Beyond Co-existence: Exploiting WiFi White Space for ZigBee Performance Assurance
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Transcript of Beyond Co-existence: Exploiting WiFi White Space for ZigBee Performance Assurance
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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
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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
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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
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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
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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