Communications of the ACM
Symbol-Synchronous Buses: Deterministic, Low-Latency Wireless Mesh Networking with LEDs
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Latency-sensitive applications for the Internet of Things often require performance guarantees that contemporary wireless networks fail to offer. The cause of this shortcoming lies in the inherent complexity and inefficiency of networking abstractions such as routing, medium access control, and store-and-forward packet switching, which coordinate multiple nodes across a wireless network. This research highlight describes a novel networking paradigm that aims to enable a new class of latency-sensitive applications by systematically breaking these abstractions. The paradigm, referred to as a symbol-synchronous bus, has nodes that concurrently transmit optical signals and thus delivers a wireless mesh network with a performance envelope resembling that of a wired bus in terms of deterministic latency and throughput. A physical prototype, called ZERO-WIRE, confirms that symbol-synchronous buses unlock a novel end-to-end performance envelope for wireless mesh networks: our 25-node test bed achieves 19kbps of contention-agnostic goodput, latency under 1 ms for two-byte frames across four hops, jitter on the order of 10μs of (is, and a base reliability of 99%. These early results suggest a bright future for the under-explored area of optical wireless mesh networks in delivering ubiquitous connectivity through a low-complexity physical layer.
1. Introduction
Attaining wire-like performance has long been a goal of wireless networking research. Indeed, successful technologies for the wireless collection of sensor data within the Internet of Things (IoT) are being marketed as achieving "wire-like reliability,"21 promising to replace costly and hard-to-maintain wired networks with a drop-in wireless solution. Under the hood, however, these networks operate quite unlike wires: the dominant type of network in these settings is the wireless mesh network (WMN), which builds on years of research examining how to reliably propagate data frames by fully receiving and then forwarding them from one node to another ("routing" by "store-and-forward packet switching"), and how to prevent different nodes from transmitting concurrently on the same channel in order to avoid destructive interference ("medium access control" (MAC)). Current IoT protocol stacks hence instantiate routing protocols, which establish the paths along which messages should be forwarded, as well as MAC protocols, which enforce schedules that determine when a node's radio should transmit a data frame, that is, a sequence of symbols that communicates data to the next hop in the path.
Store-and-forward routing and transmission scheduling result in end-to-end latency (i.e., the time it takes for a frame to be transmitted from a sending to a receiving interface across the network) that is far larger and much more variable than for wired industrial control networks.2, 13 Since low latency and jitter (i.e., variance in latency) are paramount for the latter type of network, the applicability of wireless meshes in latency-sensitive or event-driven scenarios, such as real-time control or robotics, remains limited.16 The use of low-power wide area networks (LPWANs) instead of WMNs does not offer a solution either: LPWANs may eliminate multi-hop routing, yet they do so at the expense of throughput and reliability.1
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