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

2. Networks
A handoff occurs when a mobile station moves beyond the radio range of one AP and enters another BSS.

3. Handoff delay Mobile station All APs Probe delay New AP
Probe request (broadcast)
Probe response
Probe delay
Authentication request
New AP
Authentication delay
Authentication response
Association request
Association delay
Association response

4. Handoff Delay

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

6. Selective Scanning
In most of the environments (802.11b & g), 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. Selective Scanning - Algorithm
Scan channels using channel mask
(Full scan)
Found good AP ?
Yes
Join to the AP No
Flip channel mask
Set new channel mask
New channel mask =
1, 6, 11 + scanned channels – current channel

8. Caching Background Algorithm
Spatial locality (Office, school, campus…)
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.
Key
AP1
AP2 1
Current AP
Next best AP
Second best AP …. N

9. Cache – Simulation Coverage area Detection area Handoff !! Key Best
Next 1
AP1(11) 2
Coverage area
Detection area
Channel mask
1, 6
AP1(11)
Handoff !!
AP2(6)
AP3(1)

10. Cache – Simulation Coverage area Detection area Handoff !! Cache miss!
Key
AP1
AP2 1
AP1(11) 2 -
Coverage area
Detection area
Channel mask
1, 6
AP1(11)
Handoff !!
AP2(6)
AP3(1)
Cache miss!
Selective Scanning
AP2, AP3

11. Cache – Simulation Coverage area Detection area Handoff !! Cache miss!
Key
AP1
AP2 1
AP1 (11)
AP2 (6)
AP3 (1) 2 -
Coverage area
Detection area
Channel mask
1, 11
1, 6
AP1(11)
Handoff !!
AP2(6)
AP3(1)
Cache miss!
Selective Scanning
AP2, AP3
Connect
AP2

12. Cache – Simulation Coverage area Detection area Handoff !! Key AP1 AP2-
Coverage area
Detection area
Channel mask
1, 11
Handoff !!
AP1(11)
AP2(6)
AP3(1)

13. Cache – Simulation Coverage area Detection area Handoff !! Cache miss!
Key
AP1
AP2 1
AP1 (11)
AP2 (6)
AP3 (1) 2 -
Coverage area
Detection area
Channel mask
1, 11
Handoff !!
AP1(11)
Cache miss!
AP2(6)
AP3(1)

14. Cache – Simulation Coverage area Detection area Handoff !! Cache miss!
Key
AP1
AP2 1
AP1 (11)
AP2 (6)
AP3 (1) 2 -
Coverage area
Detection area
Channel mask
1, 11
Handoff !!
AP1(11)
Cache miss!
AP2(6)
AP3(1)
Selective Scanning
AP1

15. Cache – Simulation Coverage area Detection area Handoff !! Cache miss!
Key
AP1
AP2 1
AP1 (11)
AP2 (6)
AP3 (1) 2 -
Coverage area
Detection area
Channel mask
1, 11
1, 6
Handoff !!
AP1(11)
Cache miss!
AP2(6)
AP3(1)
Selective Scanning
AP1
Connect
AP1

16. Cache – Simulation Coverage area Detection area Handoff !! Key AP1 AP2-
Coverage area
Detection area
Channel mask
1, 6
AP1(11)
Handoff !!
AP2(6)
AP3(1)

17. Cache – Simulation Coverage area Detection area Handoff !! Cache HIT!
Key
AP1
AP2 1
AP1 (11)
AP2 (6)
AP3 (1) 2 -
Coverage area
Detection area
Channel mask
1, 6
AP1(11)
Handoff !!
AP2(6)
AP3(1)
Cache HIT!

18. Cache – Simulation Coverage area Detection area Handoff !! Cache HIT!
Key
AP1
AP2 1
AP1 (11)
AP2 (6)
AP3 (1) 2 -
Coverage area
Detection area
Channel mask
1, 11
AP1(11)
Handoff !!
AP2(6)
AP3(1)
Cache HIT!
Connect
AP2

19. Cache – Simulation Coverage area Detection area Handoff !! Cache Hit!!
Key
AP1
AP2 1
AP1 (11)
AP2 (6)
AP3 (1) 2 -
Coverage area
Detection area
AP1(11)
Channel mask
1, 11
Handoff !!
AP1(11)
Cache Hit!!
AP2(6)
AP3(1)

20. Cache – Simulation Coverage area Detection area Handoff !! Cache Hit!!
Key
AP1
AP2 1
AP1 (11)
AP2 (6)
AP3 (1) 2 -
Coverage area
Detection area
Channel mask
1, 11
Handoff !!
AP1(11)
Cache Hit!!
AP2(6)
AP3(1)
Connect
AP1

21. Cache – Simulation Coverage area Detection area Handoff !! Cache Hit!!
Key
AP1
AP2 1
AP1 (11)
AP2 (6)
AP3 (1) 2 -
Coverage area
Detection area
Channel mask
1, 6
Handoff !!
AP1(11)
Cache Hit!!
AP2(6)
AP3(1)
Connect
AP1

22. Experiment Result – Handoff time

23. Experiment Result – Packet Delay

24. Experiment Result – Packet Loss

25. Experiment Result – Packet Loss

26. Conclusion Fast MAC Layer handoff using selective scanning and caching
Selective Scanning : 100~130 msec
Caching : 2~4 msec
Low Power Consumption (PDAs).
Don’t need to modify AP, infrastructure, or standard. Just need to modify the wireless card driver!

27. Thank you! For more information: