A Wireless Mesh Network Based on Multichannel Ad Hoc Connections

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A Wireless Mesh Network Based on Multichannel Ad
Hoc Connections

• Jingyi He Gary Chan
• HKUST
• Oct. 22, 2003

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Outline

• Introduction
• System Architecture
• Channel Requirement
• Illustrative System Performance
• Conclusions

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Current Wireless LAN Deployment

• Attached to the wired network
• Operates in the IEEE 802.11 Infrastructure Mode

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IEEE 802.11 Operational Modes

• Infrastructure Mode (WLAN)
• Ad Hoc Mode (MANET)

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Limitations of Current WLAN Deployment

• Depending on the existence of the wired
  infrastructure
• Limited coverage only one hop away from the
  wired network ( 250m)

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To Extend WLAN Coverage Wireless Mesh Network
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Next

• Introduction
• System Architecture
• Channel Requirement
• Illustrative System Performance
• Conclusions

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Major Building Blocks

• The Air Interface
• PHY layer
• MAC layer
• The Routing Strategy
• (the network layer)

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PHY Layer - Multichannel Operation

• All the WLANs can use the same channel with good
  planning
• Communications between APs use different channels
  than that used by the WLANs ? at least two
  wireless interfaces are needed at each AP
• Multiple channels can be used in the mesh network

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Transmission Range (TR) and Interference Range
(IR)

• TR the maximum distance between two mobile nodes
  which can communicate with each other.
• IR the maximum distance between two mobile nodes
  which disable each other from being active
  (transmitting or receiving) simultaneously.

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TR and IR contd

• Typical values
• TR 250m
• IR 550m
• For mesh communication larger values

TR
IR
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Power Control

• Since the APs are stationary, power control is
  easier than in ad hoc networks
• Power control can be used to control the
  connectivity of the mesh network
• Power control can be used to reduce the
  interferences and hence channel requirement for a
  given connectivity

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MAC Layer - Options

• IEEE 802.11
• CSMA/CA
• IEEE 802.16a Mesh Mode
• Scheduling Based (No collision)
• Other names of IEEE 802.16
• WirelessMAN
• Broadband Wireless Access (BWA) system

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Our choice 802.11

• Under the same framework of 802.11, adding the ad
  hoc functionality to the APs would require minor
  modifications
• 802.11 AP has been a mature market product with
  wide-spread deployment, while 802.16 hasnt seen
  the same commercial success.

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Network Layer - Routing in the Wireless Mesh
Network

• Fixed routing is not desirable as channel
  conditions could vary drastically even between
  two fixed points (as observed from MIT Roofnet
  testbed)
• Ad hoc routing protocols can be used, with
  reduced control overhead (e.g., less frequent
  routing information exchange)

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Modifications needed for Multichannel Operation

• A wireless interface field should be added to
  each routing table entry
• Broadcast packets (e.g., routing information
  updates) should be sent to all interfaces
• A mechanism which handles multiple channels
  between two APs is needed

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Next

• Introduction
• System Architecture
• Channel Requirement
• Illustrative System Performance
• Conclusions

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Connectivity Patterns of WMesh

• All links active (ALA)
• Spanning tree (ST)
• Spanning tree with power control (ST-Pctrl)

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All Links Active (ALA)
8 channels
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Spanning Tree (ST)
5 channels
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Spanning Tree with Power Control (ST-Pctrl)
4 channels
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Determining the Channel Requirement

• Given the connectivity of the wireless mesh
  network
• Approaches
• Step 1 Graph tranformation
• Step 2 Graph coloring

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4
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5
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3
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Topologies Considered

• Random

• Grid

a
a
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Number of APs Needed

• To set up a connected mesh of APs for the square
  region of width a (m)

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Number of Channels Needed - Random
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Number of Channels Needed - Grid
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Next

• Introduction
• System Architecture
• Channel Requirement
• Illustrative System Performance
• Conclusions

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Performance Metrics

• Throughput
• Overall
• Per-channel
• Fairness among the connections
• Jains Fairness Index
• f ? (0, 1, with f 1 meaning perfect fairness

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Simulation Setup

• Simulator ns-2 (2.1b9a)
• Mesh topology
• 16 APs in grid manner with a1250m
• ST-Pctrl connectivity

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Simulation Setup contd

• Traffic pattern
• Skewed a small number of APs (attached to the
  infrastructure) are the destinations of the
  connections from other infrastructureless APs
• Routing Protocol
• Modified AODV

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Single infrastructure AP
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9
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Throughput UDP traffic load

• Per-channel

• Overall

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Packet loss rate traffic load
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System throughput packet loss rate
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Fairness index traffic load
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Unfairness at high loads
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Multiple infrastructure APs

• 1 channel

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System throughput of Infra-APs
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Fairness index of Infra-APs
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Multiple infrastructure APs

• 9 channels

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System throughput of Infra-APs
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Fairness index of Infra-APs
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Next

• Introduction
• System Architecture
• Channel Requirement
• Illustrative System Performance
• Conclusions

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Conclusions

• A framework of the proposed multichannel wireless
  mesh network has been presented
• Channel requirement in such networks has been
  studied. Results show that a sparse connectivity
  can reduce the channel requirement
• System performance has been studied. Results show
  that multichannel network achieves higher
  throughput and better fairness than
  single-channel ad hoc network. However, with
  sparse connectivity, the channel efficiency is
  not as high.

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Future Work

• Better connectivity and channel assignment, to
  improve the channel efficiency
• Infrastructure node placement