An Approach for Solving the Unfairness Problem in WLANs Martin - - PowerPoint PPT Presentation

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An Approach for Solving the Unfairness Problem in WLANs Martin - - PowerPoint PPT Presentation

Dec 08, 2022 233 likes •472 views

An Approach for Solving the Unfairness Problem in WLANs Martin Heusse * , Yan Grunenberger * , Elena Lpez-Aguilera ** , Andrzej Duda * * LIG Grenoble Computer Laboratory ** Catalan Institute of Technology Outline WLAN unfairness problem

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SLIDE 1

An Approach for Solving the Unfairness Problem in WLANs

Martin Heusse*,

Yan Grunenberger*, Elena López-Aguilera**, Andrzej Duda* * LIG Grenoble Computer Laboratory ** Catalan Institute of Technology

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SLIDE 2

2

Outline

  • WLAN unfairness problem
  • Idea of Asymmetric Access Point
  • Implementing AAP
  • Performance of Asymmetric Access Point
  • Conclusions
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SLIDE 3

Unfairness problem

  • TCP connections to mobile stations
  • downloads, uploads
  • Sporadic UDP traffic with real-time requirements

(VoIP)

  • We assume that wireless LAN is the bottleneck

3

wired

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SLIDE 4

DCF characteristics

  • Half-duplex operation
  • One frame at a time
  • Equal channel access opportunities for all

contending entities

  • AP and any of N stations

° statistical share of 1/(N+1)

  • Independent of frame length/transmission speed
  • Exponential backoff
  • Short term unfairness for larger N
  • Increased unfairness if bad channel conditions

4

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SLIDE 5

TCP Delayed Acknowledgements

  • An ACK is delayed until (timeout or)
  • k segments are received (k=2 typical)
  • k: number of data segments per ACK

5

40 39 37 38

35 33

41 40 38 39

35 37

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SLIDE 6

N uploads

  • kN data segments at stations, N ACKs at AP
  • AP share needs to be N/(N + kN), 1/3 for k=2
  • If share of 1/(1+N)
  • Short buffer at AP: losses, but ACKs are cumulative
  • Large buffer at AP: longer RTT, limited by flow control

6

wired

N=3

ACKs DATA segments

k=2

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SLIDE 7

N downloads

  • kN data segments at AP

, N ACKs at stations

  • AP share needs to be kN/(N + kN), 2/3 for k=2
  • If share of 1/(1+N)
  • Short buffer at AP: loss, limited by congestion control
  • Large buffer at AP: longer RTT, limited by flow control

7

wired

N=3 k=2

ACK DATA segments

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SLIDE 8

DCF; 4 uploads, 1 download

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5 10 15 20 25 30 5 10 15 20 25 Time (s) Data transferred (MB) Download Uploads

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SLIDE 9

DCF; ping RTT

  • vs. 4 downloads

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RTT (ms) Frequency 10 20 30 40 50 60 10 20 30 40 50 60

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SLIDE 10

Asymmetric AP Approach

  • Give more channel access opportunity to AP
  • Asymmetric Access Point benefits from k more

share than all stations in cell (or kN than any station)

  • Corresponds to the worst case (N downloads)
  • Increases performance in mixed upload/

download scenario

  • keeps the AP buffer empty so that TCP

connections become self-clocked by the destination (short RTT over the wireless part)

10

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SLIDE 11

Asymmetric AP Approach

11

wired

N=3 k=2

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SLIDE 12

Implementing AAP

  • Stations
  • Operate according to Idle Sense
  • Adapt CW to load conditions in the cell by
  • bserving the mean number of idle slots
  • Asymmetric Access Point
  • Constant CW value, independent of N!
  • Derived for given k and 802.11 variant

12

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SLIDE 13

10 20 30 40 50 3.5 4.0 4.5 5.0 5.5 6.0 N n_i

Optimal CW

n̄i: average number of idle slots between transmission attempts n̄i

target

10 20 30 40 50 100 300 500 N

  • ptimal CW

n̄i converges quickly CW proportional to N

13

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SLIDE 14

AAP

14

  • CW proportional to N
  • AP Access probability proportional to N

➡ AP CW divided by N compared to STA CWs

➡ AP CW is a constant

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SLIDE 15

Measurements

  • Implementation of Idle Sense
  • Intel PRO/Wireless 2200BG 802.11 a/b/g

cards

  • Modified firmware, operational cards
  • AP - FreeBSD box
  • constant CW
  • Stations close to AP

, good channel conditions, 802.11a at 54 Mb/s

15

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SLIDE 16

DCF; 4 uploads, 1 download

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5 10 15 20 25 30 5 10 15 20 25 Time (s) Data transferred (MB) Download Uploads

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SLIDE 17

AAP; 4 uploads, 1 download

5 10 15 20 25 30 5 10 15 20 25 Time (s) Data transferred (MB) Download Uploads

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SLIDE 18

DCF; 1 upload, 4 downloads

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5 10 15 20 25 30 5 10 15 20 25 Time (s) Data transferred (MB) Upload Downloads

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SLIDE 19

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AAP; 1 upload, 4 downloads

5 10 15 20 25 30 5 10 15 20 25 Time (s) Data transferred (MB) Downloads Upload

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SLIDE 20

DCF; ping RTT

  • vs. 4 downloads

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RTT (ms) Frequency 10 20 30 40 50 60 10 20 30 40 50 60

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SLIDE 21

AAP; ping RTT

  • vs. 4 downloads

21

RTT (ms) Frequency 10 20 30 40 50 60 10 20 30 40 50 60

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SLIDE 22

Conclusions

  • Simple solution at MAC layer to the unfairness

problem

  • Right shares of transmission opportunity
  • Correct operation of TCP over 802.11 - self-

clocked flow control

  • Keeps empty buffer at AP - gives short delays
  • Always preference to download connections
  • Optimal solution in mixed upload/download

scenarios requires upper layer modification

  • Proper scheduling at IP/TCP layer

22