Decentralized Scattering of Wake-up Times in Wireless Sensor Networks
Amy L. MurphyITC-IRST, Trento, Italy
joint work with
Alessandro Giusti, Politecnico di Milano, ItalyGian Pietro Picco, University of Trento, Italy
A common solution to save energy on sensing devices is to periodically turn them on and off
Energy saved while turned off lead to clear gains in system lifetime
We explore duty cycling at the application level, cycling communication and/or sensing– Spreading out awake times, NOT synchronizing them
Results in lifetime increase, but at a cost…
Energy Management with Duty Cycling
times1
epochawake interval
wakeup time
Cost of Duty Cycling: response time
QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
Random Wakeup Scattered Wakeup
4 transmissionsrequired
2 transmissionsrequired
Scenario– Distributed nodes– Mobile base station– BS queries nearby nodes– Queries repeat until a node
responds– Communication duty cycled:
respond only when turned on
Scenario– Distributed sensing nodes– Sensors detect events within a
given radius, only when sensors are active
– Duty cycle the sensing capability
– Note: long wakeup intervals
Cost of Duty Cycling: event coverage
QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
AC
B
Random Wakeup
AC
B
A
Scattered Wakeup
A C B
“Less” likely todetect events
“More” likely todetect events
Wake-up Scattering: distributed protocol
All nodes discover– Their wake up time, WC
– Wake up time of the node before them, Wprev
– Wake up time of the node after them, Wnext
Calculate their new target wake up time– Wc’=(Wprev+Wnext)/2 * – Move toward this new
wakeup time in the next epoch
Key Properties– Process is entirely localized– All nodes execute in parallel– No central coordination
ABC
WC
WnextWprev
WC’
0.00
0.05
0.10
0.15
0.20
0.25
init 1 2 3 4 5 10 20 50Iteration Number
Average Response Delay (fraction of E)
Rapid convergence to goodscattering (2-3 rounds) Increasing
awake intervals
A=0.50
A=0.01
Scattering to reduce response delayA
vera
ge R
espo
nse
Del
ay (
frac
tion
of E
)
Scattering Iteration Numberinit
A=0.15
A=0.10
Same responsedelay
No scattering.Long awake time.
After scattering.33% shorter awake time.
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
init 1 2 3 4 5 10 20 50Iteration Number
Average Response Delay (fraction of E)
Effectiveness of scattering for response delay: Different network densities
Ave
rage
Res
pons
e D
elay
(fr
actio
n of
E)
Scattering Iteration Numberinit
Range=100, 8.4 neighbors
Range=110, 10.1 neighborsSame response
delay
No scattering.Large radio range.
After scattering.10% smaller radio range.
0.4
0.5
0.6
0.7
0.8
0.9
1
init 1 2 3 4 5 10 20 50Iteration Number
Sensing Coverage
Scattering to increase sensing coverage
Goal: increase percentage of events detected by scattering the awake intervals of the sensors themselves
Results are similar to those for response delay– Details in the paper
After scattering,same coverage.
20% smaller awake interval.
Visualization
http://www.elet.polimi.it/upload/giusti/scattering
Sensing radius
Node ON
Node OFF
Overlappingsensing
Node
Pairwisecommunication
Scattering & Latency in Tree-based data collection on WSN
Many WSN are used to collect data at a central location by constructing an overlay tree along which data flows
Goal: low latency for data from source to sink
In terms of wake-up times, the parent should wake up after the child to receive its data
C
X
A B
Scattering and Tree Formation
Consider a simple tree, and the wake-up times from the perspective of X– Well scattered, but X needs to send data to C
– Would be better if C wakes up immediately after X, not A
Scattering never changes the sequence of wakeup times– We introduce jumping, a simple mechanism
that allows reordering in the sequence of wakeup times
– After jumping, additional scattering is required
Jumping enabled if the Wnext is a child
– With some probability, select next wakeup time between Wnext and Wnextnext
A
XB
C
C
X
A B
0
1
2
3
4
5
6
init scattering jumping
Time To Root
A=0.01 A=0.05 A=0.1 A=0.15
Reducing Latency with Jumping
Time to root not significantly affected by scattering alone
Reducing gap betweenwake-ups reduces latency.
However….
Initial
Scattering
Jumping
4 different awake intervals
Jumping to place parentsafter children results in significant improvement
Tim
e to
Roo
t(l
ow
er is
bet
ter)
Waving: reduce gap between wakeup times
Tradeoff between “tree” and “coverage”
Reducing space betweenwake-ups reduces latency.
However….
…benefits of scattering reduced.Result: lower event coverage
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
init scattering jumping
0
1
2
3
4
5
6
init scattering jumping
Tim
e to
Roo
t(l
ow
er is
be
tter
)P
erce
nt C
over
age
(hig
he
r is
be
tter
)
Visualization: Tree
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
http://www.elet.polimi.it/upload/giusti/scattering
Discussion
Combining wakeup scattering for coverage and jumping to achieve good, tree-based data collection yields a promising complete solution
Wakeup Scattering is fully decentralized– Wake up times are determined based on local information– Epochs need not be synchronized across nodes
Simple algorithm yields significant results– Response delay: same as random wakeup times with 33% longer
awake interval– Event coverage: same as random with 20% longer awake interval– Tree: scattering + jumping improve over random from 25 to 45%
http://www.elet.polimi.it/upload/giusti/scattering
Future Directions
Modify the awake interval to meet the application needs, e.g., increased coverage
Exploit signal strength to approximate distance between sensors– Close sensors should have “more scattered” awake
times
Combine jumping to improve solutions for response delay and coverage– Avoid local minima in scattering solution
Consider applying scattering at the MAC layer
http://www.elet.polimi.it/upload/giusti/scattering