X-Sync: Cross-Technology Clock Synchronization Among Off-the-Shelf Wireless IoT Devices

Clock synchronization in distributed IoT systems is a necessary feature to allow a coherent data collection and event detection. This task is challenging, as today's IoT systems often consist of heterogeneous wireless devices using incompatible technologies. Because of this, existing solutions often make use of multi-radio gateways, which allow an indirect synchronization across heterogeneous devices, but increase end-to-end delays and suffer from an increased overhead. In this work, we present X-Sync, a novel approach allowing a direct and bidirectional clock synchronization among off-the-shelf wireless IoT devices with incompatible physical layer. X-Sync leverages cross-technology communication to convey timing information among heterogeneous devices and presents novel techniques to compensate for the inaccuracies in reliably detecting the start of a cross-technology frame. We seamlessly integrate X-Sync into the Contiki-NG operating system and evaluate its performance experimentally on off-the-shelf Bluetooth Low Energy and IEEE 802.15.4 devices, showing that X-Sync achieves a μs-level synchronization accuracy.