Link dynamics accessment in low-power wireless networksIntroductionInterference and link dynamics constitute great concerns for the stability and performance of wireless sensor network protocols. These phenomena normally manifest in link burstiness, i.e, prolonged periods of time where packet transmissions from a sender to a receiver are lost (Srinivasan et al., 2008). Such spikes of packet losses cause delays and instability in communication protocols with potentially severe consequences Multichannel communication has been proposed as alternative to adaptive (single-channel) routing protocols for mitigating the impact of interference and link dynamics in wireless sensor networks. While several studies have advocated features of both techniques (not without running up against contradicting arguments) a comprehensive study that aligns these results is still lacking. This work aims at filling this gap. Testbed
Experiment Overview
RF characteristics for point-to-point communicationsWe compare single channel communication and channel hopping in four settings: (i) packet reception ratio (PRR), (ii) maximum burst loss, (iii) temporal correlation of losses (link burstiness), and (iv) frequency correlation of losses. 1. Packet reception ratio
Observation: Our conclusion are aligned with previous results in (Ortiz and Culler, 2010, Sexton et al., 2005) (i.e. the link quality is not homogenous across channels). However, the sole links PRR analysis, as used in (Ortiz and Culler, 2010, Sexton et al., 2005), is not sufficient to characterize the statistical behavior of a packet losses of a multi-channel channel hopping MAC. To close this gap, we next analyze three alternative metrics. 2. Maximum burst lossPacket losses are often correlated in time and occur in bursts.Therefore, analyzing only the (long term average) packet reception ratio is not sufficient, as it hides important performance indicator such as link burstiness. A simple metric to describe the link RF characteristics in terms of burstiness is the maximum burst loss, defined as the maximum number of consecutive packets lost over a communication link.
3. Link burstiness: temporal correlationTo quantify the correlation of packet losses in time for single-channel communication, Srinivasan et al. defined a “link burstiness” metric referred to as the -factor (Srinivasan et al., 2008). The metric is based on the conditional packet delivery function, C(n), which describes the conditional probability of successful packet reception given that the n previous packet were received (for ) or lost (for ) where is the distance from the ideal bursty link, while and are the conditional packet delivery functions of the empirical and independent link with the same PRR, respectively. A value of identifies a link with independent packet losses i.e. following a Bernoulli process, while a value of indicates a bimodal link, i.e. a link that exists either in a good or a bad state.
Observation: we use to quantify the temporal correlation of packet losses for both singlechannel and multi-channel communication. To compute in the case of channel hopping, we consider the sequence of packets received between each transmitterreceiver pair. Thus, in this case describes the link-burstiness of a given transmitterreceiver pair across multiple channels and for a given channel hopping sequence. Both the length of the hopping sequence and the channel used will influence . 4. Correlation of losses: frequency correlationWe use the metric -factor (Srinivasan et al., 2010), to describe the inter-link reception correlation in case of single-channel communication. This metric quantifies the spatial correlation of packet reception at different nodes receiving packets from the same source. We adapt the theoretical framework of (Srinivasan et al., 2010) to characterize the inter-frequency reception correlation of a link in case of multi-channel communication. Analysis of inter-frequency correlation through the CDF of the -factor for length-2 TSCH sequences computed over all links in the network. We consider three cases: (a) adjacent 802.15.4 channels under different 802.11 channels; (b) distant-2 802.15.4 channels under different 802.11 channels; and (c) well separated 802.15.4 channels. Adaptive routing and channel-hopping in multi-hop networksIn this section we evaluate the benefit of channel hopping in multi-hop single-path routing, and present two core findings:
1. Routing: channel-hopping in multi-hop networksObservation: increasing channel robustness does not guarantee a better network performance compared to single channel solution. As long as there still exist many good links in the network, routing makes the use of channel hopping superfluous. 2. Multi-hop analysis in sparse networks
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