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An Experimental Study of ARQ Protocol in 802.11b Wireless LAN

An Experimental Study of ARQ Protocol in 802.11b Wireless LAN. Chung Ho Nam, Soung C. Liew, Cheng Peng Fu. Outline. Introduction Background Details of experiments Conclusion. Introduction. 802.11b is becoming hot Previous studies are based on simulations and old wlan technology

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An Experimental Study of ARQ Protocol in 802.11b Wireless LAN

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  1. An Experimental Study of ARQ Protocol in 802.11b Wireless LAN Chung Ho Nam, Soung C. Liew, Cheng Peng Fu

  2. Outline • Introduction • Background • Details of experiments • Conclusion

  3. Introduction • 802.11b is becoming hot • Previous studies are based on simulations and old wlan technology • Experimental approach was used in this paper

  4. Introduction • 2 main issues: • Effectiveness of ARQ in 802.11b • 3 Experiments • Loss behavior of 802.11b network • 2 more: • Self-collision of TCP traffic • Changes in collision rates due to movements of mobile hosts

  5. What is ARQ? • Automatic Repeat reQuest • Error control in data transmission • 3 common techniques: • Stop and Wait • Go back N • Selective Repeat • 802.11b employs “Stop and Wait” at link-layer

  6. ARQ in 802.11b • Stop and Wait

  7. ARQ in 802.11b • Stop and Wait

  8. ARQ in 802.11b • Not 100% reliable • Only try at most (1 + 3) times in current commercial products • e.g. Orinoco, Buffalo • Algorithm • Skip

  9. Why? • Transmission error in air • Channel noise • Channel interference • Packet collisions • No direct collision detection in 802.11b • TCP or UDP on top of 802.11b • Misinterpret packet loss as congestion loss (TCP)

  10. Set-up for Experiments

  11. Statistics from AP Manager • Multiple Transmit Retry Count (MTRC) • >= 2 retransmission attempts (successful) • Transmit Retry Count (TRC) • >= 1 retransmission attempts (successful) • Transmit Failed Count (TFC) • Still failed after 3 retransmission attempts

  12. Experiment (I) • Is ARQ adequate to compensate for packet loss due to transmission error? • Server: NetProbe • Client: NetProbe • Protocol: UDP • Packet size: 1460 bytes • Data rate: 500kbps • Number of packets: 100,000 • Distance: 5m (best), 15m (good), 25m (marginal)

  13. Result of experiment (I) Loss rate = TFC / Total number of UDP packets

  14. Result of experiment (I) • Total number of unsuccessful transmission attempts at layer 2 = Packets requiring exactly one retransmission to succeed + 2 * Packets requiring exactly two retransmissions to succeed + 3 * Packets requiring exactly three retransmissions to succeed + 4 * Packets that fail to be transmitted successfully (i.e., TFC)

  15. 1+2+3 2+3 3 4+4+4+4 Let  be this Result of experiment (I) = Packets requiring one or more retransmissions to succeed (i.e., TRC) + Packets requiring two or more retransmissions to succeed (i.e., MTRC) + 1 * Packets requiring exactly three retransmissions to succeed + 4 * TFC

  16. Result of experiment (I) Ploss

  17. Result of experiment (I)

  18. Experiment (II) • Is ARQ adequate to compensate for packet loss due to packet collisions? • Two-way traffic • Protocol: UDP • Packet size: 1460 bytes • Data rate: 500kbps • Number of packets: 100,000 • Distance: 5m (best), 15m (good), 25m (marginal)

  19. Result of experiment (II)

  20. Result of experiment (II)

  21. Result of experiment (II)

  22. TCP throughput VS Loss Prob.

  23. Implication of Experiment (II) • Collision Probability Increases with Distance • Interaction between transmission errors and collision errors • But “noise” and “interference” should be independent • More time is needed for transmission when the data rate is low • Wireless medium is much more occupied • Probability of packet loss is higher

  24. The Poor Pulling Down the Rich • e.g. • When A moves farther away from AP, packet duration increases • Less airtime for B • Both A and B degrade

  25. Experiment (III) • Interactions between link-layer ARQ in WLAN and transport-layer ARQ in TCP? • Suiteserver and Suiteclient (like FTP) • Protocol: TCP • Packet size: 1460 bytes • Data rate: Not restricted • Total data size = 1460*100,000 bytes • Distance: 5m (best), 15m (good), 25m (marginal)

  26. Result of experiment (III) > 5% of packets were retransmitted

  27. Result of experiment (III) • Recall: link-layer packets and ACKs DO NOT collide • TCP ACKs are regular link-layer packets • Collide with packets from AP • More experiments need to be done

  28. Packet Loss Pattern in 802.11b • Random Loss VS Bursty Loss • Let p = total packet loss / total packet transmitted • = the overall loss rate of a link • If packets losses are random, then the gap between two successive packet losses follows the following probability distribution: • Pj = P [j next packets transmitted successfully before the next loss] = p (1-p) j • In particular, if loss is random, P0 = p, whereas if loss is bursty, P0 > p.

  29. Experimental Evaluation • Server: NetProbe • Client: NetProbe • Protocol: UDP • Packet size: 1460 bytes • Data rate: 500kbps • Number of packets: 100,000 • Distance: 25m (marginal)

  30. p=0.0676 IDEAL y=p(1-p)x

  31. p=0.0676 REAL

  32. Conclusions • One-way traffic • No collision • ARQ is effective in keeping the loss rate to below 0.01 • Two-way traffic • Collisions exist • Loss rate may rise beyond 0.01

  33. Conclusions • Two possible improvements • Strengthen the link-layer ARQ • Modify TCP • Two issues to be studied • Self-collision in TCP • Movements of clients • Packet loss in 802.11b is not random

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