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Probing in 802.11 using Neighbor Graphs

Probing in 802.11 using Neighbor Graphs. Minho Shin, Arunesh Mishra, William Arbaugh University of Maryland. OUTLINE. Motivation & Background Neighbor Graph NG / NG-prune probing algorithm Experiment/simulation results Conclusion. Motivation. Hand-off latency is critical in WLAN infra.

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Probing in 802.11 using Neighbor Graphs

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  1. Probing in 802.11 using Neighbor Graphs Minho Shin, Arunesh Mishra, William Arbaugh University of Maryland

  2. OUTLINE • Motivation & Background • Neighbor Graph • NG / NG-prune probing algorithm • Experiment/simulation results • Conclusion

  3. Motivation • Hand-off latency is critical in WLAN infra. • small coverage • demand for multimedia or realtime applications • High hand-off latency is observed • 60 ~ 400 ms, 252 ms on avg, in experiment • expected to increase with 11i authentications • Probing latency > 90% of hand-off latency • Reducing probing latency is important

  4. probe request [6] probe response Probing [11] probe response auth request auth response reassoc req reassoc res Hand-off procedure MS AP1 [11] [6] AP2 AP3 AP2 AP3 AP4 [1] AP5 [2] [1] AP4 [11] AP1 Authentication AP5 [6] Reassociation

  5. What affects probing latency • Number of channels to probe • Standard doesn’t define • naive : all 11 channels (Full-scanning) • only used channels (Observed-scanning) • Waiting Time for probe response • Standard defines • MinChannelTime / MaxChannelTime

  6. Probing in detail AP3 AP5 ch1 ch6 CS&T MinChannelTime CS&T MinChannelTime MaxChannelTime Wasted Channel Time Wasted Probe-wait • MinChannelTime when no AP respond • MaxChannelTime when any AP respond

  7. OUTLINE • Motivation & Background • Neighbor Graph • NG / NG-prune probing algorithm • Experiment/simulation results • Conclusion

  8. Neighbor Graph Definition • [Mishra, Shin, Arbaugh, INFOCOM 2004] • NG dynamically learns the mobility patterns • NG = <V,E>, a directed graph • V : set of all APs • (APi,APj)∈Eif an STA handoffs from APi to APj • Distributed data structure • AP maintains the list of neighbor APs

  9. Neighbor Graph Generation B G Personal Neighbor Graph (PNG) J Neighbor Graph (NG) H I F D E A C A B

  10. Neighbor Graph Dynamic Mobility Pattern B K G • edge-deletion • deprecated path • AP failure/removal J H I • edge-addition • AP restore/install • new path F D E A C A B

  11. OUTLINE • Motivation & Background • Neighbor Graph • NG / NG-prune probing algorithm • Experiment/simulation results • Conclusion

  12. NG probing • Key ideas • Probe only neighbor channels • Wait for only neighbor AP’s response AP3 AP5 ch1 ch6 Next Channel MaxChannelTime CS&T MinChannelTime CS&T Saved Channel Time Saved Probe-wait

  13. NG-prune technique • By NG probing, STA waits for MaxChannelTime • AP2 and AP4 don’t overlap • If STA knows Ap2 and AP4 doesn’t overlap, STA no longer waits for response from AP2 as soon as AP4 responds • Use non-overlap graph [11] [6] AP2 AP3 [1] AP4 AP1 [11] AP5 [6]

  14. Channel 1 : 1*RTT Channel 6 : 1*RTT Channel 11: 0 Reduce both probe-count and probe-wait time Pruning algorithm can be reduced to a variant of the set cover problem NG-prune Example [6] E [1] D [11] [1] F A C [11] B [6]

  15. NG-prune Comparison • Full Scanning : • 2*MaxCT+9*MinCT • Observed Scanning : • 2*MaxCT+1*MinCT • NG probing • 1*Max + 1*Min + 1*RTT • NG-prune probing • 2*RTT [6] E [1] D [11] [1] F A C [11] B [6]

  16. OUTLINE • Motivation & Background • Neighbor Graph • NG / NG-prune probing algorithm • Experiment/simulation results • Conclusion

  17. Experiment Methodology • 20 Cisco 350 APs over two floors using channel 1, 6 ,11 • Avg # of neighbors = 3.15 • STA using laptop with Prism2 based wireless card • Probing algorithm implemented in driver & user roaming program • Full / observed scanning, NG / enhanced-NG

  18. Experiment Probing Latency NG-Pruning

  19. Simulation Probing Latency vs # of Channels

  20. Simulation Pruning vs # of Neighbor

  21. Conclusion • New efficient probing algorithms ( NG/NG-prune probing ) and evaluated by experiemt and simulations • Performance improves • as # of indep. channels increase (802.11a) • as density of access points increase (# of neighbor increase)

  22. Thank you

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