Song Min Kim

Correlated flooding in low-duty-cycle wireless sensor networks

Flooding in low-duty-cycle wireless sensor networks is very costly due to asynchronous schedules of sensor nodes. To adapt existing flooding-tree-based designs for low-duty-cycle networks, we shall schedule nodes of common parents wake up simultaneously. Traditionally, energy optimality in a designated flooding-tree is achieved by selecting parents with the highest link quality. In this work, we demonstrate that surprisingly more energy can be saved by considering link correlation. Specifically, this work first experimentally verifies the existence of link correlation and mathematically proves that the energy consumption of broadcasting can be reduced by letting nodes with higher correlation receive packets simultaneously. A novel flooding scheme, named Correlated Flooding, is then designed so that nodes with high correlation are assigned to a common sender and their receptions of a broadcasting packet are only acknowledged by a single ACK. This unique feature effectively ameliorates the ACK implosion problem, saving energy on both data packets and ACKs. We evaluate Correlated Flooding with extensive simulations and a testbed implementation with 20 MICAz nodes. We show that Correlated Flooding saves more than 66% energy on ACKs and 15%–50% energy on data packets for most network settings, while having similar performance on flooding delay and reliability.

FreeBee: Cross-technology Communication via Free Side-channel

This paper presents FreeBee, which enables direct unicast as well as cross-technology/channel broadcast among three popular wireless technologies: WiFi, ZigBee, and Bluetooth. Our design aims to shed the light on the opportunities that cross-technology communication has to offer including, but not limited to, cross-technology cooperation and coordination. The key concept of FreeBee is to modulate symbol messages by shifting the timing of periodic beacon frames already mandatory for wireless standards without incurring extra traffic. Such a generic cross-technology design consumes zero additional bandwidth, allowing continuous broadcast to safely reach mobile and/or duty-cycled devices. A new \emph{interval multiplexing} technique is proposed to enable concurrent broadcasts from multiple senders or boost the transmission rate of a single sender. Theoretical and experimental exploration reveals that FreeBee offers a reliable symbol delivery under a second and supports mobility of 30mph and low duty-cycle operations of under 5%.

CorLayer: a transparent link correlation layer for energy efficient broadcast

Wireless communication essentially occurs in a broadcast medium with concurrent receptions. Recent works [34, 41] have shown clear evidence that wireless links are not independent and that transmissions from a transmitter to multiple receivers are correlated, a phenomenon that has profound implications for the performance of network protocols such as broadcast, multi-cast, opportunistic forwarding and network coding. In this paper, we show how link correlation can significantly impact broadcast. We present the design and implementation of CorLayer, a general supporting layer for energy efficient reliable broadcast that carefully blacklists certain poorly correlated wireless links. This method uses only one-hop information, which makes it work in a fully distributed manner and introduces minimal communication overhead. The highlight of our work is CorLayers broad applicability and effectiveness. Our system effort is indeed significant. We integrate CorLayer transparently with sixteen state-of-the-art broadcast protocols specified in thirteen publications [1, 3, 18, 19, 23, 25--27, 32, 36, 38--40] on three physical testbeds running TelosB, MICAz, and GreenOrbs nodes, respectively. The experimental results show that CorLayer remarkably improves energy efficiency across a wide spectrum of broadcast protocols and that the total number of packet transmissions can be reduced consistently by 47% on average.

Link-Correlation-Aware Opportunistic Routing in Wireless Networks

Recent empirical studies have shown clear evidence that wireless links are not independent and that the packet receptions on adjacent wireless links are correlated. This finding contradicts the widely held link-independence assumption in the calculation of the core metric, i.e., the expected number of transmissions to the candidate forwarder set, in opportunistic routing (OR). The inappropriate assumption may cause serious estimation errors in the forwarder set selection, which further leads to underutilized diversity benefits or extra scheduling costs. We thus advocate that OR should be made aware of link correlation. In this paper, we propose a novel link-correlation-aware OR scheme, which significantly improves the performance by exploiting the diverse low correlated forwarding links. We evaluate the design in a real-world setting with 24 MICAz nodes. Testbed evaluation and extensive simulation show that higher link correlation leads to fewer diversity benefits and that, with our link-correlation-aware design, the number of transmissions is reduced by 38%.

Link correlation aware opportunistic routing

By exploiting reception diversity of wireless network links, researchers have shown that opportunistic routing can improve network performance significantly over traditional routing schemes. However, recently empirical studies indicate that we are too optimistic, i.e. diversity gain can be overestimated if we continue to assume that packet receptions of wireless links are independent events. For the first time, this paper formally analyzes the opportunistic routing gain under the presence of link correlation considering the loss of DATA and ACK packets. Based on the model, we introduce a new link-correlation-aware opportunistic routing scheme, which improves the performance by exploiting the diverse uncorrelated forwarding links. Our design is evaluated using simulation where we show (i) link correlation leads to less diversity gain, (ii) and with our link-correlation-aware design; improvement can be gained. We also provide a unique model to generate strings of randomly correlated receptions.

