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Exploiting Shadow Fading to DEtect CO-Moving Wireless DEvices, IEEE Transactions on Mobile Computing, 2009 Authors: Gayathri Chandrasekaran, Mesut Ergin, Marco Gruteser, Rich Martin, Jie yang, Yingying Chen Abstract: |
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We present the DECODE technique to determine whether a set of
transmitters are co-moving, i.e., moving together in close proximity.
Co-movement information can find use in applications ranging from inventory
tracking, to social network sensing, and to optimizing mobile device localization.
The positioning errors from indoor RSS based localization systems tend to be too
large making it difficult to detect whether two devices are moving together based on the inter-device distances.
DECODE achieves accurate co-movement detection by exploiting the correlations in positioning
errors over time. DECODE can not only be implemented in the position space
but also in the signal space where a correlation in shadow fading
due to objects blocking the path between the transmitter and receiver exists.
This technique requires no changes in or cooperation from the tracked devices other than sporadic
transmission of packets. Using experiments from an office environment, we show
that DECODE can achieve near perfect co-movement detection at walking-speed
mobility using correlation coefficients computed over approximately 60-second time
intervals. We further show that DECODE is generic and could accomplish detection for mixed
mobile transmitters of different technologies (IEEE 802.11b/g and IEEE 802.15.4),
and our results are not very sensitive to the frequency at which transmitters
communicate.
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High Throughput MAC Layer Multicasting over Time-Varying Channels , Elsevier Computer Communications (COMCOM) , Volume 32, Number 1, pp 94-104, Jan. 2009 Authors: Ai Chen, Gayathri Chandrasekaran, Dongwook Lee, and Prasun Sinha Abstract: |
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Efficient, scalable and robust multicasting support from the MAC layer is needed for meeting the
demands of multicast based applications over WiFi and mesh networks. However, the IEEE 802.11 pro-
tocol has no specific mechanism for multicasting. It implements multicasting using broadcasting at the
base transmission rate. We identify two fundamental reasons for performance limitations of this
approach in presence of interference and realistic time-varying channels: (a) Channel-state Indifference:
irrespective of the current quality of the channel to the receivers, the transmission always uses the base
transmission rate; (b) Demand Ignorance: packets are transmitted by a node even if children in the mul-
ticast tree have received those packets by virtue of overhearing. We propose a solution for MAC layer
multicasting called HIMAC that uses the following two mechanisms: Unary Channel Feedback (UCF) and
Unary Negative Feedback (UNF) to respectively address the shortcomings of 802.11. Our study is supported
by measurements in simulations. We observe that the end-to-end throughput of multicast sessions using
MAODV can be increased by up to 74% while reducing the end-to-end latency by up to a factor of 56.
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Association Management for Data Dissemination over Wireless Mesh Networks, Elsevier Computer Networks, 2007 Authors: Dongwook Lee, Gayathri Chandrasekaran, Mukundan Sridharan and Prasun Sinha Abstract: |
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optimize the broadcast traffic load. Traditionally, users associate with access points
(APs) with the strongest signal strength.We explore the concept of dual-association,
where the AP for unicast traffic and the AP for broadcast traffic are independently
chosen by exploiting overlapping coverages that are typical in mesh networks. The
goal of our proposed solution is to optimize the overall network load by exploiting
the flexibility provided by independent selection of unicast and broadcast APs. We
propose a novel cost metric based on ETT (Expected Transmission Time) and the
number of nodes in range of the APs, that are advertised in the beacons from
the APs. Users periodically scan and associate with the AP which has the lowest
cost metric. The proposed approach reduces the number of APs that handle the
broadcast traffic resulting in a heavy reduction in control and data packet overhead.
This leads to higher packet delivery rate and enhanced video quality measured in
terms of PSNR. Our approach allows the freed up resources at APs to increase the
unicast throughput. We compare the performance of our approach with traditional
signal strength based association using extensive simulations and real experiments
on an indoor testbed of 180 IEEE 802.11b based devices.
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JOURNAL PUBLICATIONS
CONFERENCE PUBLICATIONS
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Tracking Vehicular Speed Variations by Warping Mobile Phone Signal Strengths, IEEE Internation Conference on Pervasive Computing and Communications (PERCOM), 2011 Authors: Gayathri Chandrasekaran, Tam Vu, Alexander Varshavsky, Marco Gruteser, Rich Martin, Jie Yang, Yingying Chen Abstract: |
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In this paper, we consider the problem of tracking fine-grained speeds variations
of vehicles using signal strength traces from GSM enabled phones. Existing speed estimation techniques using mobile phone signals can provide longer-term speed averages but cannot track short-term speed variations. Understanding short-term speed variations, however, is important in
a variety of traffic engineering applications---for example, it may help distinguish slow speeds due to traffic lights from traffic congestion when collecting real time traffic information. Using mobile phones in such applications is particularly attractive because it can be readily obtained from a large number of vehicles.
