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
1
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 )
Smart electricity meter (Elster) Industrial sensor networks
2
(Intel fabrication plant)
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/
3
Empirical Study of Coexistence
WiFi interferer:
802.11g
• Change WiFi node location
• Measure ZigBee sending rate
• WiFi interference on sender
Interference
link
Data link
• Measure ZigBee packet delivery ratio
• WiFi interference on receiver
WiFi Interferer Position
ZigBee sender and recver
TelosB with CC2420
4
WiFi Hidden Terminals
• Don’t trigger backoff at ZigBee
sender
• Corrupt packets at ZigBee
receiver
5
WiFi Interferer Position
WiFi Exposed Terminals
• Defer ZigBee sender’s
transmissions
• Not strong enough to corrupt
ZigBee packets
6
WiFi Interferer Position
WiFi Blind Terminals
• Interfere both ZigBee sender
and receivers
• Severe packet loss on ZigBee
link
• WiFi sending rate not affected
7
Why Blind Terminals ?
• Power asymmetry
• Heterogeneous PHY layers
ZigBee
tx range
ZigBee sender
– WiFi only senses demodulatable signals
ZigBee recver
– Energy-based sensing?
WiFi interferer
WiFi tx range
8
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
9
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
10
Modeling WiFi White Space
• Length of white space follows iid Pareto distri.
α = 1ms
shorter intervals are
not usable for ZigBee
• Implementation
• Collect white space samples in a moving time window
• Generate model by Maximum Likelihood Estimation
11
Pareto Model: Goodness of Fit
OSDI ’06 traces
SigCOMM’08 traces
Pareto model is accurate when modeling window < 100ms
Sampling frequency is about 200Hz  20 samples are enough!12
Outline
• Motivation
• Blind Terminal Problem
• WiFi White Space Modeling
• WISE: WhIte Space-aware framE adaptation
• Experimental Results
13
Basic Idea of WISE
• Sender splits ZigBee frame into sub-frames
• Fill the white space with sub-frames
• Receiver assembles sub-frames into frame
WiFi frame cluster
ZigBee sub-frames
ZigBee
Time
sampling window
ZigBee frame
pending
Frame Adaptation
• Collision probability
Sub-Frame
size
White space
age
• Sub-frame size optimization
ZigBee data rate
250Kbps
Collision
Threshold
Maximum ZigBee
frame size
15
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
16
Frame Delivery Ratio
Broadcast
Unicast with 3 retx
17
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
18
Throughput Overhead
19
Throughput
WiFi Interference Summary
Hidden terminal
The WiFi node is located within the
interferenceDesign
range offlaw
ZigBee receiver,
but outside the CCA range of ZigBee
of CSMA
sender.
Exposed terminal
The WiFi node is located within the CCA
range of ZigBee sender, but outside the
interference range of ZigBee receiver.
CSMA supposed to work.
Why blind terminals?
Blind terminal
The WiFi node is located within both the
CCA range of ZigBee sender and the
interference range of ZigBee receiver.
21
Self-Similarity of
WiFi Frame Clusters
• Arrival process of frame cluster is self-similar
• Variance is similar at different time scales
22
WISE Protocol Design
• Original ZigBee frame
PHY Hdr
MAC Hdr
Payload
CRC
• Sub-frame layout
• WISE treat each MAC layer frame as a session
• MAC protocol independent
PHY Hdr
MAC Hdr
ID
PHY Hdr
ID
Payload
PHY Hdr
ID
Payload
CRC
• Protocol overhead?
• Small sub-frames have low collision probability
• Large sub-frames are transmission efficient
23
Frame Adaptation
• Optimal sub-frame size
Average white
space lifetime
λ and ρ are measured on-line
24
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
25
Effect of Sampling Frequency
26
CSMA is NOT White Space Aware
Collisions
CCA
ZigBee
Transmission
WiFi
channel
trace
Time
27
ZigBee Link Performance Analysis
• What’s the prob. of colliding w/ WiFi packets?
• Analytical collision probability model
– ZigBee carrier sensing model
– White space model
Why Blind Terminals ?
• Heterogeneous PHY layer
– 802.11 backoff algorithm
Choose random
waiting time T
between [1, CW]
Data ready
Carrier
Sense
802.11
modulated
packet
detected
No 802.11
modulated
packet
in channel
No
Count
down T
T=0?
Yes
Increase T
by the packet
duration
ZigBee In-friendly
Send
29