distributed applications in wireless CPS is an unsolved problem. We thus present a distributed protocol that considers the complete transmission chain including peripheral busses, memory accesses, networking interfaces, and the wireless real-time protocol. Our protocol provably guarantees that message buffers along this chain do not overflow and that all messages received at the destination application interface meet their end-to-end deadlines. To achieve this while being adaptive to unpredictable changes in the system and the real-time traffic requirements, our protocol establishes at run-time a set of contracts among all major elements of the transmission chain based on a worst-case delay and buffer analysis of the overall system. Using simulations, we validate that our analytic bounds are both safe and tight.