Mobility and Multi-channel Communications in Low-power Wireless Networks

Sammanfattning: The prospect of replacing existing fixed networks with cheap, flexible and evenmobile low-power wireless network has been a strong research driver in recent years.However, many challenges still exist: reliability is hampered by unstable and burstycommunication links; the wireless medium is getting congested by an increasingnumber of wireless devices; and life-times are limited due to difficulties in developingefficient duty-cycling mechanisms. These challenges inhibit the industry to fullyembrace and exploit the capabilities and business opportunities that low-powerwireless devices offer. In this thesis, we propose, design, implement, and evaluateprotocols and systems to increase flexibility and improve efficiency of low-powerwireless communications.First, we present MobiSense, a system architecture for energy-efficient communicationsin micro-mobility sensing scenarios. MobiSense is a hybrid architecturecombining a fixed infrastructure network and mobile sensor nodes. Simulations andexperimental results show that the system provides high throughput and reliabilitywith low-latency handoffs.Secondly, we investigate if and how multi-channel communication can mitigate theimpact of link dynamics on low-power wireless protocols. Our study is motivated bya curiosity to reconcile two opposing views: that link dynamics is best compensatedby either (i) adaptive routing, or (ii) multi-channel communication. We perform acomprehensive measurement campaign and evaluate performance both in the singlelink and over a multi-hop network. We study packet reception ratios, maximumburst losses, temporal correlation of losses and loss correlations across channels.The evaluation shows that multi-channel communication significantly reduces linkburstiness and packet losses. In multi-hop networks, multi-channel communicationsand adaptive routing achieves similar end-to-end reliability in dense topologies,while multi-channel communication outperforms adaptive routing in sparse networkswhere re-routing options are limited.Third, we address the problem of distributed information exchange in proximitybasednetworks. First, we consider randomized information exchange and assess thepotential of multi-channel epidemic discovery. We propose an epidemic neightbordiscoverymechanism that reduces discovery times considerably compared to singlechannelprotocols in large and dense networks. Then, the idea is extended todeterministic information exchange. We propose, design and evaluate an epidemicinformation dissemination mechanism with strong performance both in theory andpractice.Finally, we apply some of the concepts from epidemic discovery to the designof an asynchronous, sender-initiated multi-channel medium access protocol. Theprotocol combines a novel mechanism for rapid schedule learning that avoids perpacketchannel negotiations with the use of burst data transfer to provide efficientsupport of ’multiple contending unicast and parallel data flows.