1. 1 A Hybrid Channel Scan Mechanism for Improved VoIP Handoff Performance in 802.11 WLANs Jeng-Ji Huang, Sen-Ching Chang Department of Applied Electronics Technology National Taiwan Normal University Taiwan

2. 2 Outline Introduction Related Works The Proposed Hybrid Channel Scan Mechanism Simulation Results Conclusions

3. 3 Introduction IEEE 802.11 a/b/g WLAN –Advantages: low cost, easy deployment, high data rates –Disadvantages: small coverage, common control/data channel

4. 4 Small Coverage t t 1 t 2 AP old AP new Radius: 50~100 m handoff

5. 5 Small Coverage Cell-Search Threshold (30 dB) Delta-SNR (8 dB) Out-of-Range Threshold (12 dB) t t 1 t 2 SNR Out-of-Range Delta-SNR Cell-Search

6. 6 Common Data/Control Channel Scan –for AP discovery Re-authentication –connect to a new AP –including Authentication association 350~500 ms < 20 ms

7. 7 Common Data/Control Channel How many to scan? –11 channels in 802.11b –10 channels to be scanned, except the one currently used by the old AP How to perform the scan? –Passively listen to beacons save power –Actively exchange probe messages save time f t T  100 ms beacons

8. 8 Common Data/Control Channel No specific control channel to perform scan Use spare time to measure SNR of scanned channels AP old New APs? scan t

9. 9 Related Works Reduce the time spent on each channel –[1], [3], [7] Minimize the total number of channels to be scanned –Selective scanning [8] Remember where you’ve been before –Adjacent APs, their channels

10. 10 The Proposed Hybrid Channel Scan Mechanism Beacon Listening [4] –Voice Activity Detection –Beacon Listening performed during SSs –Prevent packet loss and increase handoff success prob. Switching-Time Calculation Probe Exchanging Three-state Markov Chain.

11. 11 Beacon Listening In [4], no neighboring APs’ information is available. –At first, no idea when beacon will arrive –A MS waits for the beacon on each channel –A lot of downlink packets losses could occur In this paper, we assume that the information is given via neighbor graph [9]. –Scheduling scans on selected channels

12. 12 Switching-Time Calculation p 1 : (t 1,  SNR 1 ) p 2 : (t 2,  SNR 2 ) Let y be the target Delta-SNR t is the switching-time t s

13. 13 Probe Exchanging Before t s, the station performs passive beacon listening to save power. After t s, the station enters into the effective period. During the period, active scan will be performed to ensure a handoff success.

14. 14 Simulation Results CSSA: [4] –Pure Beacon Listening CSSA w. prior info –Neighboring APs info is known HCSM –The proposed mechanism Comparison of packet loss rate.

15. 15 Simulation Results 66.75% 37% 26.93%

16. 16 Conclusion A hybrid channel scan mechanism is proposed to improve VoIP performance in IEEE 802.11 WLANs. In the proposed mechanism, passive scan is used first in order to save power, and immediate and effective active scan is compelled to lower handoff failure prob.

17. 17 Thanks for your attention!