Reliable Mobility Support in Low-Power Wireless Networks

The need for reliable mobility support in low-power wireless networks (LPWNs) is everincreasing in application contexts such as health-care monitoring, industrial automation,smart cities and generally in the “Internet of Things”. The wireless nature, the environmentalcontext and scalability features of these applications usually impose the useof low-power and low-cost devices. Namely, these devices feature simple single radiotransceivers, communicating at very low TX/RX power and using basic electronics andantennas. These characteristics lead to unstable, asymmetric and unreliable wireless links,which in turn affect the quality of service of the LPWN. This quality of service is evenmore difficult to achieve when the applications at stake support nodes mobility, as inheath-care monitoring and preventive/corrective maintenance in industrial environments.In this context, this Thesis aims at guaranteeing reliable and timely communication inLPWN under nodes mobility.The main research objectives of this Thesis are: (i) to devise a reliable and fasthand-off mechanism for LPWNs, using state-of-the-art (SOTA) commercial-off-the-shelf(COTS) technologies, (ii) to integrate the proposed hand-off mechanism within a SOTA/COTSprotocol stack and (iii) to support the mobility solution with analytical, simulation and experimentalevaluations.Within this research context, we designed a hard hand-off mechanism (dubbed smartHOP)based on averaged single link quality parameters — received signal strength indicator(RSSI) or signal-tonoise ratio (SNR). We fine-tuned the most relevant hand-offparameters — lower threshold level (T`), hysteresis margin (HM), window size (ws), stabilitymonitoring (m) — with preliminary experiments. We studied the impact of radiochannel parameters — path loss exponent and shadowing standard deviation — on handoffperformance (packet delivery ratio and hand-off delay) based on a probabilistic model.A simulation model was designed to further study the impact of link monitoring frequency(window size) and the stability monitoring parameters on hand-off performance. An extendedexperiment with a more realistic environmental set-up was performed to betterfine-tune the hand-off parameters.We integrated smart-HOP within the commodity RPL/6LoWPAN routing protocol(dubbed mRPL) in a simple, effective and backward compatible manner. We also developeda soft hand-off mechanism to seamlessly change parent nodes while moving. Inorder to enable soft hand-off, we activated neighbor overhearing mechanism to eliminatethe Discovery Phase in the hand-off process. We applied the soft hand-off model on topof mRPL (mRPL+) to further increase network performance.Overall, results indicate that mRPL and mRPL+ are able to provide network connectivityand responsiveness by switching between parents in a short period of time while successfully delivering most of data packets. mRPL and mRPL+ are able to provide goodreliability (≈ 100% packet delivery ratio) compared with RPL (delivering < 50% of datapackets). The average hand-off delay in RPL is ≈ 3 s, while it reduces to 85 ms in mRPLand 4 ms in mRPL+. FACULDADE DE ENGENHARIA DA UNIVERSIDADE DO PORTO Reliable Mobility Support in Low-Power Wireless Networks Mohammad Hossein Fotouhi Ghazvini