STEM: Topology Management for Energy Efficient Sensor Networks

1
STEM Topology Management for Energy Efficient
Sensor Networks

• Curt Schurgers, Vlasios Tsiatsis, Mani Srivastava
• Networked and Embedded Systems Lab (NESL)
• http//nesl.ee.ucla.edu

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Wireless Sensor Network

• Sensor network consists of autonomous sensor
  nodes
• Functionality
• Detect events
• Relay information to the user
• Applications monitoring of wildlife, intruders,
  machine conditions, earthquakes, fire,
  contaminants, office environment, participants on
  reality shows, babys diaper conditions, etc.

event
sensor network
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Sensor Node
In-node processing
Wireless communication with neighboring nodes
Event detection
Acoustic, seismic, magnetic, etc. interface
Electro-magnetic interface
sensors
radio
CPU
Limited battery supply
battery
Energy efficiency is the crucial design criterion
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Network Operations
2.4 Kbps RFM radio with R 20 m
Savvides

• Energy consumption of the radio dominates that of
  the sensors and CPU
• ? perform event detection continuously
• The only energy efficient mode of the radio is
  the sleep mode
• ? put radio to sleep as often as possible
• (we refer to this as the node being put to
  sleep)

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Energy Conservation Strategy

• Existing approaches keep enough nodes awake to
  handle the data forwarding (forwarding state),
  but for substantial energy savings we need large
  densities
• Eureka moment most of the time, the network is
  only monitoring its environment, waiting for an
  event to happen (monitoring state)
• New strategy put nodes to sleep and only wake
  them up when they need to participate in data
  forwarding

Nodes have their radio in sleep mode to conserve
energy
Yogy for president
Nodes turn on their radio, when they need to
communicate
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Wakeup Paradox

• Nodes need to wake up when
• they detect an event or want to initiate
  communication
• they need to receive packets from other nodes ???
• Paradox how can a sleeping node be reached ?
• Solutions
• low-power paging channel
• low duty cycle paging channel

zzzzzzz
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Principle of STEM
STEM Sparse Topology and Energy Management

• Low duty cycle paging channel to wake up a
  neighboring node
• Use separate radio for the paging channel to
  avoid interference with regular data forwarding
• Trades off energy savings for setup latency

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High Level Operation of STEM
Wakeup plane
Power
f1
Tx
Time
Power
Data plane
f2
Tx /Rx
Sleep
Initiator node
Target node
Rx
Wakeup plane
Power
f1
Sleep
Time
Power
Data plane
f2
Tx /Rx
Sleep
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Detailed Operation of STEM
Initiator node
f1
B1
B2
1. beacon received
Train of beacon packets
TRx
2. beacon acknowledge
T
f1
Target node
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Collision Resolution
1 initiator node

• beacon received correctly
• only intended receiver turns on the data radio
  and sends a beacon acknowledge in the wakeup plane

more initiator nodes

• upon detection of collision, a node turns on its
  data radio
• after T, the initiator node assumes the target
  node is up and contacts it on the data plane
• when an expected target node doesnt receive
  data, it times out and goes back to sleep

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Setup Delay Analysis
?
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Analysis of STEM
Wakeup plane
f1
Data plane
f2
Forwarding state
Monitoring state
Fraction of time in the forwarding state ?

• Setup latency
• Energy savings

Appropriate choice of interval sizes
Mostly monitoring state ? ltlt 1 or ? gtgt 1
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Performance Simulation of STEM
(W)
Without STEM
T 600 ms
T 1200 ms
T 3000 ms
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Energy Latency Tradeoff of STEM
? 101
? 102
TRx 0.225 s
? 103
? 104

• The tradeoff between energy and delay is
  manipulated by varying T
• T ? ? E ? TS ?
• The energy savings increase as the monitoring
  state becomes more dominant, ? ?

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Topology Management in Forwarding State
GAF Geographic Adaptive Fidelity

• Conserve traffic forwarding capacity
• Divide network in virtual grids
• Each node in a grid is equivalent from a traffic
  forwarding perspective
• Keep 1 node awake in each grid at each time

• GAF reduces the energy by a factor M
• This factor is a function of the average number
  of nodes in a grid M

Average number of neighbors of a node
   
for uniformly random node deployment
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GAF Energy Savings
Uniformly random node distribution
P(neighbors x)
? 10
?
x
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Comparison STEM - GAF
STEM
Curve of comparable energy savings
Leverage latency
?
Leverage density
GAF
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Combining STEM and GAF

• As in GAF, 1 node is active in each grid
• ? the grid can be considered a virtual node
• This virtual node runs the STEM protocol

STEM alone
? 10
? 30
GAF alone
? 60
? 100
? 200
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Conclusions

• Most of the time, sensor networks are only
  sensing the environment, without forwarding
  traffic
• STEM trades off energy savings versus wakeup
  latency
• STEM integrates well with other topology
  management schemes and provides substantial
  additional savings