HAEST: Harvesting Ambient Events to Synchronize Time across Heterogeneous IoT Devices

Synchronizing clocks is a resource-intensive and a resource-rigid task; this makes it challenging to align time across resource-constrained and heterogeneous IoT devices. Just as low-power IoT devices harvest energy from the environment, we propose to scavenge timing information from environmental events to conserve device power and bandwidth. We convert ambient events in an environment - sensed by various sensing modalities - into synchronization signals. Our approach, HAEST, leverages prevalent sensors on commodity platforms such as accelerometer, microphone, and optical sensors to timestamp commonly observed events. We present a light-weight and robust approach to detect events across different types of sensors and devices. This creates ample opportunities to align time and simultaneously avoid the clocks to drift apart. Through evaluation on a hardware testbed in a smart home and a wireless body area network, we show that HAEST achieves clock accuracy as low as sub-milliseconds with no cost for IoT devices. Importantly, our use of off-the-shelf IoT platforms in our evaluations, establishes the universality and applicability of our approach to a variety of smart spaces with heterogeneous devices.