Group-Based Discovery in Low-Duty-Cycle Mobile Sensor Networks
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2012 9th Annual IEEE Communications Society Conference on Sensor, Mesh and Ad Hoc Communications and Networks (SECON) Group-based Discovery in Low-duty-cycle Mobile Sensor Networks Liangyin Chen∗,YuGu†, Shuo Guo§,TianHe§, Yuanchao Shu‡†, Fan Zhang‡ and Jiming Chen‡ ∗College of Computer Science, Sichuan University † Singapore University of Technology and Design §Department of Computer Science and Engineering, University of Minnesota ‡ Department of Control, Zhejiang University Abstract—Wireless Sensor Networks have been used in many is to reduce duty-cycle by listening briefly and shutting down mobile applications such as wildlife tracking and participatory radios most of the time (e.g., 99% or more). Such a simple urban sensing. Because of the combination of high mobility and low-duty-cycle operation, however, leads to a challenging low-duty-cycle operations, it is a challenging issue to reduce discovery delay among mobile nodes, so that mobile nodes can issue: how nodes within physical vicinity can discovery each establish connection quickly once they are within each other’s other if they listen to the channel in an asynchronous manner. vicinity. Existing discovery designs are essentially pair-wise based, This issue becomes even more challenging when low-duty- in which discovery is passively achieved when two nodes are cycle operation is combined with the mobility of sensor nodes pre-scheduled to wake-up at the same time. In contrast, for in many applications such as in ZerbraNet [7], opportunistic the first time, this work reduces discovery delay significantly by proactively referring wake-up schedules among a group of data collection (SNIP) [19] and urban sensing [5]. Because nodes. Because proactive references incur additional overhead, mobility invalidates many assumptions implicit in low power we introduce a novel selective reference mechanism based on static designs [4], such a combination imposes a time con- spatiotemporal properties of neighborhood and the mobility of straint on how fast nodes shall finish discovery before they the nodes. Our quantitative analysis indicates that the discovery are physically disconnected. delay of our group-based mechanism is significantly smaller than that of the pair-wise one. Our testbed experiments using Node discovery in low-duty-cycle network has attracted 40 sensor nodes confirm our theoretical analysis, showing one research attention in recent years. Previous works mainly focus order of magnitude reduction in discovery delay compared with on how to ensure a pair of nodes can wake up simultaneously traditional pair-wise methods with only 0.5%∼8.8% increase in through a certain type of scheduling algorithms. Notable energy consumption. discovery designs include: stochastic-based protocols [2], [13], quorum-based protocols [11], [12], [16], [20], Disco [3] and I. INTRODUCTION U-Connect [8]. These designs successfully ensure that a pair Wireless Sensor Networks have been proposed for use in of nodes finish discovery within a bounded delay. Although lit- many challenging applications, such as military surveillance, erature is encouraging, we believe there are rooms to improve. scientific exploration and structural monitoring. Sustainable Specifically, we notice all existing designs essentially are pair- deployment of these systems calls for energy-efficient designs. wise based. Discovery is achieved only when two nodes are Extensive research has indicated that energy in low-power pre-scheduled to wake-up simultaneously. However, if nodes sensors is consumed mostly by being ready for potential can share their known schedules with each other, discovery incoming packets, a problem commonly referred to as idle can be achieved in a more proactive and fast manner with listening. For example, the widely used ChipCon CC2420 small overhead. radio [1] draws 19.7mA when receiving or idle listening, This paper presents a Group-based Discovery method as which is actually larger than the 17.4mA used when trans- a performance add-on to existing pair-wise mobile discovery mitting. More importantly, packet transmission time is usually designs. It essentially builds a schedule reference mechanism very small (e.g., about 1 millisecond to transmit a TinyOS among nodes to expedite the discovery process. The operation packet using a CC2420 radio), while the duration of idle of schedule reference is straightforward. For example, after listening for reception can be orders of magnitude longer. node A discovers node B using a traditional pair-wise method, For example, most environmental applications, such as Great node B can proactively refer (push) the wakeup-schedules of Duck Island [15], sample the environment at relatively low its known neighbors (such as node C) to node A. Consequent- rates (on the order of minutes between samples). With a ly, node A can quickly discover node C indirectly via node comparable current draw and a 3∼4 orders of magnitude B. Clearly, excessive reference operations would introduce longer duration waiting