Architecture and Evaluation of an Unplanned 802.11b Mesh Network

1
Architecture and Evaluation of an Unplanned
802.11b Mesh Network

• John Bicket, Daniel Aguayo, Sanjit Biswas, and
  Robert Morris
• MIT Computer Science and Artificial Intelligence
  Lab
• Presented by Anuradha Kadam
• February 6, 2007

2
Outline

• Introduction
• Roofnet Design
• Evaluation
• Network Use
• Conclusion

3
Introduction

• Community wireless networks
• Multi-hop network with nodes in chosen locations
  and directional antennas
• Require well-coordinated groups with technical
  expertise, result in good connectivity and
  throughput
• Hot-spot access points to which clients directly
  connect
• Do not require much coordination to deploy and
  operate, not as much coverage per wired
  connection.
• Best characteristics of both network types.

4
Introduction

• Unconstrained node placement
• Omni-directional antennas
• Multi-hop routing
• Optimization of routing for throughput
• Roofnet

5
Roofnet Design

• 37 nodes spread over four square km
• Each node hosted by a volunteer
• Most buildings are 3 or 4 story

6
Hardware

• Node PC, an 802.11b card and roof-mounted
  omni-directional antenna
• PCs ethernet port provides Internet service to
  user
• 802.11b card based on Intersil Prism 2.5 chip-set
• RTS/CTS disabled, pseudo-IBSS mode

7
Software and Auto-configuration

• Each node Linux, routing software implemented in
  Click, a DHCP server, a web server
• Software is pre-installed.
• Node acts as like a cable or DSL modem
• User connects PC or laptop to the nodes ethernet
  interface
• Node automatically configures users computer via
  DHCP
• Lists itself as default IP router

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Addressing

• Roofnet carries IP packets inside its own header
  format and routing protocol
• Node chooses address whose low 24 bits are low
  24 bits of nodes Ethernet address and high 8
  bits are an unused class-A IP address.
• Same address at both the Roofnet and IP layers
• These addresses are meaningful only inside
  Roofnet
• Allocates addresses from 192.168.1.x to users
• NAT between Ethernet and Roofnet

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Gateways and Internet Access

• Each node on startup asks for an IP address as a
  DHCP client.
• If it succeeds, the node advertises itself as an
  Internet gateway.
• Gateway acts as NAT for connections from Roofnet
  to the Internet.
• Node selects gateway to which it has the best
  route metric.
• Four Internet gateways

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Routing Protocol

• Srcr - find highest throughput route between pair
  of nodes
• Omnidirectional antennas give choice of links
• Dynamic source-routing (DSR)
• Each node maintains partial database of link
  metrics
• Dijkstras algorithm

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Routing Protocol

• Link metric learning
• Node includes links current metric in packets
  source route
• DSR-style flooded query
• Overheard queries and responses

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Routing Protocol

• Combination of link-state and DSR-style on demand
  querying
• Roofnet gateway floods dummy query
• Node sends data to a gateway gateway learns
  about links back to the node
• Nodes do not need to send flooded queries

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Routing Protocol

• Flooded queries often do not follow best route
• Srcr solution compute best route from database
• Link conditions change leading to change in best
  route
• Notification of failed link sent back to source
• New metric information sent to source

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Routing Protocol

• Source re-runs Dijkstras algorithm
• Better metric information
• Sources learn through dummy queries from gateways
  or
• Unsolicited link metric information about nearby
  links

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Routing Metric

• Srcr uses Estimated Transmission Time (ETT)
  metric
• ETT predicts total amount of time needed to send
  data packet along a route
• Srcr chooses route with lowest ETT

16
Routing Metric

• Srcr predicts that a links highest-throughout
  bit-rate is the bit-rate with the highest product
  of delivery probability and bit-rate.
• 1500-byte periodic broadcasts at each available
  802.11 bit rate
• Periodic minimum-size broadcasts at 1Mbps

