Fault Tolerant Design and Analysisfor Access Points in Public WLAN

公眾無線區域網路擷取點容錯技術之設計與分析

22

Outline

Introduction

Background

Proposed approach

Comparison

Evaluation

Simulation

Conclusion

References

33

AP

AP

AP

AP

AP

AP

AP

AP: access pointMS: mobile station

MS

MS

MS

MS

MS

MS

MS

MS

MS

MS

MS

MS

Introduction

What is public WLAN (public Wireless Local Access Network)

44

AP

AP

AP

AP

AP

APMS

MS

MS

MS

MS

MS

MS

MS

MS

AP: access pointMS: mobile station

Introduction

Motivation

MS

MS

MS

APFaulty AP

MS

MS

MS

Failure

55

Introduction

Traditional fault tolerance in public WLAN Based on the hardware or network planning support.

Goal Propose a new approach to tolerating the AP failure in

a public WLAN. No extra hardware required.

No pre-planned network required.

The vicinity and overloading are simultaneously considered.

66

ESS

...

Distribution System

AP: access pointBSS: basic service set

BSS

APMSMS

BSS

AP

MS

MS

BSS

AP

MS

MS

MS

SNMP Server

BSS

AP

MS

MS MS

MS: mobile stationESS: extended service set

Background

Network model

77

Background

Previous approaches: Access-point replication

Overlapping coverage

Link multiplexing

88

Background

Hardware cost

Hardware compatibility

Fault-tolerant capability

Previous approaches: Access-point replication

Overlapping coverage

Link multiplexing

MS

MS

Redundant AP

AP

MS

MS

Redundant AP

AP

MS

MS

MS

Redundant AP

Faulty AP

Failure

MS

MS

Redundant AP

AP

MS

[DSN’03]

99

Background

Network planning

AP overloading

Previous approaches: Access-point replication

Overlapping coverage

Link multiplexing

Fault-tolerant capability MSMS

AP

MS

AP

MS

MS

MSMS

Faulty AP

MS

AP

MS

MS

Failure

[ICC’04, WMCSA’03, MONET’02, COMCON’01, WCNC’99]

1010

Background

Previous approaches: Access-point replication

Overlapping coverage

Link multiplexing

Hardware cost

Software support

Fault-tolerant capabilityMSMS

AP

MS

APMS

AP

MSMS

MS

APMS

AP

Faulty AP

Failure

Network planning

[WORDS’03]

1111

Proposed Approach

Basic idea If an AP fails, some working (survival) APs still exist in

the system.

If the failure-affected MSs can move to the coverage

ranges of the survival APs, their wireless connectivity

can be resumed.

1212

Proposed Approach

Basic idea (Cont.) The public WLAN is usually deployed in indoor

environment. A failure-affected MS can find a survival AP without moving

too far.

The main idea of the proposed approach: To give each failure-affected MS a direction to guide it how to

move.

1313

Proposed Approach

Problems for achieving the basic idea How to select preferable survival AP as the fault-tolerant

AP ? How to avoid the preferable AP being an overloading AP

or a faulty AP ? How to forcefully direct to the coverage range of fault-

tolerant AP ?

MS

MS

AP

MS

MSMS

AP AP

MS

MS

AP APFaulty AP

Failure

MS

MS

MSHot-spotAP

1414

Proposed Approach

First problem To make each failure-affected MS quickly resume the

wireless connectivity with the least cost.

AP

MSAP

AP

Overlapping coverage range

AP

MSFaulty AP

AP with thestrongest signal

Overlapping coverage range

Failure

Non-overlapping coverage range

AP

AP

AP AP

AP AP

AP

AP AP AP AP

Non-overlapping coverage range

AP

Faulty AP

Failure

Fault-tolerant APcandidate

Fault-tolerant APcandidate

Fault-tolerant APcandidate

Fault-tolerant APcandidate

AP

AP Fault-tolerant APcandidate

Fault-tolerant APcandidate

AP

1515

Proposed Approach

Second problem To enhance the fault-tolerant AP selection with the state

consideration.

AP-1 AP-2 AP-3 AP-4

AP-5

AP-11AP-8

AP-6 AP-7

AP-9 AP-10

Faulty AP-6

Failure

Failure-tolerant AP set Faulty AP list

AP - 2 , 3 , 5 , 7 , 9 , 10 AP - 6

AP - 3 , 4 , 10 , 11 AP - 6 , 7Faulty AP-7

Failure

MS

OverloadingAP-10

1616

Proposed Approach

Third problem To make each failure-affected MS follow the given

direction without randomly moving. To prevent inter-AP interference:

• The channels used by neighboring APs be separated by at least

five channels

To set failure-affected MS:

• Scanning mode: Active mode.

