In this paper, we investigate schemes for energy-efficient multi-hop broadcasting in large-scale dense Wireless Sensor Networks. We begin with an initial simplified study of the schemes for relay selection. Our first finding is that MPR-based (Multipoint Relay) mechanisms work poorly in a dense network while the recently proposed Multicast Protocol for Low power and Lossy Networks (MPL) protocol based on Trickle performs better. However, Trickle requires to overhear packet retransmissions in the vicinity, while sensor nodes try to avoid overhearing by periodically waking up and going to sleep to save energy.We propose Beacon-based Forwarding Tree (BFT), a new scheme that achieves similar performance to MPL, although it fits better the case of nodes with low radio duty cycling MACs of the type of beacon-enabled IEEE 802.15.4. Our scheme also guarantees network coverage and its optimized version results in the shortest path distance to the broadcast source at a cost of lesser load mitigation. We compare and discuss the measured performance of MPL on top of Contiki-MAC and BFT over beacon-enabled 802.15.4 on a Contiki testbed. The experimental results of the comparisons show that BFT may achieve very good performance for a range of broadcast intensity, it has a predictable power consumption, a remarkable low power consumption for leaf nodes, and low loss rates. On the other hand, MPL over ContikiMAC can obtain very low duty cycles for low broadcast traffic.
In dense wireless sensor networks, a multichannel MAC is a good means to reduce channel contention and increase frame reception probability. In this paper, we report on experiments with transmissions on various channels in the 2.4GHz ISM band and find more channel diversity than expected: this effect is particularly exacerbated at a short range, but it also has a significant impact at any distance. Moreover, we find that wireless sensor nodes have a radiation pattern that changes significantly with the frequency channel. This feature is inherent to the size of the sensor node, in which the antenna necessarily interferes with other components. The first consequence of this finding is that frequency diversity in sensor networks is even more effective than generally thought, and conversely, single channel communication schemes should be avoided as long as the power budget is not very comfortable.
Reliable wireless communication even in adverse conditions is the key for building the energy efficient and dependable Internet of Things. In this paper, we explore the benefits of channel diversity for enabling efficient wireless communication: we propose MRR (Multi-channel Round-Robin), a backwardcompatible evolution of beacon-enabled IEEE 802.15.4 in which energy constrained nodes take advantage of additional active periods operating on different channels in a round-robin way. Each active period starts with a beacon sent on a cyclically changing channel, which then allows an associated device to transmit data on the channel used for the beacon. MRR schedules the additional active periods at carefully selected instants to avoid direct beacon collisions. To motivate our work, we first experimentally corroborate previous findings that channel diversity is an effective way of mitigating variable or poor transmission conditions. Then, we observe that channel diversity improves the quality of transmission even better than expected-it appears that wireless sensor nodes have a radiation pattern that changes significantly from one frequency channel to another, which often results in a considerably improved gain when using the right communication channel. The evaluation of the MRR scheme through measurements on a real indoor multihop testbed shows that the proposed scheme results in significantly improved Packet Reception Ratio even without resorting to e.g. channel blacklisting. These results confirm the benefits of multichannel operation and exhibit a fully functional solution that does not add a large overhead compared to using a single channel.
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