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Beyond Co-existence: Exploiting WiFi White Space

for ZigBee Performance Assurance

Jun Huang 1, Guoliang Xing 1, Gang Zhou 2, Ruogu Zhou 1  

1 Michigan State University, 2 College of William and Mary

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ZigBee Networks

• Low communication power (10~50 mw)• Application domains

– Smart energy, healthcare IT, Industrial/home automation, remote controls, game consoles….

– Ex: 10 million smart meters installed in the US by 2010

Smart thermostat (HAI ) Industrial sensor networks(Intel fabrication plant)

Smart electricity meter (Elster)

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Challenge & State of the Art

• Interference in open radio spectrum– Numerous devices in 2.4 GHz band: WiFi, bluetooth…– AT&T public WiFi usage: 300% up Q1/09~Q1/10 [1]

• Multi-channel assignment– WiFi interferes with 12 of total 16 ZigBee channels

• Co-existence on same/overlapping channels– Carrier sense multiple access (CSMA)

[1] http://attpublicpolicy.com/wireless/the-summer%E2%80%99s-hottest-hotspot/

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Empirical Study of Coexistence

• Change WiFi node location

• Measure ZigBee sending rate• WiFi interference on sender

• Measure ZigBee packet delivery ratio• WiFi interference on receiver

WiFi interferer:802.11g 

ZigBee sender and recverTelosB with CC2420

Interferencelink

Data link

WiFi Interferer Position

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WiFi Hidden Terminals

• Don’t trigger backoff at ZigBee sender

• Corrupt packets at ZigBee receiver

WiFi Interferer Position

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WiFi Exposed Terminals

• Defer ZigBee sender’s transmissions

• Not strong enough to corrupt ZigBee packets

WiFi Interferer Position

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WiFi Blind Terminals

• Interfere both ZigBee sender and receivers

• Severe packet loss on ZigBee link

• WiFi sending rate not affected

Why Blind Terminals ?

• Power asymmetry

• Heterogeneous PHY layers

– WiFi only senses de-modulatable signals

– Energy-based sensing?

ZigBee sender

ZigBee recver

WiFi interferer

WiFi tx range

ZigBee tx range

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White Space in Real-life WiFi Traffic• Large amount of channel idle time

• WiFi frames are clustered white space: cluster gaps  that can be utilized by ZigBee

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Self-Similarity of Cluster Arrivals• Variance is similar at different time scales

• Rigorously tested via rescaled range statistics and periodogram-based analysis

# clusters/5s

# clusters/s

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Modeling WiFi White Space• Length of white space follows iid Pareto distri.

• Implementation• Collect white space samples in a moving time window• Generate model by Maximum Likelihood Estimation

α = 1ms shorter intervals are not usable for ZigBee

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Pareto Model: Goodness of Fit

Pareto model is accurate when modeling window < 100ms

OSDI ’06 traces SigCOMM’08 traces

Sampling frequency is about 200Hz 20 samples are enough!

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Outline

• Motivation

• Blind Terminal Problem

• WiFi White Space Modeling

• WISE: WhIte Space-aware framE adaptation

• Experimental Results

Basic Idea of WISE• Sender splits ZigBee frame into sub-frames• Fill the white space with sub-frames• Receiver assembles sub-frames into frame

ZigBee

Time

WiFi frame cluster ZigBee sub-frames

ZigBee frame pending

sampling window

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Frame Adaptation

• Collision probability

• Sub-frame size optimizationCollision 

Threshold

Maximum ZigBee frame size

ZigBee data rate250Kbps

Sub-Frame size

White space age

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Experiment Setting• ZigBee configuration

• TelosB with ZigBee-compliant CC2420 radios• Good link performance without WiFi interference

• WiFi configuration• 802.11g netbooks with Atheros AR9285 chipset

• D-ITG for realistic traffic generation

• Baseline protocols• B-MAC and Opportunistic transmission (OppTx)

• Evaluation metrics• Modeling accuracy, sampling frequency, delivery ratio,

throughput, overhead

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Frame Delivery Ratio

Broadcast Unicast with 3 retx

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Conclusions

• Empirical study of WiFi and ZigBee coexistence• Blind terminal problem

• WiFi white space modeling

• Rigorous statistic analysis on real WiFi traffic

• WISE: White space aware frame adaptation• Implemented in TinyOS 2.x on TelosB • Significant performance gains over B-MAC and OppTx

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Throughput Overhead

Throughput

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WiFi Interference Summary

Hidden terminalThe WiFi node is located within the interference range of ZigBee receiver, but outside the CCA range of ZigBee sender.

Exposed terminalThe WiFi node is located within the CCA range of ZigBee sender, but outside the interference range of ZigBee receiver.

Blind terminalThe WiFi node is located within both the CCA range of ZigBee sender and the interference range of ZigBee receiver.

Design flaw of CSMA

CSMA supposed to work.Why blind terminals?

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Self-Similarity of WiFi Frame Clusters

• Arrival process of frame cluster is self-similar

• Variance is similar at different time scales

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WISE Protocol Design• Original ZigBee frame

• Sub-frame layout• WISE treat each MAC layer frame as a session• MAC protocol independent

• Protocol overhead?• Small sub-frames have low collision probability• Large sub-frames are transmission efficient

PayloadPHY Hdr MAC Hdr CRC

PayloadPHY Hdr MAC Hdr ID PHY Hdr ID PayloadPHY Hdr ID CRC

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Frame Adaptation

• Optimal sub-frame size

λ and ρ are measured on-line

Average white space lifetime

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Measure the White Space Model

• WiFi white space sampling• Sampling the interrupt on CCA pin of CC2420:

sampling frequency 4K~8KHz

• Record white space sample if • Signal cannot be decoded • Interval between signals is longer than 1ms

• Impact of ZigBee interference

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Effect of Sampling Frequency

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CSMA is NOT White Space Aware

TransmissionZigBee

WiFi channel 

trace

CCACollisions

Time

ZigBee Link Performance Analysis

• What’s the prob. of colliding w/ WiFi packets?• Analytical collision probability model

– ZigBee carrier sensing model– White space model

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Why Blind Terminals ?

• Heterogeneous PHY layer

– 802.11 backoff algorithm

Send

Choose random waiting time T

between [1, CW]

Count down T T=0?

Carrier Sense

Increase T by the packet

duration

No 802.11 modulated

packet in channel

802.11 modulated

packet detectedData ready

No

Yes

ZigBee In-friendly