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.

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

Handoff DelayHandoff Delay

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600

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Experiments

Handoff Time (msec) Asso + AuthScanning

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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 !!

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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)

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

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

Sept 16 2005 Sangho Shin

Expreriment Result Expreriment Result – Handoff – Handoff timetime

Handoff Time

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Experiments

mse

cOriginal HandoffSelective ScanningCaching

Sept 16 2005 Sangho Shin

Expreriment Result Expreriment Result – Packet – Packet DelayDelay

Packet Delay in Sender

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50

100

150

200

250

Normal Handoff Selective Scanning Caching

Del

ay (

mse

c)

Sept 16 2005 Sangho Shin

Expreriment Result Expreriment Result – Packet – Packet LossLoss

Packet Loss in Receiver

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Normal Handoff Selective Scanning Caching

# o

f pac

kets

Sept 16 2005 Sangho Shin

Expreriment Result Expreriment Result – Packet – Packet LossLoss

Bridging Delay

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Experiments

Dela

y (m

sec)

Sept 16 2005 Sangho Shin

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!