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

Outline

Introduction Network Model and Assumptions Main Design Simulation Conclusions

Introduction

Low Duty Cycle wireless sensor networks • Long Network Lifetime

t

…

Active State

Dormant State

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

Introduction

Motivation• Existing solutions are not suitable to be directly applied

to low-duty-cycle wireless sensor networks.

• Unreliable wireless links

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

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.

Main Idea Energy-Optimal Tree

• No redundant transmissions• Long flooding delay

Main Idea

Adding opportunistically early links into the energy-optimal routing tree

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)

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)

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

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

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

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

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

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.

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

Simulation

Simulation

Simulation

Simulation

Simulation

Simulation

Simulation

Implementation and Evaluation

Test-bed Implementation• 30 MicaZ nodes form a 4-hop network

• Randomly generated working schedules

• Duty cycle from 1% to 5%

Implementation and Evaluation

Flooding delay vs. Duty Cycle Energy Cost vs. Duty Cycle

Implementation and Evaluation

Ratio of Opportunistically Early Packets

Hop Count 1 Hop Count 2 Hop Count 3 Hop Count 4

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.

Thanks~~