Our approach is founded on the observation that the large-scale path loss and shadow fading components of signal strength readings (signal profile) obtained from the mobile phone on any given road segment appears similar over multiple trips along the same road segment except for distortions along the time axis due to speed variations. We therefore propose a speed tracking technique that uses a Derivative Dynamic Time Warping (DDTW) algorithm to realign a given signal profile with a known training profile from the same road. The speed tracking technique than translates the warping path (i.e., the degree of stretching and compressing needed for alignment) into an estimated speed trace. Using 6.4 hours of GSM signal strength traces collected from a vehicle, we show that our algorithm can estimate vehicular speeds with a median error of +/- $4 mph$ compared to that of using a GPS and can capture significant speed variations on road segments with a precision of 68% and a recall of 84% Download complete paper |
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Vehicular Speed Estimation using Received Signal Strength from Mobile Phones, 12th ACM International Conference on Ubiquitous Computing(UBICOMP), 2010 Authors: Gayathri Chandrasekaran, Tam Vu, Alexander Varshavsky, Marco Gruteser, Rich Martin, Jie Yang, Yingying Chen Abstract: |
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This paper introduces an algorithm that estimates the speed of a mobile phone by matching time-series signal strength
data to a known signal strength trace from the same road. Knowing a mobile phone's speed is useful, for example, to
estimate traffic congestion or other transportation performance metrics. The proposed algorithm can be implemented
in the carrier's infrastructure with Network Measurement Reports obtained by a base station or on a mobile phone with
signal strength readings obtained by the handset and depending on implementation choices, promises lower energy consumption than
Global Positioning System~(GPS) receivers. We evaluate the effectiveness of our algorithm on highway and arterial
roads using GSM signal strength traces obtained from several phones over a one month period. The results show that
the Correlation algorithm is significantly more accurate than existing techniques based on handoffs or phone localization.
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Detecting Identity Spoofs in IEEE 802.11e Wireless Networks, IEEE GLOBECOMM, Dec 2009 Authors: Gayathri Chandrasekaran, John-Austen Francisco, Vinod Ganapathy, Marco Gruteser, Wade Trappe Abstract: |
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Wireless networks are vulnerable to identity spoofing attacks, where an
attacker can forge the MAC address of his wireless device to assume the
identity of another victim device on the network. Identity spoofing allows an
attacker to avail network services that are normally restricted to
legitimate users. Prior techniques to detect such attacks rely on
characteristics such as progressions of MAC sequence numbers. However, these
techniques can wrongly classify benign flows as malicious with newer 802.11e
wireless devices that allow multiple progressions of MAC sequence numbers from
the same device. Several other techniques that rely on physical properties of
transmitting devices are ineffective when the attacker
and the victim are mobile. In this paper, we propose an architecture to
robustly detect identity spoofing attacks under varying operating conditions.
Our architecture employs a series of
increasingly powerful detectors to identify or eliminate the possibility of an
attack, culminating in a powerful, RSSI-based per-packet localizer that
reliably detects identity spoofing attacks. We implemented this architecture
and used it to detect a variety of identity spoofing attacks. Our experiments
show that it can effectively detect identity spoofs with a low false positive
rate of 0.5%.
Download complete paper |
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Empirical Evaluation of the Limits on Localization Using Signal Strength, IEEE SECON 2009 (Acceptance Rate ~ 19%) Authors: Gayathri Chandrasekaran, Mesut Ergin, Jie yang, Song Liu, Yingying Chen, Marco Gruteser, Rich Martin Abstract: |
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This work investigates the lower bounds of wireless localization
accuracy using signal strength on commodity hardware. Our work
relies on trace-driven analysis using an
extensive indoor experimental infrastructure. First, we
report the best experimental accuracy, twice the best prior reported
accuracy for any localization system. We experimentally show that adding more and more
resources (e.g., training points or landmarks) beyond a certain limit, can degrade the localization performance for lateration-based
algorithms, and that it could only be improved further by ``cleaning'' the data.
However, matching algorithms are more robust to poor quality RSS measurements.
We next compare with a theoretical lower bound using standard Cram\'{e}r Rao
Bound (CRB) analysis for unbiased estimators, which is frequently used to
provide bounds on localization precision. Because many localization
algorithms are based on different mathematical foundations, we apply a diverse
set of existing algorithms to our packet traces and found that the variance of
the localization errors from these algorithms are smaller than the variance bound
established by the CRB. Finally, we found that there exists a wide discrepancy
from what free-space models predict in the signal to distance function even in
an environment with limited shadowing and multipath, thereby imposing a
fundamental limit on the achievable localization accuracy indoors.
