Enhanced Backoff Scheme for IEEE 802'11 DCF

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Enhanced Backoff Schemefor IEEE 802.11 DCF

• 7. 13
• Dong-Hun Kang
• Dept. of Computer Engineering,
• Kyungnam University
• (esy2k_at_net.kyungnam.ac.kr)

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Outline

• IEEE 802.11 WLAN Fairness Issue
• New Method 1
• New Method 2
• Simulation Result

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IEEE 802.11 DCF

• It is based on carrier sense multiple access with
  collision avoidance (CSMA/CA).
• Before a station starts a transmission, it must
  sense the channel idle for a duration, called DCF
  interframe space (DIFS), plus an additional
  backoff time.
• Only when the channel remains idle during all
  this time can the station initiate the
  transmission.
• The backoff time is an integer multiple of basic
  time slot, drawn randomly between zero and the
  so-called CW.
• For each successfully received frame, the
  receiving station immediately replies with an
  acknowledgment frame (ACK).
• After an unsuccessful transmission attempt, the
  CW is doubled and another attempt with backoff is
  performed.

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IEEE 802.11 WLAN Fairness Issue

• It doesnt consider number of backoff
• It never consider to differentiate between many
  transmission attempted station and first
  transmission attempted station.
• In spite of many long backoff time, transmission
  opportunity is small.
• That is short term unfairness.
• According to guarantee that fairness , IEEE
  802.11 DCF need to improvement.

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New method 1 Differentiated DIFS

• To supply fairness, we can use different DIFS
• Long DIFS is low priority, Short DIFS is high
  priority
• DIFS will change by number of backoff stage

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New method 1 Differentiated DIFS (contd)
difs0 0.000050s (50ms)
0,0
0,CW0

CWmin_ 31 CWmax_ 1023 ShortRetrylimit_
7 LongRetrylimit_ 4
difs1 0.000045s

1,CW1
1,0
2,CW2
2,0

difs2 0.000040s
3,CW3
3,0

difs3 0.000035s
4,CW4
4,0

difs4 0.000030s

difs5 0.000025s
5,CW5
5,0
6,CW6
6,0

difs6 0.000020s
discard
gt ShortRetryLimit
7,0
difs7 0.000015s

• Default DIFS of legacy DCF is 50ms in DSSS.
• As Number of backoff increased, short DIFS is
  matched.

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New method 1 Differentiated DIFS (contd)

• For example, When Station 1 is at two backoff
  stage and third transmission attempt, and when
  Station 3 is at zero backoff stage and first
  transmission attempt,
• As DIFS decreased, many backoff stage and many
  transmission attempted station have more
  opportunity.

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New method 2 Start Stage

• Start-Backoff Stage is not zero stage.
• Which Stage is Start-Backoff Stage ?
• End stage Start-Backoff Stage 6(Retry Limit)
• Start-Backoff Stage can move

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Simulation Environment

• Simulator NS2
• Number of Station 10 70
• Performance Measure
• Throughput
• Delay
• Number of Backoff for each packet
• Simulation time 300s
• Simulation Parameter
• RTS-CTS option
• IEEE 802.11b MAC, PHY Parameter, DataRate 11Mbps
• Simulation Assumption
• There are always frames in the queue
• Simulation Object
• When fairness solved, we show performance measure
  enhancement

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Throughput

• As the number of station increasing, Saturation
  Throughput decreasing.
• However, the original method is faster decreased
  than new method.
• New method 2 is faster decreased than old
  proposed method in 5

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Average Delay

• As the number of station increasing, Total Delay
  increasing.
• However, the original method is faster increased
  than new method.

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Standard deviation of Delay

• As the number of station increasing, Total Delay
  increasing.
• However, the original method is faster increased
  than new method.
• Especially, new method 1 is the most slow
  increased than others.

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Bakoff for each packet

• As the number of station increasing, Backoff for
  each packet increasing.
• However, the original method is faster increased
  than new method.

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Successful Transmission Rate for each packet

• As the number of station increasing, successful
  transmission rate for each packet decreasing.
• However, the original method is faster decreased
  than new method.

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Reference

• 1 G. Bianchi, Performance Analysis of the IEEE
  802.11 Distributed Coordination Function, IEEE
  JSAC, vol. 18, no. 3, pp.535-547,March 2000.
• 2 H. Wu et al., Performance of Reliable
  Transport Protocol over IEEE 802.11 Wireless LAN
  Analysis and Enhancement, IEEE Infocom, 2002.
• 3 E. Ziouva and T. Antonakopoulos, CSMA/CA
  Performance under High Traffic Conditions
  Throughput and Delay Analysis, Computer
  Communications, vol. 25, no. 3, pp. 313-321, 2002
• 4 S. Ci and H. Sharif, Evaluating Saturation
  Throughput Performance of the IEEE 802.11 MAC
  under Fading Channels, BROADNETS, pp.726-731,
  2005.
• 5 W. Kuo and C. J. Kuo, Enhanced Backoff
  Scheme in CSMA/CA for IEEE 802.11, VTC, pp.
  2809-2813, 2003
• 6 IEEE Standard for Wireless LAN Medium Access
  Control (MAC) and Physical Layer (PHY)
  Specifications, IEEE Std 802.11, 1999(R2003)
• 7 IEEE Standard for Wireless LAN Medium Access
  Control (MAC) and Physical Layer (PHY)
  Specifications, Higher-Speed Physical Layer
  Extension in the 2.4GHz Band, IEEE 802.11b, 1999

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Thanks for your attentions.