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Reliable Mobility Support in Low-Power Wireless Networks
Ref: CISTER-TR-150502       Publication Date: 16, Apr, 2015

Abstract:
The need for reliable mobility support in low-power wireless networks (LPWNs) is ever increasing in application contexts such as health-care monitoring, industrial automation, smart cities and generally in the “Internet of Things”. The wireless nature, the environmental context and scalability features of these applications usually impose the use of low-power and low-cost devices. Namely, these devices feature simple single radio transceivers, communicating at very low TX/RX power and using basic electronics and antennas. 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 even more difficult to achieve when the applications at stake support nodes mobility, as in heath-care monitoring and preventive/corrective maintenance in industrial environments. In this context, this Thesis aims at guaranteeing reliable and timely communication in LPWN under nodes mobility. The main research objectives of this Thesis are: (i) to devise a reliable and fast hand-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/COTS protocol stack and (iii) to support the mobility solution with analytical, simulation and experimental evaluations. 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-to-noise ratio (SNR). We fine-tuned the most relevant hand-off parameters — lower threshold level (T` ), hysteresis margin (HM), window size (ws), stability monitoring (m) — with preliminary experiments. We studied the impact of radio channel parameters — path loss exponent and shadowing standard deviation — on handoff performance (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 extended experiment with a more realistic environmental set-up was performed to better fine-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 developed a soft hand-off mechanism to seamlessly change parent nodes while moving. In order to enable soft hand-off, we activated neighbor overhearing mechanism to eliminate the Discovery Phase in the hand-off process. We applied the soft hand-off model on top of mRPL (mRPL+) to further increase network performance. Overall, results indicate that mRPL and mRPL+ are able to provide network connectivity and 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 good reliability (≈ 100% packet delivery ratio) compared with RPL (delivering < 50% of data packets). The average hand-off delay in RPL is ≈ 3 s, while it reduces to 85 ms in mRPL and 4 ms in mRPL+.

Hossein Fotouhi

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