ZiFi: Wireless LAN Discovery via ZigBee Interference Signatures

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ZiFi: Wireless LAN Discovery via ZigBee Interference Signatures Ruogu Zhou 1 , Yongping Xiong 2 , Guoliang Xing 1 Limin Sun 2 , Jian Ma 3 1 Michigan State University 2 Chinese Academy of Sciences 3 Nokia Research Center, China

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ZiFi: Wireless LAN Discovery via ZigBee Interference Signatures. Ruogu Zhou 1 , Yongping Xiong 2 , Guoliang Xing 1 Limin Sun 2 , Jian Ma 3 1 Michigan State University 2 Chinese Academy of Sciences 3 Nokia Research Center, China. Mobile WiFi Access. WiFi usage from mobile users is soaring - PowerPoint PPT Presentation

Transcript of ZiFi: Wireless LAN Discovery via ZigBee Interference Signatures

Page 1: ZiFi: Wireless LAN Discovery via ZigBee Interference Signatures

ZiFi: Wireless LAN Discovery via ZigBee Interference Signatures

Ruogu Zhou1, Yongping Xiong2, Guoliang Xing1

Limin Sun2, Jian Ma3

1Michigan State University2Chinese Academy of Sciences3Nokia Research Center, China

Page 2: ZiFi: Wireless LAN Discovery via ZigBee Interference Signatures

Mobile WiFi Access

• WiFi usage from mobile users is soaring– AT&T public WiFi: 85.5 mi connections in 2009, 121.2 mi in

the first half 2010, 300% up from Q1 2009 to Q1 of 2010 [1]

• Growth of WiFi hotspots has fell behind– 5.2% up from Q1 to Q2 of 2010 [2]

• Reality of urban WiFi coverage for mobile users– Long uncovered periods, poor association success rate…– High power consumption due to active scan or listening

2[1] http://attpublicpolicy.com/wireless/the-summer%E2%80%99s-hottest-hotspot/[2] JiWire Q2 2010 Mobile Audience Insights Report

Page 3: ZiFi: Wireless LAN Discovery via ZigBee Interference Signatures

• Predict WiFi availability by contexts – Location, cellular towers, BT contact patterns– Reliance on historical info

• Discover through out-of-band signaling– Significant modification to WiFi infrastructure

WiFi Discovery: State of the Art

3WiFi Access Point (AP)WiFi Access Point (AP)

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Motivation for a New Approach• Many mobile devices have multiple interfaces

– 802.11 b/g/n and Bluetooth– ZigBee for industrial/home automation

• Most these radios share the open 2.4GHz band– Significant interference between different radios– Ex: WiFi interferes 12 of total 16 ZigBee channels

• Can we turn the curse into blessing?– Detect WiFi through unique interference signatures

on ZigBee/BT receivers4

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• Low-power radio remains active– Sample wireless signals via Received Signal Strength

(RSS) indicator– Identify unique features of WiFi RSS samples

• Activate WiFi interface if WiFi hotspot detected

Basic Idea

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BT/ZigBeeWiFi

RSS samples

TM

ZigBee sensor

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RSS Features of WiFi Signals • Signal magnitude varies significantly• Frame duration and inter-arrival gap vary with

apps and 802.11 versions• Resemblance with signals of other RF sources

6RSS samples

magnitudeduration gap

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802.11 Beacons as WiFi Signature

Beacons broadcasted at the lowest rateAll WiFi APs periodically broadcast beacons

Beacon period is unknown!– Default 102.4 ms, typically 100 – 200 ms

Low SNR: up to 103 data frames bw beacons!False positives and negativesNoise/data frames may also exhibit periodicityBeacons may become aperiodic due to contentions

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ZiFi System Architecture

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RSS Sampling & Shaping

RSS Sampling & Shaping

Common Multiple Folding

Common Multiple Folding

Beacon DetectorBeacon DetectorWiFi

Access Point

ZigBee radio WiFi radio

Mobile Device

threshold

100

amplify periodic signals

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Basic Idea of Folding• Amplify a signal of period P in N samples• Total number of additions is N-P, N>104

• Folding on many periods is expensive!

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1 2 3 4 5 6 7 1 2 3

folding

An example of folding on period 3

321

8 9

7 8 9

64 5

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Common Multiple Folding• Key observation: folding on period P can be computed from folding on Q if Q mod P = 0

• 6+9 additions for folding on 6 and 3 separately• 6+3 additions for folding on 6 first, then on 3

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folding on period 6 folding on period 3

RSS sample index

121110987

654321987

321

121110

654

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Apply This Idea Recursively • Fold on LCM of a period set and then partition• Encode the process by a tree

– A node represents a period set and its LCM– Children’ period sets are partition of parent’s set

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(2,3,….,9,10)2520

(2,3,4,6,8,9)72

(5,7,10)70

(3,6,9)18

(8,4,2)8

(7) (5,10)10

(3,6)6

(9)9

(3)

(4,2)4

(2) (5)

# of additions N-2520

2520-72 2520-70

72-18 70-772-8 70-10

18-6 18-9

6-3

8-4 8-2 10-5

Example:Folding N samples on periods (2,3,….9,10)

Opt CM tree is a binary tree!

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Challenges of Using Beacons as WiFi Signature

Beacon period is unknown!– Default 102.4 ms, typically 100 – 200 ms

Low SNR: up to 103 data frames bw beacons!False positives and negatives

Noise/data frames may also exhibit periodicityBeacons may become aperiodic due to contentions

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ZiFi System Architecture

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RSS sampling & shaping

RSS sampling & shaping

Common Multiple Folding

Common Multiple Folding

Beacon DetectorBeacon DetectorWiFi

Access Point

ZigBee radio WiFi radio

Mobile Device

beacons?

thresholdpeaks caused by data framespeaks caused by data frames

beacons?

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Modeling FP and FN Rates• Derive FP/FN rate based on 802.11 CSMA model

– FP rate = Prob. (max. folding result ≥ λ | no beacons)– FN rate = Prob. (max. folding result < λ | beacons)

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scheduled beacon TX time

actual beacon TX time X1+X2

)()Prob( 20

1

FNfXXRateFN

withinbeaconsProb)( bFN Tf

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Evaluation Test-bed• Test-bed consists of 4 802.11g APs, 4 netbooks,

2 TelosB motes, and a Nokia N73 phone.

15MiniSD based

ZigBee module MiniSD based

ZigBee module USB based

TelosB mote USB based

TelosB mote

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Evaluation Metrics

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WiFi client running traffic generator D-ITG

WiFi access point

ZiFi node

• Detection delay, accuracy, impact of ZigBee interference, computation/energy overhead

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Detection Delay• Beacon period = 98.3 ms• Error rate = FP rate + FN rate

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Avg. error rate < 5% when delay >786 msAvg. error rate < 5% when delay >786 ms

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Detection Accuracy• ZiFi-opt: choose opt threshold using FP bound• ZiFi-α: threshold fixed to α

– High α low FP rate, high FN rate– Low α low FN rate, high FP rate

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ZiFi-0.6

ZiFi-0.9

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Conclusions

• ZiFi: a novel system to detecting WiFi hotspots using ZigBee radio – A novel Digital Signal Processing algorithm to

amplifying beacons in interference signals – A beacon detector to improving stochastic

detection performance• Evaluation on Linux netbooks and a Nokia N73

smartphone

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