Presenter: Kevin Wine WINLAB, Wireless Information Networks Laboratory, Rutgers University

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• Presenter Kevin Wine WINLAB, Wireless
  Information Networks Laboratory, Rutgers
  University
• PI Badri Nath
• Site Visit Briefing
• August 2001
• http//www.cs.rutgers.edu/dataman/webdust
• badri_at_cs.rutgers.edu
• Co-PIs Tomasz Imielinski, Rich Martin

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Mobility in Sensor Networks - Motivation

• We are interested in mobility in sensor networks
  because
• Users interested in sensor data will not always
  be stationary
• Mobile users will need to inject a sensor query
  into the sensor network while they are mobile.
• Mobile users will need to retrieve a sensor
  response from the sensor network while they are
  mobile.

Vehicle travel path
Query submitted to sensor network
Response retrieved from sensor network
Sensor Field
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Mobility in Sensor Networks Mobile Channel

• Mobility does bad things to the channel

Path Loss
As much as 50db!!
On the order of milliseconds
Time
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Mobility in Sensor Networks Mobile Channel

• Time between dips is a function of velocity and
  carrier frequency

(60 MPH 5280 ft/mile)
88 feet/sec
Mobile Velocity
(60 min 60 seconds)
(186,000 MPH 5280 ft/mile)
1.09 feet
Wavelength
(900MHz carrier)
Rule of Thumb One dip for every one half
wavelength of travel. So that would be a dip for
every 6 inches of travel.
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5.7ms
Time between dips
(88 ft/sec 2 dips/ft)
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Mobility in Sensor Networks Packet Sizing

• Packets should fit between the dips
• Large packets that cover one or more dips
  probably wont get through!
• General idea the smaller the packet the better.

These packets will probably get through.
5.7 ms (900MHz, 60MPH)
This packet will probably NOT get through
unless youre very close.
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Mobility in Sensor Networks Mobile Channel

• The previous mobile channel plot I showed you is
  a best case example controlled environment
  line of site.
• Add stationary obstacles (trees, buildings) and
  obstacles in motion (cars, trucks, people) and
  the situation gets much worse!!
• Result widely varying channel conditions that
  change quickly from good to bad.

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Mobility in Sensor Networks Mobile Channel
Truck in the way
Behind a building
Path Loss
Time

• Further measures are usually required in a mobile
  environment to achieve same throughput as in a
  fixed environment.
• We want to get as much throughput as possible
  given this channel and power-constrained radios.

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Mobility in Sensor Networks Adaptive Redundancy

• Transmit the same packet multiple times to
  increase chances of reception.
• Separate the multiple packets in time to overcome
  extended channel outages (time diversity).
• Adapt the number of multiple packets and their
  time separation to the current channel conditions
• Good channel Fewer duplicates, closer in time
• Bad channel More duplicates, further in time

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Mobility in Sensor Networks Adaptive
Retransmission

• Simple re-transmission use acknowledgements to
  cause lost packets to be retransmitted.
• Adaptive re-transmission - Measure the channel
  and retransmit lost packets depending on channel
  conditions
• Good channel retransmit packet once
• Bad channel retransmit packet several times
• Should be implemented at the link layer for best
  possible channel condition tracking.

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Mobility in Sensor Networks Adaptive Data Rate
Switching

• Lower data rates are more reliable, but less
  throughput
• Higher data rates give more throughput, less
  reliability
• Adjust the link data rate depending on channel
  conditions
• Good channel use a higher data rate
• Bad channel use a lower data rate
• Again, should be implemented at the link layer
  for best possible channel condition tracking.

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Mobility in Sensor Networks Summary

• The trick is to find the optimal combination of
• Packet size
• small packets cause more overhead but more get
  through
• Packet redundency and retransmission
• How many copies to send
• When to give up
• How much coding to apply
• Data rates and switching thresholds
• At what link quality do you change rates
• What data rates do you use

Retransmission
Solution
Packet Size
Parameter Space
Coding and Redundency
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Mobility in Sensor Networks Adding Mobility
Using a Sensor Network Gateway
Gateway acts as bridge between two networks
Other wireless network ie. Wireless LAN
Gateway and Sensor Network Bridge
Sensor Network
Mobile network System
Video Camera
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Mobility in Sensor Networks Magnetic Field
Acceleration Sensors

• We are in the process of building magnetic field
  and acceleration sensors for the mote platform.

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Status of Work / Plan

• Task1 Evaluated impact of mobility on sensor
  nodes (motes) (Completed July 01)
• Task2 Design, evaluate, implement schemes for
• Supporting mobility in sensor networks
• Size, coding, rate adaptive schemes (Year 2)
• Task3 Implement schemes to support
• mobility on sensoria nodes. Experiment, evaluate
  choices
• For mobility support in sensor networks.
  Scalability issues (Year 3)

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Information

• http//www.cs.rutgers.edu/dataman/webdust
• E-mail kevinw_at_winlab.rutgers.edu