Modeling Media Access in Embedded TwoFlow Topologies of Multihop Wireless Networks

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Modeling Media Access in Embedded Two-Flow
Topologies of Multi-hopWireless Networks

• Jingpu Shi
• (joint work with Michele Garetto and Edward
  Knightly)
• Department of Electrical and Computer Engineering
  
• Rice University
• April 27, 2005

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Background Introduction Wireless Medium Access
Control
b
A

• In a wireless network, how stations access
  channel to accomplish transmissions ?
• The MAC (Medium Access Control) is a set of rules
  to determine how to access the medium (channel).
• Performance Metrics
• Throughput
• Fairness etc.

a
B
(a) Ad Hoc network
(b) Hot Spot
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Background Introduction Hidden Terminal Problem
b
A

• A simple MAC solution a station starts
  transmitting only after it senses the channel is
  idle.
• Hidden terminal can not be sensed, therefore
  packets could collide, and be destroyed.

a
B
(a)
a
b
B
A
(b)
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Background Introduction IEEE 802.11 MAC

• A distributed algorithm.
• 4-way or 2-way exchange for every data packet
  transmission.
• 4-way exchange Control packet transmissions
  precede data packet transmissions to avoid
  collision.
• Maintain a value CW (contention window) using a
  exponential increase linear decrease algorithm.

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Background Introduction IEEE 802.11 MAC
(continued)
Sender
RTS
DATA
CTS
ACK
Receiver
Others
RTS
CTS
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Motivation of This Work

• In Multi-hop wireless networks, scenarios similar
  to the hidden terminal problem lead to fairness
  problems.
• Those problems have not been very well addressed
  and understood.
• In this work, we view a network as a set of
  sub-graphs consisting two flows and characterize
  its media access.

Source
Destination
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Outline

• Background and Motivation
• Scenario identifications and their likelihood to
  occur
• Fairness simulations
• Media access modeling

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Twelve topologies

• Identical transmission range and interference
  range.
• We only consider one-way flows.
• A link is established when two stations are in
  radio range.

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All Possible Topologies
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Scenario Classification

• Senders Connected (SC) scenarios 2-7, where
  senders of each flow are in radio range.
• Asymmetric Incomplete State (AIS), scenarios 11
  and 12, where senders are disconnected,
  asymmetric connections between the two flows.
• Symmetric Incomplete State (SIS), scenario 8, 9
  and 10, where senders are disconnected, symmetric
  connections between the two flows.

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Scenario LikelihoodAssumptions and illustration

• Whats the probability of each scenario occurring
  ?
• Spatial analysis, assuming the two flows are
  uniformly distributed in a region and border
  effect is negligible.

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Scenario LikelihoodResults for each scenario

• Scenario 11 dominates when distance becomes large

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Scenario LikelihoodResults for each group
AIS and SIS class are highly likely to occur when
distance between two hops becomes large.
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Outline

• Motivation
• Scenario identifications and their likelihood
• Fairness simulations
• Media access modeling

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Performance Simulations With CSMA/CA protocol

• Observations
• SC-No fairness problem.
• AIS-Both short-term and long-term fairness
  problems.
• SIS-Long-term fair, short-term unfair.
• Root cause different information about the
  channel.

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Outline

• Motivation
• Scenario identifications and their likelihood
• Fairness simulations
• Modeling media access

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Modeling Framework

• Identify 4 different state
• idle channel
• channel occupied by successful transmissions
• channel occupied by a collision
• busy channel due to activity of other stations
• Define probabilities
• Probability of the four stats and throughput of
  the station

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Model AIS ClassSample topology and modeling
strategy

• Use decoupling technique.
• Independently study the behavior of each
  transmitting node.
• Consider flow B first, and then flow A.

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Model AIS Class Strategies and steps

• For the first flow, the only unknown parameter is
  collision probability, which can be computed from
  the plot above.
• For the second flow, the only unknown parameter
  is b, which can be computed easily given
  throughput of flow A is known.

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Model AIS ClassResults

• With RTS/CTS
  Without RTS/CTS

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Model SIS ClassSample topology and modeling
strategy

• We analyze short-term unfairness.
• Main difficulty the two transmitting nodes are
  tightly correlated.
• A Markov chain model using bi-dimensional state
  description.

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Model SIS Class Strategies and steps

• We represent the system state as pair (SA, SB),
  where SA and SB denote the backoff stage of
  Sender A and B respectively.
• Transition probability of the Markov chain.

• ri is the probability that a station transmits
  after one slot in backoff stage i. f is the
  duration of the first packets (RTS or DATA)
  transmitted.
• After solving the Markov chain, we can compute
  the transition time from state (m, 0) to (0,m),
  where m is the maximum backoff stage.

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Model SIS ClassResults (cont.)
(C1) RTS/CTS access, m 6, CWmax 1024. (C2)
RTS/CTS access, m 8, CWmax infinity. (C3)
Basic access, m 3, CWmax 1024. (C4) Basic
access, m 6, CWmax 1024.
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Outline

• Background and Motivation
• Scenario identifications and their likelihood to
  occur
• Fairness simulations
• Media access modeling

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• More Questions or Comments ?
• Email jingpu_at_rice.edu