On the Performance Behavior of IEEE 802'11 Distributed Coordination Function

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On the Performance Behavior of IEEE 802.11
Distributed Coordination Function

• M.K.Sidiropoulos, J.S.Vardakas and M.D.Logothetis
• Wire Communications Laboratory,
• Department of Electrical Computer Engineering,
• University of Patras,
• 265 00 Patras, Greece
• E-mail m-logo_at_wcl.ee.upatras.gr

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Outline

• Purpose of the paper.
• DCF-An Example.
• Mathematical Analysis ( Assumptions ).
• 802.11 DCF Markov Chain Model and Steady State
  Analysis
• leading to a Saturation Throughput formula.
• Simulation results ( IEEE 802.11b network).
• Conclusion.

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Purpose of the paper

• We propose a new Markov model for the DCF of
  IEEE 802.11
• based on Bianchis, Wus and Ziouvas
  models.
• and derive an analytical formula for the
  Saturation Throughput
• for both Basic and RTS/CTS access schemes.
• Simulation Study
• Validation of our new Markov model based on
  throughput results by the NS-2.
• Average end-to-end packet delay for both access
  schemes.

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DCF-An Example

• DCF employs 2 mechanisms
• Basic access scheme A 2-way handshaking
  technique.

• Note that
• After a DIFS time interval each station defers
  for an additional random backoff time.
• The backoff counter is frozen if a transmission
  is detected on the channel
• BACKOFF
  SUSPENSION

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DCF-An Example (cont.)

• RTS/CTS
• Request-To-Send / Clear-To- Send.
• It is a 4-way handshaking technique.
• Introduced to tackle the hidden terminal
  problem.
• Improve throughput performance in case of long
  packets.

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Mathematical Analysis

• Assumptions
• Ideal channel conditions ( error-free channel).
• Finite number of stations, each of which has
  always a packet
• available for transmission. (saturation
  conditions)
• Constant and independent collision probability
  p.
• Probability pb independent of the backoff
  procedure.

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802.11 DCF Markov Chain Model
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Saturation Throughput model.
Normalized Throughput ( fraction of the channel
time used for payload transmissions)

• Ps a successful transmission in a slot.

• Ptr at least one transmission in a slot.

• EP average packet payload.

• s duration of an empty slot time

• Ts average time of a successful
  transmission

• Tc average duration of a collision .

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Steady State Analysis
Stationary Distribution of the chain ( Steady
State )
From the chain we have
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Steady State Analysis (cont.)
Normalization condition
Contention Window
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Steady State Analysis (cont.)
Channel access probability
Collision probability
Probability of channel being busy
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Saturation Throughput model.
Normalized Throughput ( fraction of the channel
time used for payload transmissions)

• Ps a successful transmission in a slot.

• Ptr at least one transmission in a slot.

• EP average packet payload.

• s duration of an empty slot time

• Ts average time of a successful
  transmission

• Tc average duration of a collision .

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Simulation Study

• Performance metrics measured
• by simulation
• Saturation throughput.
• End-to-end average packet delay.
• Simulations in NS-2
• IEEE 802.11b single-hop
• network.
• Network Topology
• No hidden stations, all have LOS.
• CBR traffic over UDP links towards the AP.
• No mobility.

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Model Validation Simulation vs. Analysis
1Mbps.

• Basic and RTS/CTS
• Close match of analytical
• model and simulation results.
• Our model is closer to
• simulation than Wus.
• The RTS/CTS gives higher
• throughput than Basic due to
• the short RTS frames.
• ( Only exception for n 5).

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Model Validation Simulation vs. Analysis 5.5
and 11 Mbps.

• In both cases analysis and
• simulation are in satisfactory
• agreement.
• Basic access scheme gives
• higher throughput than
• RTS/CTS when channel bit
• rate ?. ( RTS, CTS packets
• are transmitted at 1Mbps).
• Throughput ? as bit rate?
• ( DIFS,SIFS, Backoff delay
• remain unchanged)

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Average Delay Simulation

• As network size ? delay ? for both
  access schemes.
• As channel bit rate ? delay ?.
• RTS/CTS delay is lower than Basic delay only
  for 1Mbps.
• Not efficient to use RTS/CTS for high data rates

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Conclusion

• We have developed an analytical model to enhance
  Bianchis and Wus analytical model for the
  saturation throughput of the DCF of the IEEE
  802.11 protocol.
• Our model gives greater throughput results than
  Wus model for both access schemes, Basic and
  RTS/CTS.
• Via numerous simulations with NS-2 we have shown
  that our model is close to simulation, for all
  network sizes .
• As channel bit rate increases
• throughput decreases
• Average delay decreases.
• Basic vs. RTS/CTS
• In low rates RTS is better than Basic.
• In higher rates Basic is preferable than RTS (
  gives greater throughput and lower delay).

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THANK YOU!