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Mobile Computing, IEEE Transactions on

Issue 11 • Date Nov. 2010

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  • [Front cover]

    Page(s): c1
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  • [Inside front cover]

    Page(s): c2
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  • Editorial: Inttroduction of New Associate Editors

    Page(s): 1505 - 1507
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  • Duty Cycle Control for Low-Power-Listening MAC Protocols

    Page(s): 1508 - 1521
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2527 KB) |  | HTML iconHTML  

    Energy efficiency is of the utmost importance in wireless sensor networks. The family of Low-Power-Listening MAC protocols was proposed to reduce one form of energy dissipation-idle listening, a radio state for which the energy consumption cannot be neglected. Low-Power-Listening MAC protocols are characterized by a duty cycle: a node probes the channel every t_i {rm s} of sleep. A low duty cycle favors receiving nodes because they may sleep for longer periods of time, but at the same time, contention may increase locally, thereby reducing the number of packets that can be sent. We propose two new approaches to control the duty cycle so that the target rate of transmitted packets is reached, while the consumed energy is minimized. The first approach, called asymmetric additive duty cycle control (AADCC), employs a linear increase/linear decrease in the t_i value based on the number of successfully received packets. This approach is easy to implement, but it cannot provide an ideal solution. The second approach, called dynamic duty cycle control (DDCC) utilizes control theory to strike a near-optimal balance between energy consumption and packet delivery successes. We generalize both approaches to multihop networks. Results show that both approaches can appropriately adjust t_i to the current network conditions, although the dynamic controller (DDCC) yields results closer to the ideal solution. Thus, the network can use an energy saving low duty cycle, while delivering up to four times more packets in a timely manner when the offered load increases. View full abstract»

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  • Exploiting Microscopic Spectrum Opportunities in Cognitive Radio Networks via Coordinated Channel Access

    Page(s): 1522 - 1534
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    Under the current opportunistic spectrum access (OSA) paradigm, a common belief is that a cognitive radio (CR) can use a channel only when this channel is not being used by any neighboring primary radio (PR). Therefore, the existence of a spectrum opportunity hinges on the absence of active cochannel PRs in a macroscopic region. In this paper, we propose the concept of microscopic spectrum opportunity and show that CRs can still utilize this type of opportunities without interfering with active cochannel PRs, even when these PRs are close to them. As a result, a channel may at the same time present different levels of availability to different CRs. Channel access needs to be carefully coordinated between these CRs to avoid collisions, and more importantly, ensure efficient utilization of the spectrum opportunity from a network's standpoint. In this paper, we formulate the coordinated channel access as a joint power/rate control and channel assignment optimization problem, with the objective of maximizing the sum-rate achieved by the cognitive radio network (CRN). We develop both centralized and distributed algorithms to solve this problem. Our simulation results show that even when accounting for the implementation overhead, significant throughput gain is achieved under our designs. View full abstract»

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  • Rate Adaptation in Congested Wireless Networks through Real-Time Measurements

    Page(s): 1535 - 1550
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    Rate adaptation is a critical component that impacts the performance of IEEE 802.11 wireless networks. In congested networks, traditional rate adaptation algorithms have been shown to choose lower data-rates for packet transmissions, leading to reduced total network throughput and capacity. A primary reason for this behavior is the lack of real-time congestion measurement techniques that can assist in the identification of congestion-related packet losses in a wireless network. In this work, we first propose two real-time congestion measurement techniques, namely an active probe-based method called Channel Access Delay, and a passive method called Channel Busy Time. We evaluate the two techniques in a testbed network and a large WLAN connected to the Internet. We then present the design and evaluation of Wireless cOngestion Optimized Fallback (WOOF), a rate adaptation scheme that uses congestion measurement to identify congestion-related packet losses. Through simulation and testbed implementation we show that, compared to other well-known rate adaptation algorithms, WOOF achieves up to 300 percent throughput improvement in congested networks. View full abstract»

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  • Accuracy Performance Analysis between War Driving and War Walking in Metropolitan Wi-Fi Localization

    Page(s): 1551 - 1562
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (3444 KB) |  | HTML iconHTML  

    Wi-Fi ingerprint-based localization is currently the most promising approach to building metropolitan-scale localization systems for both indoor and outdoor environments. A Wi-Fi fingerprint localization system makes use of a radio map that contains Wi-Fi signals received at various locations, and uses the map for location estimation. Thus, the accuracy of a Wi-Fi fingerprint localization system greatly depends on the quality of its radio map. This paper explores various properties of a metropolitan radio map that may affect the accuracy of Wi-Fi fingerprint localization systems. In this study, metropolitan-scale radio maps are obtained using war walking and war driving. These maps contain hundreds of thousands of access points and signal samples. A detailed comparison analysis of selected radio map properties reveals how different map properties affect the difference between the positional accuracies of the driving and walking radio maps in Wi-Fi fingerprint-based localization. View full abstract»