PSR: Practical synchronous rendezvous in low-duty-cycle wireless networks

Low-duty-cycle radio operations have been proposed for wireless networks facing severe energy constraints. Despite energy savings, duty-cycling the radio creates transient-available wireless links, making communication rendezvous a challenging task under the practical issue of clock drift. To overcome limitations of prior work, this paper presents PSR, a practical design for synchronous rendezvous in low-duty-cycle wireless networks. The key idea behind PSR is to extract timing information naturally embedded in the pattern of radio duty-cycling, so that normal traffic in the network can be utilized as a “free” input for drift detection, which helps reduce (or even eliminate) the overhead of traditional time-stamp exchange with dedicated packets or bits. To prevent an overuse of such free information, leading to energy waste, an energy-driven adaptive mechanism is developed for clock calibration to balance between energy efficiency and rendezvous accuracy. PSR is evaluated with both test-bed experiments and extensive simulations, by augmenting and comparing with four different MAC protocols. Results show that PSR is practical and effective under different levels of traffic load, and can be fused with those MAC protocols to improve their energy efficiency without major change of the original designs.

C-Morse: Cross-technology communication with transparent Morse coding

Recent research on CTC (cross-technology communication) demonstrates the viability of direct coordination among heterogeneous devices (e.g., WiFi and ZigBee) with incompatible physical layers. Although encouraging, current solutions suffer from either severe inefficiency in channel utilization or low throughput using limited beacons. To address these limitations, this paper presents C-Morse, which leverages all traffic (such as through data packets, beacons and other control frames) to achieve a high cross-technology communication throughput. The key idea of C-Morse is to slightly perturb the transmission timing of existing WiFi packets to construct recognizable radio energy patterns without introducing noticeable delays to upper layers. At the receiver side, ZigBee captures such patterns by sensing the RSSI value, and then decodes the transmitted symbols. C-Morse also introduces a novel timing-based multiplexing technique to allow the coexistence of multiple C-Morse access points and reject other interference, showing a reliable symbol delivery ratio. As a result, C-Morse achieves a free side-channel, whose CTC throughput is as much as 9 χ of the present state of the art, while maintaining the through traffic within a negligible delay that goes unnoticed by applications and end-users.

Exploiting causes and effects of wireless link correlation for better performance

Contradicting the widely believed assumption of link independence, recently the phenomenon of reception correlation among nearby receivers has been revealed and exploited for varieties of protocols [3], [8], [17], [21], [23], [24]. However, despite the diversified correlation-aware designs proposed up to date, they commonly suffer from a shortcoming where link correlation is inaccurately measured, which leads them to sub-optimal performance. In this work we propose a general framework for accurate capturing of link correlation, enabling better utilization of the phenomenon for protocols lying on top of it. Our framework uses SINR (Signal to Interference plus Noise Ratio) to detect correlations, followed by modeling the correlations for in-network use. We show that our design is light-weight, both computation and storage-wise. We apply our model to opportunistic routing and network coding on a physical 802.15.4 test-bed for energy savings of 25% and 15%.

cETX: Incorporating Spatiotemporal Correlation for Better Wireless Networking

In this work, we experimentally observe the existence of spatiotemporal correlation among adjacent wireless links within short time intervals. Such an observation calls attention to potential errors in existing popular metrics built upon the assumption of link independence. Specifically we propose cETX (correlated ETX), a generalized metric, to compensate for estimation errors suffered by the widely-adopted ETX in the presence of correlated interference. To the best of our knowledge, this is the first work to introduce a unified metric embracing both temporal and spatiotemporal correlations. The highlight of the cETX metric is its broad applicability and effectiveness. Evaluations on ZigBee (802.15.4) and Wi-Fi (802.11b/g/n) testbeds deployed in a lab, corridor, and on a bridge reveal that: Simply replacing ETX with cETX (i) cuts down the error by 70.2% and 62.1%, respectively, and (ii) saves averages of 22% and 37% communication cost in three unicast [4, 13, 17] and nine broadcast protocols [7, 18, 23, 24, 27, 30, 40] at the price of only 0.7% additional overhead.

Prediction Based Mobile Data Aggregation in Wireless Sensor Network

A wireless sensor network consists of many energy-autonomous micro-sensors distributed throughout an area of interest. Each node has a limited energy supply and generates information that needs to be communicated to a sink node. To reduce costs, the data sent via intermediate sensors to a sink, are often aggregated. The existing energy-efficient approaches to in-network aggregation in sensor networks can be classified into two categories, the centralized and distributed approaches, each having its unique strengths and weaknesses. In this paper, we introduce PMDA (Prediction based Mobile Data Aggregation) scheme which uses a novel data aggregation scheme to utilize the knowledge of the mobile node and the infrastructure (static node tree) in gathering the data from the mobile node. This knowledge (geo-location and transmission range of the mobile node) is useful for gathering the data of the mobile node. Hence, the sensor nodes can construct a near-optimal aggregation tree by itself, using the knowledge of the mobile node, which is a similar process to forming the centralized aggregation tree. We show that the PMDA is a near-optimal data aggregation scheme with mobility support, achieving energy and delay efficiency. This data aggregation scheme is proven to outperform the other general data aggregation schemes by our experimental results.