Network-wide Consensus Utilizing the Capture Effect in Low-power Wireless Networks

Paper in proceeding, 2017

In low-power wireless networking, new applications such as cooperative robots or industrial closed-loop control demand for network-wide agreement at low-latency and high reliability. Distributed consensus protocols is a mature field of research in a wired context, but has received little attention in low-power wireless settings. In this paper, we present A2: Agreement in the Air, a system that brings distributed consensus to low-power multi-hop networks. We introduce Synchrotron, a synchronous transmissions kernel that builds a robust mesh by exploiting the capture effect, frequency hopping with parallel channels, and link-layer security. We build A2 on top of this reliable base layer, and enable the two- and three-phase commit protocols, as well as network services such as joining, hopping sequence distribution and re-keying. We evaluate A2 on four public testbeds with different deployment densities and sizes. A2 requires only 475 ms to complete a two-phase commit over 180 nodes. The resulting duty cycle is 0.5 percent for 1-minute intervals. We show that A2 achieves zero losses end-to-end over long experiments, representing millions of data points. When further adding controlled failures, we show that two-phase commit ensures transaction consistency in A2 while three-phase commit provides liveness at the expense of inconsistency under specific failure scenarios.