A Case for Adapting Channel Width inWireless Networks

1
A Case for Adapting Channel Width inWireless
Networks

• Ranveer Chandra Ratul Mahajan Thomas
  Moscibroda Ramya Raghavendra Paramvir Bahl

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Briefings

• An attempt to consider the forgotten variable in
  802.11 MAC channel bandwidth
• A reconsideration of an old problem in an
  established scenario
• Presented opportunities, obstacles and solutions

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Outline

• How can we change channel width with atheros
  cards
• Benefit of change channel width to the basic
  transmission link
• Peak throughput, Transmission range, Energy
  consumption
• Benefit of change channel width to the 802.11
  network
• Power range, throughput, fairness, network
  capacity
• A simple algorithm to let two nodes agree on an
  optimal channel bandwidth SampleWidth
• -Approach
• -Optimality and convergency
• -Evaluation

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Change the channel width on atheros card
Yes we can!
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The hack
Madwifi driver
Controlling registers
Trans receiver
Frequency generator (VCO)
Mac control (back off, slot time, etc)
Available bandwidths 5MHz, 10MHz, 20MHz, 40MHz
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Impact of Channel width to peak throughputtheory
Peak throughput packet size 1/t
Packet loss is not considered here
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Impact of Channel width to peak throughput
measurement
A channel emulator is used to rule out channel
dynamics and ensure good received singal
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Impact of Channel width to peak throughput
comparison
close
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Benefit of channel width to transmission range

• Its not a new topic
• Narrower bandwidth for long range transmission,
  wider for close range
• Reason
• Constraint of transmission power
• Resilience to delay spread

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Constraint of transmission power
High SNR
Red signal Black noise The area of red Is the
total Transmission power
Low SNR
In FCC regulation, The total transmission Power
is fixed
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Constraint of transmission power result
Channel emulator is used to imposed attenuation
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Resilience to delay spread
Narrow band
Black first symbol arrival Red second symbol
arrival
! T1/F
Wide band
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Resilience to delay spread result
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Impact of channel bandwidth on power consumption
Power
Wireless card
Small resistor
Votage
Data acquisition system
The power measurement system
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Impact of channel bandwidth on power consumption
result
How about a Power per bit plot?
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Impact of channel bandwidth on 802.11 network

• Reduce power increase range at the same time!
• Improve flow throughput different range requires
  different channel bandwidth

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Impact of channel bandwidth on 802.11 network
Improve fairness allocate more bandwidth to more
loaded aps, and less bandwidth to less
loaded ones.
T throughput
FI fairness indicator
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Impact of channel bandwidth on 802.11 network

• Improve network capacity solve the rate anomaly
  problem
• Rate anomaly problem slow link would hinder fast
  link since 802.11 mac tries to make sure
  contending links have the same amount of channel
  time

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Improve network capacity result
Two flows, three indoor scenarios
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SampleWidth algorithm

• Goal
• Select the best channel width between two nodes
  for given objective(throughput, power, etc)

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SampleWidth algorithm

• Approach
• A empirical local optimization

Current point
throughput
Modulation (average rate From autorate algorithm)
Channel bandwidth
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SampleWidth algorithm
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Optimality and convergence

• Empirical analysis
• Assume objective function is concave and prove in
  measurement
• Concaveness indictor smoothness
• Also decide the best parameters experimentally
  compare algorithm result to the optimum

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Performance evalution

• Comparisons between SampleWidth and optimum ones

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Performance evaluation

• Comparisons between SampleWidth and optimum ones

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Performance evalution

• Energy savings

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Performance evalution

• Efficiency of autorate
• smoothness

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Performance evalution

• Channel switching overhead

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Interoperability of DifferentWidths
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Questions?

• How applicable is this?
• Is optimization such as SampleWidth necessary?