Wireless Review- Probing. William Arbaugh University of Maryland

Transcription

1 Wireless Review- Probing William Arbaugh University of Maryland

2 Joint work with students Min-ho Shin Arunesh Mishra

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

4 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

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

6 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

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

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

9 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 (AP i, AP j ) is in E iff a station can hand-off from AP i to AP j Distributed data structure AP maintains the list of neighbor APs

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

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

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

13 Key ideas NG probing Probe only neighbor channels Wait for only neighbor AP s response ch1 ch6 AP3 AP5 Next Channel CS&T MinChannelTime CS&T MaxChannelTime Saved Channel Time Saved Probe-wait

14 NG-prune technique [11] AP2 AP3 [6] By NG probing, STA waits for MaxChannelTime [1] AP1 AP5 [6] AP4 [11] 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

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

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

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

18 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

19 Experiment Probing Latency Full Obsvd NG NGP Probe Count Probe Wait 10.9ms 11.0 ms 6.3 ms 4.4 ms NG-Pruning

20 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)

21 Summary of Publications Mishra, Shin, Petroni, Clancy, Arbaugh, Proacti ve Key Distribution Using Neighbor Graphs, IEEE Wireless Communications, vol. 11, Februar y Shin, Mishra, Arbaugh, An Efficient Handoff Sch eme in IEEE using Neighbor Graphs, MobiSys Mishra, Shin, Arbaugh, Context Caching using N eighbor Graphs for Fast Handoffs in a Wireless N etwork, INFOCOM 2004, March 2004.

22 Questions?

23 Back up slides

24 Simulation Probing Latency vs # of Channels

25 Simulation Pruning vs # of Neighbor

26 SNR Handoff Threshold Hand-off Decision High Threshold : SNR from AP1 Probing start Hand-off - Too early probing, so high probing overhead Probing overhead Low Threshold : - Too late probing, so suffer from bad signal High Hysteresis : - Too late hand-off, so high probing overhead Low Hysteresis : Bad Signal - Too early hand-off, so ping-pong effect SNR from AP2 hysteresis A B C D AP1 AP2

27 NG-prune Non-overlap Graph Overlap Graph(OG) is an undirected graph <V,E> ; V = set of access points <AP i, AP j > E if their coverages overlap APi, APj overlap S i (x) T h S j (x) T h Non-overlap Graph(NOG) = OG c OG is easier to generate than NOG Also distributed structure stored at each AP

28 Simulation Simulation Model Identical coverages Randomly chosen : # of neighbors positions STA s direction Optimaly chosen channel assignments Variables # of Nb : 2,3,,8 # of Chnl : 3,5,8,12