Synchronous and Concurrent Transmissions for Consensus in Low-Power Wireless
Doktorsavhandling, 2019

With the emergence of the Internet of Things, autonomous vehicles and the Industry 4.0, the need for dependable yet dynamic network protocols is arising.
Many of these applications build their operation on distributed consensus.
For example, networked cooperative robots and UAVs agree on maneuvers to execute, and industrial control systems agree on set-points for actuators.
Many applications are mission- and safety-critical, too.
Failures could cost lives or precipitate economic losses.

Any wireless network connecting mission-critical devices must be dependable, and often energy-efficient, as many devices are battery powered and we expect them to last for years.
Such a network protocol shall be self-forming and self-fixing as well, to allow for dependable autonomous operation; as many applications cannot afford to stop and wait for external (re)configuration.
In this thesis, we design, implement and evaluate network protocols as a step towards enabling such challenging ubiquitous connectivity in the Internet of Things.

We make four main contributions:

– We introduce Orchestra that addresses the challenge of bringing TSCH (Time Slotted Channel Hopping MAC) to dynamic networks as envisioned in the Internet of Things.
We focus on low-power IPv6 and RPL networks, and introduce Orchestra.
In Orchestra, nodes autonomously compute their own, local schedules.
They maintain multiple schedules, each allocated to a particular traffic plane (application, routing, MAC), and updated automatically as the topology evolves.
Orchestra (re)computes local schedules without signaling overhead, and does not require any central or distributed scheduler.
Instead, it relies on the existing network stack information to maintain the schedules.
We demonstrate the practicality of Orchestra and quantify its benefits through extensive evaluation in two testbeds, on two hardware platforms.
In long running experiments of up to 72 hours we show that Orchestra achieves end-to-end delivery ratios of over 99.99%.

– We present A2 : Agreement in the Air, a system that brings distributed consensus to low-power multi-hop networks.
A2 introduces Synchrotron, a synchronous transmissions kernel that builds a robust mesh by exploiting the capture effect, frequency hopping with parallel channels, and link-layer security.
A2 builds on top of this reliable base layer and enables the two- and three-phase commit protocols, as well as network services such as group membership, hopping sequence
distribution and re-keying.
Evaluations of A2 on public testbeds show that it requires only 475 ms to complete a two-phase commit over 180 nodes.
The resulting duty cycle is 0.5% for 1-minute intervals.
We show that A2 achieves zero losses end-to-end over long experiments, representing millions of data points.

– Fault-tolerance is a necessity in most practical systems and a must in mission-critical systems.
In systems that build their operations on consensus, it is paramount to guarantee agreements despite failures.
We present Wireless Paxos, a fault-tolerant, network-wide consensus primitive for low-power wireless networks.
It is a new flavor of Paxos, the most-used consensus protocol today, and is specifically designed to tackle the challenges of low-power wireless networks.
By building on top of concurrent transmissions, it provides low-latency, high reliability, and guarantees on the consensus.
Our results show that Wireless Paxos requires only 289 ms to complete a consensus between 188 nodes in testbed experiments.
Furthermore, we show that Wireless Paxos stays consistent even when injecting controlled failures.

– We present BlueFlood, a protocol that adapts concurrent transmissions, as introduced by Glossy, to Bluetooth.
The result is fast and efficient network-wide data dissemination in multi-hop Bluetooth networks.
Moreover, we show that BlueFlood floods can be reliably received by off-the-shelf Bluetooth devices such as smartphones, opening new applications of concurrent transmissions and a seamless integration with existing technologies.
We present an in-depth experimental feasibility study of concurrent transmissions over Bluetooth PHY in a controlled environment.
Further, we build a small-scale testbed where we evaluate BlueFlood in real-world settings of a residential environment.
We show that BlueFlood achieves a 99% end-to-end delivery ratio in multi-hop networks with a duty cycle of 0.13% for 1-second intervals.

Distributed Computing







Industrial Internet of Things

Capture Effect

Wireless Networks


Concurrent Transmissions

HA2 (lecture hall), Hörsalsvägen 4, Campus Johanneberg, Chalmers.
Opponent: Kay Römer, Professor, Institute of Technical Informatics, Graz University of Technology, Austria


Beshr Al Nahas

Chalmers, Data- och informationsteknik, Nätverk och system

Orchestra: Robust Mesh Networks Through Autonomously Scheduled TSCH

SenSys'15: Proceedings of the 13th ACM Conference on Embedded Networked Sensor Systems,; (2015)p. 337-350

Paper i proceeding

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

Proceedings of the Conference on Embedded Networked Sensor Systems (ACM SenSys),; (2017)

Paper i proceeding

Paxos Made Wireless: Consensus in the Air

; (2019)

Paper i proceeding

AgreeOnIT: Lättvikts konsensus och distribuerat datakunskap i resursbegränsade sakernas Internet

Vetenskapsrådet (VR), 2019-01-01 -- 2022-12-31.

ChaosNet: Distributed Computing for Low-Power Wireless Networks

Vetenskapsrådet (VR), 2015-01-01 -- 2018-12-31.

Ultralåg fördröjning, effektsnåla trådlösa nät

Stiftelsen för Strategisk forskning (SSF), 2017-01-01 -- 2021-12-31.






Informations- och kommunikationsteknik


Innovation och entreprenörskap



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4647

Technical report D - School of Computer Science and Engineering, Chalmers University of Technology: 176D


Chalmers tekniska högskola

HA2 (lecture hall), Hörsalsvägen 4, Campus Johanneberg, Chalmers.

Opponent: Kay Römer, Professor, Institute of Technical Informatics, Graz University of Technology, Austria

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