Project: Designing a High Capacity Wireless Mesh Network

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ECE 191 Spring 06

• Project Designing a High Capacity Wireless Mesh
  Network
• Sponsor CalIT2
• Professors Dr. Pankaj Das Dr. Clark Guest
• Mentor Dr. B. S. Manoj Prof. Ramesh Rao
• Team members
• Sanjay Gidvani sgidvani_at_ucsd.edu
• Vasilios Ikosipentarhos vikosipe_at_ucsd.edu
• Wo Chio Lao c0lao_at_ucsd.edu
• Vishal Sidhpura vsidhpur_at_ucsd.edu
• Michael Tsui mitsui_at_ucsd.edu

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Agenda

• Gantt Chart
• Tasks performed by members
• Technical content on queuing theory, CSMA/CA,
  802.11 DCF, GloMoSim
• Summary
• Objectives for Next Week

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(No Transcript)
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Tasks

• Queing Theory Vasilios Ikosipentarhos
• CSMA/CA Michael Tsui
• 802.11 DCF - Wo Chio Lao
• Glomosim Testing Vishal Sidhpura
• - Sanjay Gidvani

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Queueing Theory

• A network of m interconnected queues is known as
  a Jackson Network if it meets the following
  conditions
• 1. Customers arriving from outside the system
  arrive as a Poisson process.
• 2. The servers each act as a Poisson process
  (exponentially distributed service times).
• 3. A customer leaving queue i will either move
  to some new queue j with probability Pij or leave
  the system with probability
• These probabilities are independent and
  identically distributed.
• 4. The utilization of all of the queues is
  less than one.

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Jacksons Theorem

• In such a network, Jackson's Theorem applies and
  the distribution of customers in each queue when
  the system is in equilibrium is exactly the
  distribution of an Queueing theory with the same
  utilization.
• The Theorem

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Process of Carrier Sense Multiple Access with
Collision Avoidance (CSMA/CA)

• Physical channel sensing
• If channel is free, begin transmission
• If channel is busy
• Each node selects a random back off number
  0,cw-1 and waits that number of slots before
  monitoring the channel
• If channel stays idle and reaches zero, begin
  transmission
• If channel becomes active, wait until
  transmission is over then start counting again

contention window (randomized back-offmechanism)
DIFS
DIFS
medium busy
next frame
t
direct access if medium is free ? DIFS
slot time
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Process of CSMA/CA

• If channel is free
• Sender sends RTS that declares the amount time
  needed for transmission
• Receiver acknowledges request with CTS
• Sender sends data and Receiver acknowledges the
  transmission once completed
• Other stations store medium reservations
  announced by RTS and CTS and waits for contention

DIFS
data
RTS
sender
SIFS
SIFS
SIFS
ACK
CTS
receiver
DIFS
NAV (RTS)
data
other stations
NAV (CTS)
t
defer access
contention
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802.11 Distributed Coordination Function

• The fundamental access mechanism in 802.11 MAC
• Based on the CSMA/CA protocol with binary
  exponential backoff.
• Issues
• The presence of RF interference caused the
  throughput of the network to drop significantly
• Hidden terminals resulting in more collisions
  and wasted channels
• Binary exponential backoff algorithm throughput
  performance strongly depends on the initial
  backoff window size and the number of terminals
  in the network

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Solutions to Improve the Performance of 802.11 DCF

• Maximize the use of directional antennas to
  minimize the reception of RF signals.
• Implement RTS/CTS to minimize collisions among
  hidden stations. (only if hidden channel are
  present)
• Enhanced Backoff Algorithms of 802.11 DCF
• Dynamic Contention Window size resetting scheme
• CW min2CW, CWmax1 on a collision
• CW maxCW/2, CWmin on a success
• Exponential Increase Exponential Decrease Backoff
  Algorithm
• CW minrICW, CWmax on a collison
• CW maxCW-rD, CWmin on a success

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GloMoSim(Global Mobile Information Systems
Simulation Library)

• Build a simple string topology
• Requirements
• 7 nodes
• 350m apart
• Transmission Range gt Inter-node Distance

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GloMoSim-Parameters(Global Mobile Information
Systems Simulation Library)

• Changes made to existing code
• Terrain Dimensions
• ( 3000 m, 3000 m)
• Number of Nodes
• 7 nodes
• Node Placement
• Specified by points on a grid
• Radio Transmission Power
• 16 dBm
• Radio Bandwidth
• 500,000 bits/second

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GloMoSim-Results(Global Mobile Information
Systems Simulation Library)
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Summary

• Queuing Theory
• Process of CSMA/CA
• Issues with and Improvement of 802.11 DCF
• Glomosim Testing

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Objectives for Next Week

• Continue Phase II-Experiment
• Experiment with different topologies
• Find solutions to keep throughput consistent
  through additional nodes