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Opportunistic Flooding in Low-Duty-Cycle Wireless Sensor Networks with
Unreliable Links
Shuo Goo, Yu Gu, Bo Jiang and Tian He
University of Minnesota, Twin Cities
ACM MobiCom 2009
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Outline
Introduction Network Model and Assumptions Main Design Simulation Conclusions
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Introduction
Low Duty Cycle wireless sensor networks • Long Network Lifetime
t
…
Active State
Dormant State
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Introduction
Flooding in low-duty-cycle WSNs. • No longer consists of a number of broadcasts. • Instead, it consists a number of unicasts.
Active StateDormant State
B
CD
A
BCD
At
B C D
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Introduction
Motivation• Existing solutions are not suitable to be directly applied
to low-duty-cycle wireless sensor networks.
• Unreliable wireless links
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Introduction
Design Goal• Fast data dissemination: shorter flooding delay• Less transmission redundancy: less energy cost
Three challenging issues
C
B
A
Unreliable links Redundant transmissions Collisions
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Network Model and Assumptions
Time synchronization of all sensor nodes. Pre-determined working schedules shared with all
neighbors. Unreliable wireless links
• The probability of a successful transmission depends on the link quality q.
Flooding packets are only forwarded to a node with larger hop-count to avoid flooding loops.
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Main Idea Energy-Optimal Tree
• No redundant transmissions• Long flooding delay
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Main Idea
Adding opportunistically early links into the energy-optimal routing tree
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Delay Distribution Computation
...
...
0.1 0.9 0.1 0.1 0.9
0.9 0.8
0.9 0.2 0.8 0.09 0.8
0.9
0.8
Probability mass function (pmf)
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Decision Making Process
1 1 1 1 1Time
B’s pmf0.5
0.30.1
0.04
168 24 32
Early Packets Late Packets
Early packets are forwarded to reduce delay Late packets are not forwarded to reduce energy cost
For p = 0.8
Dp = 16A B
0.5
p-quantile
DpA receives packet
Expected Packet Delay (EPD)
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Expected Packet Delay Computation
1 1 1 1 1Time
B’s pmf0.5
0.30.1
0.04
Dp= 16
168 24 32
A is expected to transmit twice!
A receives packet
A’s first try to B
A’s second try to B
EPD = 24
EDP = 16x0.5 + 24x0.5x0.5 + 32x0.5x0.52 +… …
A B0.5
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Decision Making Process
1 1 1 1 1Time
B’s pmf0.5
0.30.1
0.04
Dp = 16< EPD = 24.
A will not start the transmission to B!
168 24 32
Dp= 16EPD = 24
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p-quantile
DpEarly Packets’ EPD Late Packets’ EPD
t
Small p value: smaller Dp, fewer early packets, longer flooding delay, less energy cost => Energy-Sensitive
Large p value: larger Dp, more early packets, shorter flooding delay, more energy cost => Time-Sensitive
Decision Making Process
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Decision Conflict Resolution The selection of flooding senders
• Only a subset of neighbors are considered as a node’s flooding packet senders.
• Flooding senders have a good enough link quality between each other.
lth
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Decision Conflict Resolution
Link-quality based back-off scheme• Better link quality, higher chance to send first• Further avoids collision when two nodes can hear each other and
make the same decision• Further saves energy since the node with the best link quality has the
highest chance to send
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Simulation
Simulation Setup• Randomly generated network, 200~1000 nodes
• Randomly generated working schedules
• Duty cycle from 1%~20%
• 300m × 300m field
• The simulation results are based on 10 network topologies and 1000 flooding packets for each topology.
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Simulation
Baseline 1: optimal performance bounds• Delay optimal: collision-free pure flooding
• Energy optimal: tree-based solution
Baseline 2: improved traditional flooding• Two techniques are added to avoid collisions:
Link-quality based back-off scheme p-persistent back-off scheme
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Simulation
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Simulation
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Simulation
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Simulation
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Simulation
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Simulation
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Simulation
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Implementation and Evaluation
Test-bed Implementation• 30 MicaZ nodes form a 4-hop network
• Randomly generated working schedules
• Duty cycle from 1% to 5%
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Implementation and Evaluation
Flooding delay vs. Duty Cycle Energy Cost vs. Duty Cycle
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Implementation and Evaluation
Ratio of Opportunistically Early Packets
Hop Count 1 Hop Count 2 Hop Count 3 Hop Count 4
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Conclusions
The flooding process in low-duty-cycle networks consists of a number of unicasts. This feature calls for a new solution.
Opportunistically early packets are forwarded outside the energy-optimal tree to reduce the flooding delay.
Late packets are not forwarded to reduce energy cost.
Evaluation reveals this approaches both energy- and delay-optimal bounds.
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Thanks~~