Opportunistic Media Access for Multirate Ad Hoc Networks

1
Opportunistic Media Access for Multi-rate Ad Hoc
Networks
B. Sadeghi, V. Kanodia, A. Sabharwal and E.
Knightly Rice Networks Group
http//www.ece.rice.edu/networks
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Motivation

• Wireless channel is variable

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Goal

• Exploit the variations inherent in wireless
  channel to increase throughput

user 1
channel gain ?
user 2
time ?
user 1
user 2

• Maintain temporal shares of different flows

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Outline

• IEEE 802.11 multi-rate
• Temporal fairness
• Opportunistic Auto Rate (OAR)
• Protocol
• Sources of gain
• Simulation results
• Conclusions

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IEEE 802.11 Multi-rate

• Support of higher transmission rates in better
  channel condition
• 802.11b
• available rates 2, 5.5, 11 Mbps
• 802.11a
• available rates up to 54 Mbps
• Auto Rate Fallback (ARF)
• Monteban et al. 97
• Use history of previous transmissions to
  adaptively select future rates

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Temporal vs. Throughput Fairness

• Equivalent in single-rate networks
• Throughput fairness results in significant
  inefficiency in multi-rate networks
• Example

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Temporal vs. Throughput Fairness

• Equivalent in single-rate networks
• Throughput fairness results in significant
  inefficiency in multi-rate networks
• Example

user 1
user 3
access point
user 2
Even 1 user with low transmission rate results in
a very low network throughput
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Temporal vs. Throughput Fairness

• Equivalent in single-rate networks
• Throughput fairness results in significant
  inefficiency in multi-rate networks
• Example

user 1
user 3
access point
user 2
Same time-shares of the channel for different
flows, also higher throughput
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Opportunistic Scheduling

• Goal
• Exploit short-time-scale variations inherent in
  wireless channel to increase throughput in
  wireless ad hoc networks
• Issue
• Maintaining temporal share of each node
• Challenge
• Channel info available only upon transmission

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Opportunistic Auto Rate (OAR)

• Main observation Coherence time in order of
  multiple packet transmissions time
• If a node accesses the channel and has a good
  channel, let it keep it longer
• Given a node with channel access, determine
  number of packets to transmit as a function of
  channel quality
• OAR High throughput, while maintaining temporal
  fairness properties of single rate IEEE 802.11

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OAR Protocol

• Rates in IEEE 802.11b 2, 5.5, and 11 Mbps

• Number of packets transmitted by OAR

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RBAR Protocol
Reservation Sub-Header

• Receiver Based AutoRate(RBAR) Bahl01

• Receiver controls the senders transmission rate
• Control messages sent at Base Rate

destination
source
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OAR Protocol
Reservation Sub-Header

• OAR

• Once access granted, it is possible to send
  multiple packets if the channel is good

destination
source
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Performance Comparison

• IEEE 802.11

R
D1
Transmitter
C
A
Receiver
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Performance Comparison

• IEEE 802.11

R
D1
Transmitter
C
A
Receiver
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Analytical results

• Analysis to study source of gain for RBAR and OAR
• time spent in contention
• average transmission rate
• Challenge both MAC and channel random processes
  with memory
• Comparison of simulation and analysis results

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Integration with IEEE 802.11

• Applicable to both sender/receiver-based
  protocols
• To hold the channel
• Contention window set to zero
• Packet burst
• Fragmentation
• A mechanism in IEEE 802.11 to send multiple
  frames
• Each frame/ACK acts as virtual RTS/CTS
• Use of more-fragment-flag

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Simulation Experiments

• Implemented OAR and RBAR in ns-2 with extension
  of Ricean fading model Punnoose et al 00
• Simulation experiments
• Fully connected networks
• all nodes within radio range of each other
• Node density, channel condition, mobility, node
  location
• Asymmetric topology
• Random topologies
• Integration with TCP

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Simulation Results Number of Flows

• OAR has 42 to 56 gain over RBAR
• Increase in gain as number of flows increases

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Simulation ResultsIntegration with TCP

• Can the increased throughput provided by OAR be
  exploited by TCP flows?
• A more variable rate channel
• Required buffering downstream

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Simulation ResultsIntegration with TCP

• Queue size larger than 20
• OAR has approximately 30 gain over RBAR

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Conclusions

• OAR Opportunistic MAC for multi-rate IEEE 802.11
• Nodes with good channels are allowed multiple
  packet transmissions
• OAR exploits variations inherent in wireless
  channels to increase throughput
• OAR ensures time-shares equal to those of
  single-rate IEEE 802.11
• Analytical model characterized the impact of
  channel conditions
• Simulation results show significant gain over RBAR