Fast Wireless Handoff in 802.11 Networks
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Transcript of Fast Wireless Handoff in 802.11 Networks
Fast Wireless Handoff in Fast Wireless Handoff in 802.11 Networks802.11 Networks
Sangho ShinAndrea G. Forte
Anshuman S. RawatHenning Schulzrinne
Sept 16 2005 Sangho Shin
802.11 Networks802.11 Networks
A handoff occurs when a mobile station moves beyond the radio range of one AP and enters another BSS.
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Handoff delayHandoff delay
Mobile station All APs
Probe request(broadcast)
Probe response
New APAuthentication request
Authentication response
Reassocation request
Reassociation response
Probe delay
Authentication delay
Reassociation delay
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Handoff DelayHandoff Delay
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Experiments
Handoff Time (msec) Asso + AuthScanning
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ProblemsProblems
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%).
Selective Scanning Caching
SolutionsSolutions
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Selective ScanningSelective 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 !!
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Selective Scanning - Selective Scanning - AlgorithmAlgorithm
Scan channels using channel mask
Found good AP ?
Flip channel mask
Channel mask = 1, 6, 11
Join to the APYes
No
Set new channel mask
New channel mask = 1,6,11 + scanned channels – current channel
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CachingCaching 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
Key AP1 AP2
1 Current AP
Next best AP The second best AP
…. …. ….
N
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Cache Cache – Simulation – Simulation Key Best Next
1 AP1(11)
2
Handoff !!
Coverage area
Detection area
AP1(11)
AP2(6)
Channel mask 1, 6
AP3(1)
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Cache Cache – Simulation – Simulation Key AP1 AP2
1 AP1(11)
2 - -
Cache miss!
Handoff !!
Selective Scanning
Channel mask 1, 6
AP1(11)
AP2(6) AP3(1)
AP2, AP3
Coverage area
Detection area
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Cache Cache – Simulation – Simulation Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (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
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Cache Cache – Simulation – Simulation Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) -
Handoff !!
Channel mask 1, 11
AP3(1)
AP1(11)
AP2(6)
Coverage area
Detection area
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Cache Cache – Simulation – Simulation Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) -
Cache miss!
Handoff !!
Channel mask 1, 11
AP3(1)
AP1(11)
AP2(6)
Coverage area
Detection area
Sept 16 2005 Sangho Shin
Cache Cache – Simulation – Simulation Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Cache miss!
Handoff !!
Selective Scanning
AP3(1)
AP1(11)
AP2(6)
Channel mask 1, 11
AP1
Coverage area
Detection area
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Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Cache miss!
Handoff !!
Connect
Cache Cache – Simulation – Simulation
AP3(1)
AP1(11)
AP2(6)
Selective ScanningAP1
AP1
Channel mask 1, 111, 6
Coverage area
Detection area
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Cache Cache – Simulation – Simulation
Handoff !!AP3(1)
AP1(11)
AP2(6)
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Channel mask 1, 6
Coverage area
Detection area
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Cache Cache – Simulation – Simulation
Cache HIT!
Handoff !!AP3(1)
AP1(11)
AP2(6)
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Channel mask 1, 6
Coverage area
Detection area
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Cache Cache – Simulation – Simulation
Cache HIT!
Handoff !!
Connect
AP3(1)
AP1(11)
AP2(6)
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Channel mask 1, 11
AP2
Coverage area
Detection area
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Cache Cache – Simulation – Simulation
Handoff !!
Cache Hit!!
AP3(1)
AP1(11)
AP2(6)
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Channel mask 1, 11
AP1(11)
Coverage area
Detection area
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Cache Cache – Simulation – Simulation
Cache Hit!!
Handoff !!
Connect
AP3(1)
AP1(11)
AP2(6)
Key AP1 AP2
1 AP1 (11) AP2 (6) AP3 (1)
2 AP2 (6) AP1 (11) -
Channel mask 1, 11
AP1
Coverage area
Detection area
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Expreriment Result Expreriment Result – Handoff – Handoff timetime
Handoff Time
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Experiments
mse
cOriginal HandoffSelective ScanningCaching
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Expreriment Result Expreriment Result – Packet – Packet DelayDelay
Packet Delay in Sender
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Normal Handoff Selective Scanning Caching
Del
ay (
mse
c)
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Expreriment Result Expreriment Result – Packet – Packet LossLoss
Packet Loss in Receiver
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Normal Handoff Selective Scanning Caching
# o
f pac
kets
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Expreriment Result Expreriment Result – Packet – Packet LossLoss
Bridging Delay
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Experiments
Dela
y (m
sec)
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ConclusionConclusion 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!