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  • On Nodal Encounter Patterns in Wireless LAN Traces

    Page(s): 1563 - 1577
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (3181 KB) |  | HTML iconHTML  

    In this paper, we analyze multiple wireless LAN (WLAN) traces from university and corporate campuses. In particular, we consider important events between mobile nodes in wireless networks-encounters. We seek to understand encounter patterns in the mobile network from a holistic view, using a graph analysis approach. Such an analysis sheds light on the diverse, nonhomogeneous nature of users in the given environments in terms of their encounter events with other nodes. Furthermore, we evaluate the feasibility of forming an infrastructureless network to reach most of the nodes utilizing time-varying internode connectivity through encounters, and the robustness of such an ad hoc communication network. Our analysis shows that while the encounter events are “sparse” (i.e., any given node does not encounter with many other nodes), the connectivity of the whole network is well-maintained, and a Small World pattern of nodal encounter emerges for the observation periods longer than one day. More interestingly, the encounter events collectively form a robust communication network, in which store-carry-forward message dissemination can be successful even with over 20 percent noncooperative nodes or removal of short-lived (up to minutes) encounter events. View full abstract»

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  • XLP: A Cross-Layer Protocol for Efficient Communication in Wireless Sensor Networks

    Page(s): 1578 - 1591
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1862 KB) |  | HTML iconHTML  

    Severe energy constraints of battery-powered sensor nodes necessitate energy-efficient communication in Wireless Sensor Networks (WSNs). However, the vast majority of the existing solutions are based on the classical layered protocol approach, which leads to significant overhead. It is much more efficient to have a unified scheme, which blends common protocol layer functionalities into a cross-layer module. In this paper, a cross-layer protocol (XLP) is introduced, which achieves congestion control, routing, and medium access control in a cross-layer fashion. The design principle of XLP is based on the cross-layer concept of initiative determination, which enables receiver-based contention, initiative-based forwarding, local congestion control, and distributed duty cycle operation to realize efficient and reliable communication in WSNs. The initiative determination requires simple comparisons against thresholds, and thus, is very simple to implement, even on computationally constrained devices. To the best of our knowledge, XLP is the first protocol that integrates functionalities of all layers from PHY to transport into a cross-layer protocol. A cross-layer analytical framework is developed to investigate the performance of the XLP. Moreover, in a cross-layer simulation platform, the state-of-the-art layered and cross-layer protocols have been implemented along with XLP for performance evaluations. XLP significantly improves the communication performance and outperforms the traditional layered protocol architectures in terms of both network performance and implementation complexity. View full abstract»

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  • Multihop Range-Free Localization in Anisotropic Wireless Sensor Networks: A Pattern-Driven Scheme

    Page(s): 1592 - 1607
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (3511 KB) |  | HTML iconHTML  

    This paper focuses on multihop range-free localization in anisotropic wireless sensor networks. In anisotropic networks, geometric distance between a pair of sensor nodes is not always proportional to their hop count distance, which undermines the assumption of many existing range-free localization algorithms. To tolerate network anisotropy, we propose a pattern-driven localization scheme, which is inspired by the observation that in an anisotropic network the hop count field propagated from an anchor exhibits multiple patterns, under the interference of multiple anisotropic factors. Our localization scheme therefore for different patterns adopts different anchor-sensor distance estimation algorithms. The average anchor-sensor distance estimation accuracy of our scheme, as demonstrated by both theoretical analysis and extensive simulations, is improved to be better than 0.4r when the average sensor density is above eight, and the sensor localization accuracy thus is approximately better than 0.5r. This localization accuracy can satisfy the needs of many location-dependent protocols and applications, including geographical routing and tracking. Compared with previous localization algorithms that declares to tolerate network anisotropy, our localization scheme excels in 1) higher accuracy stemming from its ability to tolerate multiple anisotropic factors, including the existence of obstacles, sparse and nonuniform sensor distribution, irregular radio propagation pattern, and anisotropic terrain condition, 2) localization accuracy guaranteed by theoretical analysis, rather than merely by simulations, and 3) a distributed solution with less communication overhead and enhanced robustness to different network topologies. View full abstract»

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  • Integrity Regions: Authentication through Presence in Wireless Networks

    Page(s): 1608 - 1621
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    Despite years of intensive research, the main deterrents of widely deploying secure communication between wireless nodes remains the cumbersome key setup process. In this paper, we address this problem and we introduce Integrity (I) regions, a novel security primitive that enables message authentication in wireless networks without the use of preestablished or precertified keys. Integrity regions are based on the verification of entity proximity through time-of-arrival ranging techniques. IRegions can be efficiently implemented with ultrasonic ranging, in spite of the fact that ultrasound ranging techniques are vulnerable to distance enlargement and reduction attacks. We further show how IRegions can be used for key establishment in mobile peer-to-peer wireless networks and we propose a novel automatic key establishment approach, largely transparent to users, by leveraging on IRegions and nodes' mobility. We analyze our proposals against a multitude of security threats and we validate our findings via extensive simulations. View full abstract»

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  • Cross-Layer Sleep Scheduling Design in Service-Oriented Wireless Sensor Networks

    Page(s): 1622 - 1633
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    Service-oriented wireless sensor networks have recently been proposed to provide an integrated platform, where new applications can be rapidly developed through flexible service composition. In wireless sensor networks, sensors are periodically switched into the sleep mode for energy saving. This, however, will cause the unavailability of nodes, which, in turn, incurs disruptions to the service compositions requested by the applications. Thus, it is desirable to maintain enough active sensors in the system to provide each required service at any time in order to achieve dependable service compositions for various applications. In this paper, we study the cross-layer sleep scheduling design, which aims to prolong the network lifetime while satisfying the service availability requirement at the application layer. We formally define the problem, prove that the problem is NP-hard, and develop two approximation algorithms based on the LP relaxation and one efficient reordering heuristic algorithm. The proposed work will enhance the dependability of the service composition in service-oriented wireless sensor networks. View full abstract»

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  • Comparison of Data-Driven Link Estimation Methods in Low-Power Wireless Networks

    Page(s): 1634 - 1648
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2372 KB) |  | HTML iconHTML  

    Link estimation is a basic element of routing in low-power wireless networks, and data-driven link estimation using unicast MAC feedback has been shown to outperform broadcast-beacon-based link estimation. Nonetheless, little is known about how different data-driven link estimation methods affect routing behaviors. To address this issue, we classify existing data-driven link estimation methods into two broad categories: L-NT that uses aggregate information about unicast and L-ETX that uses information about the individual unicast-physical-transmissions. Through mathematical analysis and experimental measurement in a testbed of 98 XSM motes (an enhanced version of MICA2 motes), we examine the accuracy and stability of L-NT and L-ETX in estimating the ETX routing metric. We also experimentally study the routing performance of L-NT and L-ETX. We discover that these two representative, seemingly similar methods of data-driven link estimation differ significantly in routing behaviors: L-ETX is much more accurate and stable than L-NT in estimating the ETX metric, and accordingly, L-ETX achieves a higher data delivery reliability and energy efficiency than L-NT (for instance, by 25.18 percent and a factor of 3.75, respectively, in our testbed). These findings provide new insight into the subtle design issues in data-driven link estimation that significantly impact the reliability, stability, and efficiency of wireless routing, thus shedding light on how to design link estimation methods for mission-critical wireless networks which pose stringent requirements on reliability and predictability. View full abstract»

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  • A Biased Random Walk Routing Protocol for Wireless Sensor Networks: The Lukewarm Potato Protocol

    Page(s): 1649 - 1661
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    Low-latency data delivery is an important requirement for achieving effective monitoring through wireless sensor networks. When sensor nodes employ duty cycling, sending a message along the shortest path, however, does not necessarily result in minimum delay. In this paper, we first study the lowest latency path problem, i.e., the characteristics of a path with minimum delay that connects a source node to the sink under random duty cycling nodes. Then, we propose a forwarding protocol based on biased random walks, where nodes only use local information about neighbors and their next active period to make forwarding decisions. We refer to this as lukewarm potato forwarding. Our analytical model and simulation experiments show that it is possible to reduce path latency without significantly increasing the number of transmissions (energy efficiency) needed to deliver the message to the destination. In particular, although deviating from the shortest path requires additional transmissions, and hence, higher energy consumption, this increase is compensated by a lighter duty cycle. Our experiments show that, overall, we can save up to 15 percent of energy while obtaining the same data delivery delay as shortest path routing. Additionally, the proposed solution is tunable. By changing the value of just one threshold parameter, it can be tuned to operate anywhere in the continuum from hot potato/random walk forwarding protocol to a deterministic shortest path forwarding protocol. View full abstract»

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  • IEEE Computer Society Computing Now [advertisement]

    Page(s): 1662
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    Page(s): 1663
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    Page(s): 1664
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  • TMC Information for authors

    Page(s): c3
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  • [Back cover]

    Page(s): c4
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Aims & Scope

Mobile Computing, as proposed in this Transactions, focuses on the key technical issues related to (a) architectures, (b) support services, (c) algorithm/protocol design and analysis, (d) mobile environments, (e) mobile communication systems, (f) applications, and (g) emerging technologies.

Full Aims & Scope

Meet Our Editors

Editor-in-Chief
Prasant Mohapatra
Interim Vice-Provost and CIO
Professor, Dept. Computer Science
University of California, Davis, USA
pmohapatra@ucdavis.edu