Download An Efficient Peer-to-Peer File Sharing Exploiting

Fault Tolerant Design and Analysis
for 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
Outline

Introduction

Background

Proposed approach

Comparison

Evaluation

Simulation

Conclusion

References
2
Introduction

What is public WLAN (public Wireless Local Access Network)
MS
MS
MS
MS
AP
AP
AP
MS
MS
AP
MS
MS
MS
AP
AP
AP
MS
MS
MS
MS: mobile station
AP: access point
3
Introduction

Motivation
MS
MS
MS
MS
AP
AP
AP
MS
MS
AP
MS
MS
MS
Failure
AP
AP
AP AP
Faulty
MS
MS
MS
MS: mobile station
AP: access point
4
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.
5
Background

Network model
SNMP Server
Distribution System
MS
MS
AP
MS
MS
MS
BSS
...
AP
AP
MS
MS
MS
AP
MS
MS
BSS
BSS
BSS
ESS
BSS: basic service set
ESS: extended service set
AP: access point
MS: mobile station
6
Background

Previous approaches:

Access-point replication

Overlapping coverage

Link multiplexing
7
Background

Previous approaches:

Access-point replication

Overlapping coverage

Link multiplexing
[DSN’03]
Faulty
AP AP
Hardware cost
AP
Failure
Hardware compatibility
Fault-tolerant capability
MS
MS
MS
Redundant AP
MS
Redundant AP
MS
8
Background

Previous approaches:

Access-point replication

Overlapping coverage

Link multiplexing
[ICC’04, WMCSA’03, MONET’02, COMCON’01, WCNC’99]
Network planning
Faulty
AP AP
AP overloading
Fault-tolerant capability
AP
MS
MS
Failure
MS
MS
MS
9
Background

Previous approaches:

Access-point replication

Overlapping coverage

Link multiplexing
[WORDS’03]
AP
Network planning
Hardware cost
Faulty
AP AP
MS
AP
MS
Failure
Software support
MS
MS
Fault-tolerant capability
10
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.
11
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.
12
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 faulttolerant AP ?
MS
MS
MS
MS
AP
Hot-spot
MS AP
Failure
Faulty
AP AP
MS
MS
AP
MS
MS
MS
AP
AP
13
Proposed Approach

First problem

To make each failure-affected MS quickly resume the
wireless connectivity with the least cost.
AP with the
strongest
AP signal
AP
Failure
Fault-tolerant
AP AP
candidate
Fault-tolerant
AP AP
candidate
AP
MS
Faulty
AP AP
Failure
Fault-tolerant
AP AP
candidate
Faulty
AP AP
Fault-tolerant
AP AP
candidate
AP
AP
Overlapping coverage range
Fault-tolerant
AP AP
candidate
Fault-tolerant
AP AP
candidate
AP
Non-overlapping coverage range
14
Proposed Approach

Second problem

To enhance the fault-tolerant AP selection with the state
consideration.
AP-1
AP-2
AP-3
AP-4
MS
AP-5
AP-8
Failure
Failure
AP-6
Faulty
AP-6
Faulty
AP-7
AP-7

AP-9
AP-10
Overloading
AP-10

Failure-tolerant AP set
Faulty AP list

AP - 2 , 3 , 5 , 7 , 9 , 10
AP - 6

AP - 3 , 4 , 10 , 11
AP - 6 , 7
AP-11
15
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.
16
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
17
Proposed Approach

System architecture (Cont.)
SNMP Server
Fault-tolerant AP
recommendation
procedure
Distribution System
loading of APs
Request new
fault tolerant AP
Response new
fault tolerant AP


Loading control
procedure
Loading
inquiry routine
Overloading record
( recommending AP & lifetime )

AP
MS
Probe request

Normal probe response
(ACCEPT)



MS
AP
MS


MS

Extended probe response
with new fault tolerant AP
2nd stage
fault-tolerant procedure
Failure
New fault tolerant AP
Fault tolerant AP works

Faulty
AP AP
1st stage
fault-tolerant procedure
MS
MS
Fault tolerant AP fails
Map
direction
procedure
Fault tolerant AP

18
Comparison
19
Evaluation
Failurefree
overhead
The cost of downloading the map and table
The cost of the loading inquiry routine
 S map  S table   S map  S table 
  

Tdata _ load  
B
B
wiredline
wireless

 

Tloading  Tinquiry  Tresponse
The cost of the two-stage
fault-tolerant procedure
The performance affection
on a survival AP

Cexec 
Faulttolerant
overhead
1




c


1
 

thrumax
n

thruMS



n
 a 
 
 r 
n!
i
 a 
 
c 
 r 

i!
i 0










The increase of collision probability (DCF mode):
e
1  Ps 

n


 a 




 


 r


 c n

n!

1

i
 n 0

 a 






c 
r 




i!
i 0


 1  Ps 
 thrumax 


 thru 1 
MS


The increase of transmission waiting interval (PCF mode):
n

 a 

 

 r 

c
n!
 thru max  n 
i
 thru MS 
n 0
 a 

 
c 

 r 


i!
i 0





t





20
Simulation
Simulation model
Software:

Topology:
Mobile
station
SNMP Server
Application
server 1
Application
server 2
Access
Point 1
100 MBps Ethernet
Access
Point 2
Mobile
station
……

NS-2 2.27(Network Simulator version 2) on Linux
...

...

Mobile
station
Application
server 10
Access
Point 25
Mobile
station
[Huan-Yun Wei et al., "Co-DRR: An Integrated Uplink and Downlink Scheduler for Bandwidth Management over Wireless LANs"]
21
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 (

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
a
r
) is controlled to be 10, 30, 60, and 90.
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. [D. Chen et al., "Dependability Enhancement for IEEE 802.11 Wireless LAN with Redundancy Techniques"]
22
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)
23
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
24
Simulation
Simulation results (Cont.)
350
300
250
200
150
100
50
0
Execution cost (μs )
The cost of the two-stage fault-tolerant procedure
Execution cost (μs )
Analysis
Simulation
10
30
60
350
300
250
200
150
100
50
0
90
Analysis
Simulation
10
MS intensity
Number of simultaneous AP failures = 1
350
300
250
200
150
100
50
0
Analysis
Simulation
10
30
60
30
60
90
MS intensity
Number of simultaneous AP failures = 2
Execution cost (μs )

Execution cost (μs )

90
MS intensity
Number of simultaneous AP failures = 4
350
300
250
200
150
100
50
0
Analysis
Simulation
10
30
60
90
MS intensity
Number of simultaneous AP failures = 8
25
Simulation
Simulation results (Cont.)

The performance affection on a survival AP
10%
0.30
8%
Analysis
6%
Simulation
4%
2%
0%
Performance degradation
in the PCF mode (sec)
Performance degradation
in the DCF mode

0.25
Analysis
0.20
Simulation
0.15
0.10
0.05
0.00
10
30
60
MS intensity
DCF mode
90
10
30
60
90
MS intensity
PCF mode
26
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.
27
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.
28
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 PublicArea 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).
29
References
[15] D. Tipper, S. Ramaswamy, T. Dahlberg, PCS Network Survivability, In Proceedings of IEEE
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/
30