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Routing Metric

• ETT metric for a link is the expected time to
  send a 1500 byte packet at that links highest
  throughput bit-rate.
• ETT metric for a route is the sum of the ETTs for
  the routes links.
• t 1 / Si 1/ti
• t routes end-to-end throughput
• ti throughput of routes hop

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Bit-Rate Selection

• SampleRate Roofnets algorithm to choose among
  802.11b transmit bit rates.
• Adjusts bit-rate as it sends data packets over a
  link
• Adjusts choice more accurately and quickly than
  ETT
• Bases choice on actual data transmission v/s on
  periodic broadcast probes
• Sends packets at bit-rate which currently
  provides highest throughput

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Evaluation

• Method
• Basic Performance
• Link Quality and Distance
• Effect of density
• Mesh Robustness
• Architectural Alternatives
• Inter-hop Interference

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Method

• Multi-hop TCP data set
• 84-byte pings once per second for 10 seconds
• Route established and latency measured
• Throughput number of bytes delivered to
  receiving application
• 10 pairs no working routes
• Single-hop TCP data set
• Measure throughput on direct radio link between
  pair of nodes

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Method

• Loss matrix data set
• Measure loss rate between pair of nodes
• 1500-byte broadcasts at each 802.11b bit-rate
• Multi-hop density data set
• Measure throughput between fixed set of four
  nodes
• Vary number of nodes participating in routing
• Some of the analyses involve simulated route
  throughput calculated from the single-hop TCP.

22
Basic Performance
Average throughput is 627 kbits/sec
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Basic Performance
TCP throughput to each node from its chosen
gateway
24
Link Quality and Distance

• Srcr favors short links of a few hundred meters.
• Fast, short hops are the best policy

25
Link Quality and Distance

• Median 0.8
• Single-hop route with 40 loss can deliver more
  data than a two-hop route with perfect links.

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Effect of density

• Mesh networks are effective only if the node
  density is sufficiently high.
• Simulate different size subsets of Roofnet
• Estimate multi-hop throughput between pairs in
  the subset

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Effect of density
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Mesh Robustness
Most nodes have many neighbors
Majority of nodes use many neighbors
Roofnet makes good use of the mesh architecture
in ordinary routing
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Mesh Robustness

• Extent to which network is vulnerable to loss of
  its most valuable links
• Dozens of the best links must be eliminated
  before throughput is reduced by half.

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Mesh Robustness

• Effect on throughput of cumulatively eliminating
  the best-connected nodes.
• Best two nodes are important for performance.

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Architectural Alternatives Optimal Choice

• Comparison with single-hop (access point)
  network.
• Single-hop 5 gateways to cover all nodes
• Multi-hop forwarding provides higher average
  throughput
• Sequence of short high quality links

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Architectural Alternatives Random Choice

• If Roofnet were single-hop, 25 gateways would be
  required to cover all nodes.
• Multi-hop routing improves connectivity and
  throughput.
• Careful gateway choice improves throughput for
  both multi-hop and single-hop routing.

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Inter-hop Interference

• Measured multi-hop throughput lower than
  expected.
• Concurrent transmissions on different hops
  collide and cause packet loss.

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Inter-hop Interference

• 802.11 RTS/CTS mechanism prevent collisions
• RTS/CTS does not improve performance

35
Network Use

• Measurements of user activity on Roofnet
• One of the four gateways monitored packets
  forwarded between Roofnet and the Internet.
• In one 24-hr period Average of 160 kbits/sec
• Gateways radio busy 70 of the monitoring period
• Less than 1 UDP. Rest were TCP.
• 16 Roofnet nodes accessed the Internet

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Conclusion

• Ease of deployment
• 37 nodes in one year with little administrative
  or installation effort
• Average throughput 627 kbits/sec
• Position of internet gateways determined by
  convenience
• Multi-hop mesh increases both connectivity and
  throughput

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• Questions?