1. Fast Wireless Handoff in 802.11 Networks Sangho Shin Andrea G. Forte Anshuman S. Rawat Henning Schulzrinne

2. Sept 16 2005 Sangho Shin 802.11 Networks A handoff occurs when a mobile station moves beyond the radio range of one AP and enters another BSS.

3. Sept 16 2005 Sangho Shin Handoff delay Mobile station All APs Probe request(broadcast) Probe response New AP Authentication request Authentication response Reassocation request Reassociation response Probe delay Authentication delay Reassociation delay

4. Sept 16 2005 Sangho Shin Handoff Delay

5. Sept 16 2005 Sangho Shin Problems Handoff latency is too big for VoIP. Seamless VoIP requires less than 90ms latency. Handoff delay is from 200ms to 400msec. The biggest component of handoff latency is probing (over 90%). n Selective Scanning n Caching Solutions

6. Sept 16 2005 Sangho Shin Selective Scanning In most of the environments (802.11b & 802.11g), only channel 1, 6, 11 are used for APs. Two APs that have the same channel are not adjacent (Co-Channel interference). Scan 1, 6, 11 first !!

7. Sept 16 2005 Sangho Shin Selective Scanning - Algorithm Scan channels using channel mask Found good AP ? Flip channel mask Channel mask = 1, 6, 11 Join to the AP Yes No Set new channel mask New channel mask = 1,6,11 + scanned channels – current channel

8. Sept 16 2005 Sangho Shin Caching Background Spatial locality Algorithm After scanning, store the candidate AP info into cache (key=current AP) Use the AP info in cache for association without scanning when handoff happens KeyAP1AP2 1Current APNext best APThe second best AP …. N

9. Sept 16 2005 Sangho Shin Cache – Simulation KeyBestNext 1AP1(11) 2 Handoff !! Coverage area Detection area AP1(11) AP2(6) Channel mask 1, 6 AP3(1)

10. Sept 16 2005 Sangho Shin Cache – Simulation KeyAP1AP2 1AP1(11) 2-- Cache miss! Handoff !! Selective Scanning Channel mask 1, 6 AP1(11) AP2(6) AP3(1) AP2, AP3 Coverage area Detection area

11. Sept 16 2005 Sangho Shin Cache – Simulation KeyAP1AP2 1AP1 (11)AP2 (6)AP3 (1) 2AP2 (6)-- Cache miss! Handoff !! Selective Scanning Connect AP2, AP3 AP2 AP3(1) Channel mask 1, 6 AP1(11) AP2(6) 1, 11 Coverage area Detection area

12. Sept 16 2005 Sangho Shin Cache – Simulation KeyAP1AP2 1AP1 (11)AP2 (6)AP3 (1) 2AP2 (6)- Handoff !! Channel mask 1, 11 AP3(1) AP1(11) AP2(6) Coverage area Detection area

13. Sept 16 2005 Sangho Shin Cache – Simulation KeyAP1AP2 1AP1 (11)AP2 (6)AP3 (1) 2AP2 (6)- Cache miss! Handoff !! Channel mask 1, 11 AP3(1) AP1(11) AP2(6) Coverage area Detection area

14. Sept 16 2005 Sangho Shin Cache – Simulation KeyAP1AP2 1AP1 (11)AP2 (6)AP3 (1) 2AP2 (6)AP1 (11)- Cache miss! Handoff !! Selective Scanning AP3(1) AP1(11) AP2(6) Channel mask 1, 11 AP1 Coverage area Detection area

15. Sept 16 2005 Sangho Shin KeyAP1AP2 1AP1 (11)AP2 (6)AP3 (1) 2AP2 (6)AP1 (11)- Cache miss! Handoff !! Connect Cache – Simulation AP3(1) AP1(11) AP2(6) Selective Scanning AP1 Channel mask 1, 111, 6 Coverage area Detection area

16. Sept 16 2005 Sangho Shin Cache – Simulation Handoff !! AP3(1) AP1(11) AP2(6) KeyAP1AP2 1AP1 (11)AP2 (6)AP3 (1) 2AP2 (6)AP1 (11)- Channel mask 1, 6 Coverage area Detection area

17. Sept 16 2005 Sangho Shin Cache – Simulation Cache HIT! Handoff !! AP3(1) AP1(11) AP2(6) KeyAP1AP2 1AP1 (11)AP2 (6)AP3 (1) 2AP2 (6)AP1 (11)- Channel mask 1, 6 Coverage area Detection area

18. Sept 16 2005 Sangho Shin Cache – Simulation Cache HIT! Handoff !! Connect AP3(1) AP1(11) AP2(6) KeyAP1AP2 1AP1 (11)AP2 (6)AP3 (1) 2AP2 (6)AP1 (11)- Channel mask 1, 11 AP2 Coverage area Detection area

19. Sept 16 2005 Sangho Shin Cache – Simulation Handoff !! Cache Hit!! AP3(1) AP1(11) AP2(6) KeyAP1AP2 1AP1 (11)AP2 (6)AP3 (1) 2AP2 (6)AP1 (11)- Channel mask 1, 11 AP1(11 ) Coverage area Detection area

20. Sept 16 2005 Sangho Shin Cache – Simulation Cache Hit!! Handoff !! Connect AP3(1) AP1(11) AP2(6) KeyAP1AP2 1AP1 (11)AP2 (6)AP3 (1) 2AP2 (6)AP1 (11)- Channel mask 1, 11 AP1 Coverage area Detection area

21. Sept 16 2005 Sangho Shin Expreriment Result – Handoff time

22. Sept 16 2005 Sangho Shin Expreriment Result – Packet Delay

23. Sept 16 2005 Sangho Shin Expreriment Result – Packet Loss

24. Sept 16 2005 Sangho Shin Expreriment Result – Packet Loss

25. Sept 16 2005 Sangho Shin Conclusion Fast MAC Layer handoff using selective scanning and caching Selective Scanning : 100~130 msec Caching : 2~4 msec Don’t need to modify AP, infrastructure, or standard. Just need to modify wireless card driver!