Data Link Layer Architecture for Wireless Sensor Networks

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Data Link Layer Architecture for Wireless Sensor
Networks

• Charlie Zhong
• September 28, 2001

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Outline

• Background
• Proposed solution
• Future work

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I. Background

• Differentiation
• Sensor network requirements
• Data link layer functions and requirements
• Challenges
• Existing work

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How is wireless sensor network different?

• Energy consumption control
• Disaster mitigation
• Traffic management
• Not much Qos, mobility, data rate
• but easy maintenance, long operation time,
  minimal human involvement

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Sensor Network Requirements

• Desired performance (e.g. how good the
  environment control is)
• Easy setup
• Simple maintenance/diagnostics
• Low cost
• Security
• Scalability, size etc.

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Data Link Layer
Application

• Design requirements
• Supports required functions
• Communications with required reliability
• Location as part of information
• Power-efficient
• Distributed
• Requires no global synchronization
• Scalable, robust
• Easy setup and maintenance

• DLL functions
• transfers data between network and physical
  layers
• power control, error control, access control
• computes location
• maintains neighborhood info

Transport
Network
Data Link
Physical
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Challenges

• What to optimize
• The design of a subsystem is very dependent on
  that of another
• How to compare two different designs
• Need a design method and analysis

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Existing Works

• 802.11 power management need global
  synchronization
• UCLA distributed scheduling
• GTE topology control
• Tu-berlin combined tuning of RF power and MAC
• Metrics proposed are not accurate, very few
  qualitative comparison, not for entire data link
  layer

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Metrics Proposed

• Power on time
• Time spent on utilizing TX and RX
• Total number of correctly transmitted packets
  during the lifetime of battery
• Signal energy per successfully transmitted bits

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What is power?

• Communications
• Value comes from information bits, not from OH
  bits, acks, handshakes to setup
• Values comes from the transfer from source to
  destination, not every hop
• Processing
• Value comes from how you use information
• Encoding/compression/redundancy

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Our Solution

• A systematic approach
• Identify the goal for optimization
• Perform functional break down
• Understand the complicated inter-dependency
  between the design of different subsystems
• Quantify design metrics, know the tradeoffs
• Compare different algorithms
• Predict the direction for improvements

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Clean the Mess
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Outline

• Background
• Proposed solution
• Future work

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Our Solution

• Optimization goal
• UML functional description
• Case study power control subsystem
• Interactions between subsystems
• Design metrics
• Comparison of two algorithms
• Direction for improvements

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Optimization Goal

• Cost local battery source
• Value desired functions
• Goal the time the desired functions can be
  maintained should be as long as possible for
  fixed energy cost

Network Life
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Network Life

• Definition the time network stays functioning
  for given power supply
• How to quantify it
• The time network stays connected (T1) max power
  consumption
• T1the time network is still functioning after
  the 1st node dies

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Functional Description
Unified Modeling Language
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VCC Implementation
Virtual Component Co-design
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System Operation
Initialization
Maintenance
Data Communication
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Case study power control

• Controls the transmit power
• Topology control for desired connectivity
• Compensate topology changes incurred by mobility
  and dead nodes
• Controls a nodes neighborhood

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Interactions with other Subsystems
Collisions
Load balancing
Connectivity
Spatial reuse
Retransmissions
Performance degradation
Battery drain
Error performance
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Design Metrics
Simplified model
BER
Connectivity
Radius r Receiver sensibility C BER p
Max of retransmissions N
modulation
Collision rate
Power Control
coding
Interference
ch available
Transmit power PT
For given d, find minimum PT

• Assumptions
• Number of neighbors, or degree d, is used to
  approximate node connectivity
• Nodes are uniformly distributed with density D
• Channel assignment ensures every interferer is
  using a different channel
• There is no interference between channels

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Target BER
p BER pkt packet error rate
M of bits/packet N max of
retransmissions Reliability prob. of packet loss
after N retransmissionspktN1 Assumptions BPSK
modulation, no coding, BSC channel Target BER
10-3 -gt packet loss rate 0.45
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Tradeoffs

• Node connectivity kgt1 is required kgt1 is
  desired to give network layer enough paths to
  balance loads with
• The higher PT, the higher the connectivity
• At higher PT, it is harder for channel assignment
  to control collisions and interference
• For given link-level reliability, there exists
  optimum BER

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Data Link Layer Tradeoffs
Reliability
Redundancy
Network
Transport
Connectivity
Traffic density
Link-level reliability
NBs
BER
NBs
interferers
Data link data rate
Collision rate
Power Control
MAC
Interference
TX power
channels
Data rate
Physical layer
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Comparison of two algorithms

• Topology control only
• Our algorithm

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Direction for improvement

• Pick target BER based on link-level reliability,
  modulation and error control coding scheme
• Jointly optimize across network and data link
  layers for longer network life

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Outline

• Background
• Proposed solution
• Future work

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Future Work

• More accurate modeling
• Simulations
• Network implementation
• Better quantification of network life