BUTLER: Increasing the Availability of Low-Power Wireless Communication Protocols.

Over the past years, various low-power wireless protocols based on synchronous transmissions (ST) have been developed to meet the high dependability requirements of emerging cyber-physical applications. For example, Wireless Paxos provides consensus, a key mechanism for building fault-tolerant systems through replication. However, Wireless Paxos and other ST-based protocols are themselves not fault-tolerant: They suffer from a single point of failure that fundamentally impairs the availability of the communication service in the presence of node crashes and network partitions. We present BUTLER, a mechanism that allows removing the single point of failure in many ST-based protocols. BUTLER synchronizes all nodes in the network so that the communication process can be jointly started by multiple randomly chosen nodes rather than a single dedicated node. We analyze and formally prove the correctness of BUTLER and implement it on the state-of-the-art nRF52840 platform. Experiments on the FlockLab testbed demonstrate that BUTLER reliably synchronizes the network to within ± 3 µs despite large initial offsets, unpredictable node failures, and network partitions. BUTLER's temporal overhead ranges well below 1 %. Because of this efficiency and effectiveness, our results further indicate that BUTLER can dramatically improve the availability of an existing ST-based protocol without any noticeable impact on the overall communication reliability and efficiency.