RT-Link: A Time-Synchronized Link Protocol for Energy- Constrained Multi-hop Wireless Networks

We propose RT-link, a time-synchronized link protocol for real-time wireless communication in industrial control, surveillance and inventory tracking. 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. We identify three key observations in the design and deployment of RT-link: (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. Through analysis and simulation, we show that RT-link outperforms energy-efficient link protocols such as B-MAC 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 and have integrated it within the nano-RK real-time sensor OS. A 42-node network with sub-20 mus synchronization accuracy has been deployed for 3 weeks in the NIOSH Mining Research Laboratory and within two 5-story campus buildings