• Scanning channel: Same as fault-tolerant AP.

1717

Proposed Approach

System architecture Data Structures:

SNMP Server: AP deployment map and AP location table with

loading information AP: Overloading record MS: AP deployment map and AP location table

Procedures: SNMP Server: The loading inquiry routine and the fault-tolerant

AP recommendation procedure AP: The loading control procedure MS: The fault-tolerant procedure and the map direction

procedure

1818

SNMP Server

Distribution System

AP AP

MS

MS

MS

MS

APMS

Loading inquiry routine

Fault-tolerant APrecommendation

procedure

2nd stagefault-tolerant procedure

Map direction procedure

1st stagefault-tolerant procedure

MS

Loading control procedure

SNMP Server

Distribution System

AP Faulty AP

MS

MS

MS

MS

APMS

Loading inquiry routine

Fault-tolerant APrecommendation

procedure

2nd stagefault-tolerant procedure

Map direction procedure

1st stagefault-tolerant procedure

MS

Loading control procedure

Failure

Proposed Approach

System architecture (Cont.)

loading of APs

Fault tolerant AP fails

Fault tolerant AP works

Probe request

Overloading record( recommending AP & lifetime )

Request newfault tolerant AP

Response newfault tolerant AP

Extended probe responsewith new fault tolerant AP

Normal probe response

(ACCEPT)

Fault tolerant AP

New fault tolerant AP

1919

Comparison

2020

Evaluation

Failure-free

overhead

The cost of downloading the map and table The cost of the loading inquiry routine

Fault-tolerant

overhead

The cost of the two-stagefault-tolerant procedure

The performance affection on a survival AP

wireless

tablemap

wiredline

tablemaploaddata B

SS

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SST _ responseinquiryloading TTT

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The increase of collision probability (DCF mode):

The increase of transmission waiting interval (PCF mode):

2121

Simulation

Simulation model Software:

NS-2 2.27(Network Simulator version 2) on Linux

Topology:SNMP Server

Applicationserver 1

Applicationserver 2

Applicationserver 10

...

……

AccessPoint 1

Mobilestation

AccessPoint 2

AccessPoint 25

...

Mobilestation

Mobilestation

Mobilestation

100 MBps Ethernet

[Huan-Yun Wei et al., "Co-DRR: An Integrated Uplink and Downlink Scheduler for Bandwidth Management over Wireless LANs"]

2222

Simulation

Simulation model (Cont.) Parameters:

The arrivals of MSs to an AP follow a Poisson distribution.

The association time of an MS with an AP is random.

The MS intensity ( ) is controlled to be 10, 30, 60, and 90.

The max. number of MSs associated with an AP is set to 100.

The overloading threshold of an AP is set to 90.

Each MS in an AP randomly issues a data service to an

application server, and the service time is also random.

The ratio between the failure rate and recovery rate of an AP is

set to 0.0033.

r

a

[D. Chen et al., "Dependability Enhancement for IEEE 802.11 Wireless LAN with Redundancy Techniques"]

2323

Simulation

Performance metrics concerned Failure-free overhead

The cost of downloading the map and table

The cost of the loading inquiry routine

Fault-tolerant overhead The cost of the two-stage fault-tolerant procedure

The performance affection on a survival AP

• The increase of collision probability (DCF mode)

• The increase of transmission waiting interval (PCF mode)

2424

Simulation

Simulation results The cost of downloading the AP deployment map (32KB)

and AP location table (4KB): 1.17 second

The cost of the loading inquiry routine: 0.027 second

2525

Simulation

Simulation results (Cont.) The cost of the two-stage fault-tolerant procedure

Number of simultaneous AP failures = 1 Number of simultaneous AP failures = 2

Number of simultaneous AP failures = 4 Number of simultaneous AP failures = 8

050

100150

200250

300350

10 30 60 90

MS intensity

Exec

utio

n co

st (μ

s )

Analysis

Simulation

050

100150

200250

300350

10 30 60 90

MS intensity

Exec

utio

n co

st (μ

s )

Analysis

Simulation

050

100150

200250

300350

10 30 60 90

MS intensity

Exec

utio

n co

st (μ

s )

Analysis

Simulation

050

100150

200250

300350

10 30 60 90

MS intensity

Exec

utio

n co

st (μ

s )

Analysis

Simulation

2626

0.00

0.05

0.10

0.15

0.20

0.25

0.30

10 30 60 90

MS intensity

Perf

orm

ance

deg

rada

tion

in th

e PC

F m

ode

(sec

)

Analysis

Simulation

Simulation

Simulation results (Cont.) The performance affection on a survival AP

DCF mode PCF mode

0%

2%

4%

6%

8%

10%

10 30 60 90

MS intensity

Perf

orm

ance

deg

rada

tion

in th

e D

CF

mod

e Analysis

Simulation

2727

Conclusion

An efficient approach to tolerating AP failures in a

public WLAN Not requiring the hardware support.