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DECODE : Detecting Co-Moving Wireless Devices, IEEE MASS 2008 (Short Paper, Acceptance Rate ~ 20%) Authors: Gayathri Chandrasekaran, Mesut Ergin, Marco Gruteser, Rich Martin,jie yang, yingying chen Abstract: |
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We present the DECODE technique to determine from a remote receiver whether a set of
transmitters are co-moving, i.e., moving together in close proximity.
Co-movement information can find use in applications ranging from inventory
tracking, to social network sensing, and to optimizing mobile device localization.
DECODE detects co-moving transmitters by identifying correlations in
communication signal strength due to shadow fading. Unlike localization systems,
it can operate using measurements from only a single receiver. It requires no
changes in or cooperation from the tracked devices other than sporadic
transmission of packets. Using experiments from an office environment, we show
that DECODE can achieve near perfect co-movement detection at walking-speed
mobility using correlation coefficients computed over approximately 60-second time
intervals.
Download complete paper |
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Bootstrapping a Location Service Through Geocoded Postal Addresses, LoCA (held with UbiComp), 2007. (Acceptance Rate ~ 33%) Authors: Gayathri Chandrasekaran, Mesut Ergin, Marco Gruteser, Rich Martin Abstract: |
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We analyze the feasibility of boostrapping a location service
through geocoded postal addresses rather than the common wardriving
technique. A location service that contains the MAC addresses and ge-
ographic position of wireless LAN access points enables positioning ser-
vices for WLAN devices and location-aware networking protocols. This
work thus compares the accuracy of access point position estimates ob-
tained based on RF signal strengths readings (wardriving) with the ac-
curacy of the geocoded postal address. The results show similar accuracy
for geocoding in comparison to typical wardriving studies, with signif-
icantly reduced effort if postal addresses of access point positions are
known.
Download complete paper |
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HIMAC: High Throughput MAC Layer Multicasting in Wireless Networks, IEEE MASS, Vancouver, Canada Oct. 2006. Authors: Ai Chen, Dongwook Lee, Gayathri Chandrasekaran, and Prasun Sinha Abstract: |
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Effcient, scalable and robust multicasting support from the MAC layer is needed for meeting the demands of multicast based applications over WiFi and
mesh networks. However, the IEEE 802.11 protocol has no specifc mechanism for multicasting. It implements multicasting using broadcasting at the base transmission
rate.We identify two fundamental reasons for performance limitations of this approach: (a) Channel-state Indifference:
irrespective of the current quality of the channel to the receivers, the transmission always uses the base transmission
rate; (b) Demand Ignorance: packets are transmitted by a node even if children in the multicast tree have received
those packets by virtue of overhearing. We propose a solution for MAC layer multicasting called HIMAC that uses
the following two mechanisms: Unary Channel Feedback (UCF) and Unary Negative Feedback (UNF) to respectively
address the shortcomings of 802.11. Our study is supported by measurements in a testbed, and simulations.We observe
that the end-to-end throughput of multicast sessions using MAODV can be increased by up to 74% while reducing
the end-to-end latency by up to a factor of 56
Download complete paper |
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Optimizing broadcast load in Mesh Networks using Dual-Association, WiMESH (IEEE Workshop on Wireless Mesh Networks) Authors: Dongwook Lee, Gayathri Chandrasekaran, and Prasun Sinha Abstract: |
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This paper systematically studies the problem of optimizing the broadcast traffic load in a mesh network. Traditionally,
association is based on the strongest signal strength. We propose the concept of multi-association, where the access
point (AP) for unicast traffic and the AP for broadcast traffic are independently chosen by exploiting multiple coverages that
are typical in mesh networks. Our focus in this paper is on the problem of distributively selecting the AP for broadcast traffic
for reducing the load in the mesh network. We propose a novel metric called normalized-cost that is advertised in the beacons
from APs. We show that by greedily associating with the AP with minimum normalized cost, the broadcast traffic load can
be reduced. The proposed approach has 25.4% more APs for broadcast traffic than the optimal number of APs computed by
ILP. Simulation results show that the proposed approach reduces the number of APs that handle the broadcast traffic by up to a
factor of 6 in comparison to the traditional signal strength based association. This results in 43.7% lower control messages and
54.9% lower broadcast data transmission in backbone network, leading to 21% higher packet delivery ratio.
Download complete paper |