for reception, idle listening is a major high overhead in communication. The challenging issue of energy drain that, if not optimized, accounts for most energy our work is to design a selective reference mechanism to trade in communication. off between reducing discovery delay and overhead to achieve Therefore, the most effective energy conservation technique that. 978-1-4673-1905-8/12/$31.00 ©2012 IEEE 542 In summary, our contributions are as follows: a centralized solution. To provide a distributed solution in an • To the best of our knowledge, all previous work focuses asymmetric design, Disco [3] introduces a neighbor discovery on scheduling designs for pair-wise discovery. We inves- method based on the Chinese Reminder Theorem [14], in tigate how group-based discovery can reduce discovery which each node selects a pair of primes as period inde- delay with small overhead. We are the first to provide pendently based on the requirement of its duty cycle. For theoretical analysis of group-based discovery delay for example, if node i select Ti0 and Ti1 as its working periods, mobile low-duty-cycle networks and compared with pair- after node i start to work, once its time counter can divide by wise one. Ti0 or Ti1, it will wake up, or else be in sleep. In addition to • Utilizing spatiotemporal properties of neighborhood, we Disco [3], CQS-pair [12] and GQS-pair [11] present wakeup propose a selective reference mechanism that can avoid scheduling schemes for heterogeneous quorum-based system. unnecessary references while still speeding up overall Using heterogeneous quorums, nodes can have different cycle discovery process. lengths and hence different duty-cycle settings. Recently, U- • We implement and evaluate our design in a physical test- connect [8] proposes a unified neighbor discovery protocol bed consisting of 40 nodes, indicating that our design is for symmetric and asymmetric duty cycle settings. Specifi- suitable for resource constrained sensor nodes. One order cally authors show that U-Connect is an 1.5-approximation of magnitude reduction in discovery delay indicates our algorithm for the symmetric asynchronous discovery scenario, design is very effective for mobile sensor network with and the existing protocols such as Quorum and Disco are 2- high density. approximation algorithms. The rest of the paper is organized as follows: Section II Although neighbor discovery techniques are diverse, all of discusses the related work. Section III introduces the network them focus on pair-wise discovery. None of aforementioned model and assumptions. Section IV introduces the basic design works investigate how to increase discovery probability and and makes a theoretical analysis of discovery delay. Section V decrease discovery delay by sharing schedule information introduces an advanced selective reference design. Section VI among a group of nodes during the discovery process. In this presents the experiment results using a 40-node testbed. Sec- work, we introduce a generic reference mechanism on top of tion VII concludes the work. current discovery methods. It serves as a performance add-on to existing discovery methods, therefore is complementary to II. RELATED WORK the state-of-the-art. Node discovery is nothing new and has a rich literature in both ad hoc and wireless sensor networks. Discovery in III. SYSTEM MODELS AND ASSUMPTIONS always-awake networks mainly focuses on network models In this section, we define the network model and as- with directional antennas [17], [6], [18], while solutions for sumptions related to group-based discovery design for mobile discovery in duty-cycled networks are highly diverse, especial- networks. ly in mobile environments which impose time constraints on how fast discovery should be finished. Notable ones includes: A. Network Model stochastic-based protocols [13], [2], quorum-based protocol- s [16], [20], [12], [11], Disco [3], U-Connect [8], multi- We assume a network with n mobile nodes running under channel discovery [9] and collision-aware discovery [10]. In a low-duty-cycle mode, i.e., a node remains dormant most of birthday protocol [13], nodes listen, transmit or sleep in a time and becomes active only briefly (e.g., less than 5%) to probabilistic round-robin fashion, which statistically trades sense and communicate. When a node is in the active state, it off between discovery energy with discovery latency. Due to can receive packets transmitted from neighboring nodes. When the stochastic nature of its operation, there is no guarantee a node is in the dormant state, it turns off all function modules on the worst-case discovery latency. Quorum-based protocol- except a timer for the purpose of waking itself up. In other s [16], [20], [12], [11] address this limitation by ensuring words, a node can wake up to transmit a packet at any time, existence of overlapped wake-up durations between pair-wise but can receive packets only when it is in its active state. nodes within a bounded time. For instances, in [16], Tseng The working schedule of a mobile node denotes the active- construct a m×m grid matrix within contiguous slots.