Avoiding overloading situation.

Having the best fault-tolerant capability.

Numerical analysis and simulation experiments

results The failure-free and fault-tolerant overheads of the

proposed approach are small.

2828

References

[1] Hector Velayos, Victor Aleo, Gunnar Karlsson, Load Balancing in Overlapping Wireless LAN

Cells, ICC 2004 - IEEE International Conference on Communications (2004) 3833-3836.

[2] Colubris Networks Inc., Data Sheet for Colubris Networks Management System (CNMS), 2004.

[3] D. Chen, C. Kintala, S. Garg, K. S. Trivedi, Dependability Enhancement for IEEE 802.11 Wireless

LAN with Redundancy Techniques, Proceedings of the International Conference on Dependable

Systems and Networks (2003) 521-528.

[4] Gandhi R., Tolerance to Access-Point Failures in Dependable Wireless Local-Area Networks, The

Ninth IEEE International Workshop on Object-Oriented Real-Time Dependable Systems (2003)

136-143.

[5] Proxim Corp., User's Guide for ORiNOCO AP-2500, June 2003.

[6] Carlos Oliveira, Jaime Bae Kim, Tatsuya Suda, Long-Range Dependence in IEEE 802.11b

Wireless LAN Traffic: An Empirical Study, Computer Communications, 2003. CCW 2003.

Proceedings. 2003 IEEE 18th Annual Workshop on (2003) 17-23.

[7] F. K. Al-Bin-Ali, P. Boddupalli, N. Davies, An Inter-Access Point Handoff Mechanism for Wireless

Network Management: The Sabino System, In Proceedings of The 2003 International Conference

on Wireless Networks (2003) 225-230.

2929

References

[8] Huan-Yun Wei, Ching-Chuang Chiang, Ying-Dar Lin, Co-DRR: An Integrated Uplink and Downlink

Scheduler for Bandwidth Management over Wireless LANs, IEEE Symposium on Computers and

Communications (2003) 1415-1420.

[9] A. Malloy, U. Varshney, A. P. Snow, Supporting Mobile Commerce Applications Using

Dependable Wireless Networks, Mobile Networks and Applications (2002) 225-234.

[10] Anand Balachandran, Geoffrey M. Voelker, Paramvir Bahl, P. Venkat Rangan, Characterizing

User Behavior and Network Performance in a Public Wireless LAN, In Proceedings of the ACM

Sigmetrics Conference on Measurement and Modeling of Computer Systems (2002) 195-205.

[11] Anand Balachandran, Paramvir Bahl, Geoffrey M. Voelker, Hot-Spot Congestion Relief in Public-

Area Wireless Networks, Mobile Computing Systems and Applications, 2002. Proceedings Fourth

IEEE Workshop on (2002) 70-80.

[12] Gast, Matthew S., 802.11 Wireless Networks: The Definitive Guide, O'Reilly & Associates, April

2002.

[13] Cisco Systems Inc., Data Sheet for Cisco Aironet 350 Series Access Points, June 2001.

[14] I. Papanikos, M. Logothetis, A Study on Dynamic Load Balance for IEEE 802.11b Wireless LAN,

In Proceedings of the 8th International Conference on Advances in Communication & Control,

COMCON 8, Rethymna,Crete/Greece (2001).

3030

References

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Wireless Communications and Networking Conference (1999) 1028-1032.

[16] Shiann-Tsong Sheu, Chih-Chiang Wu, Dynamic Load Balance Algorithm (DLBA) for IEEE 802.11

Wireless LAN, Tamkang Journal of Science and Engineering (1999) 45-52.

[17] IEEE Std 802.11b-1999, Part 11: Wireless LAN Medium Access Control (MAC) and Physical

Layer (PHY) Specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band, IEEE,

September 1999.

[18] J. Case, M. Fedor, M. Schoffstall, J. Davin, A Simple Network Management Protocol (SNMP),

Technical Report IETF RFC 1157, May 1990.

[19] D. Gross, C. M. Harris, Fundamentals of Queuing Theory, John Wiley & Sons, 1985.

[20] NS-2 Network Simulator, Available: http://www.isi.edu/nsnam/