RT-Link: A global time-synchronized link protocol for sensor networks

For real-time wireless communication in industrial control, surveillance and inventory tracking, we propose RT-Link, a time-synchronized link protocol. RT-Link provides predictable lifetime for battery-operated embedded nodes, bounded end-to-end delay across multiple hops, and collision-free operation. We investigate the use of hardware-based time-synchronization for infrastructure nodes by using an AM carrier–current radio for indoors and atomic clock receivers for outdoors. Mobile nodes are synchronized via in-band software synchronization within the same framework. Through the design and deployment of RT-Link, we identify three key observations: (a) Hardware-based global time-synchronization is a robust and scalable option to in-band software-based techniques. (b) Achieving global time-synchronization is both economical and convenient for indoor and outdoor deployments. (c) RT-Link achieves a practical lifetime of over 2 years. By analysis and simulation, we show that RT-Link outperforms energy-efficient low-power-listen CSMA protocols in terms of node lifetime and end-to-end latency. The protocol supports flexible services such as on-demand end-to-end rate control and logical topology control. We implemented RT-Link on the CMU FireFly sensor platform as a link layer running on the Nano-RK real-time sensor OS. We also discuss deployment experiences using RT-Link in a coal mine safety application.