multi-class qos in 802.11 networks using gdmc
DESCRIPTION
Multi-Class QoS in 802.11 Networks Using GDMC. Authors: Bushra Anjum and Zartash Afzal Uzmi School of Science and Engineering, LUMS, Pakistan. Bushra Anjum North Carolina State University. IEEE Globecom 2007 – Washington, DC Friday, November 30, 2007. Outline. Introduction - PowerPoint PPT PresentationTRANSCRIPT
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Multi-Class QoS in 802.11 Networks Using GDMC
IEEE Globecom 2007 – Washington, DCFriday, November 30, 2007
Bushra AnjumNorth Carolina State University
Authors:Bushra Anjum and Zartash Afzal Uzmi
School of Science and Engineering, LUMS, Pakistan
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC2
Outline
• Introduction– 802.11 and DCF mechanism– Motivations for the new GDMC scheme
• Previous work on CW management• Description of GDMC Scheme
– GDMC Parameters– Window Management Procedure
• Simulation Scenarios and Results– Throughput Results– Delay Characteristics– Support for many traffic classes
• Conclusions
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC3
802.11 and DCF
• IEEE 802.11 Standard– Medium Access Control (MAC)
Layer– Physical (PHY) Layer
• 802.11 uses “Shared Medium”– Multiple Access using DCF
• DCF principle– Carrier Sense Multiple Access
(CSMA)– Medium Idle?
•Yes Transmit !•No Defer for backoff time
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC4
DCF: Contention Window
• CWcur may vary from CWmin to CWmax
• Backoff time is random from CW• Single CW for all traffic in DCF
– No support for multiple traffic classes
CWmin CWcur CWmax (31) (1023)
Backoff time
ContentionWindow (CW)
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC5
DCF: CW Management
CWmin CWmax (31) (1023)
ContentionWindow (CW)
CWmin CWmax (31) (1023)
ContentionWindow (CW)
Failed Attempt to Transmit
CWcur
CWcur
After Successful Transmission
CWcur is doubled
CWcur is reset to CWmin
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC6
802.11 and Multi-Class Traffic
• Single CW in DCF for all traffic– Each traffic type backs off “in the same
way”– No service differentiation
• Evolution of Network Traffic– Multi-Class (Urgent, Regular, Background)– Multi-Class QoS is needed !
• 802.11 Solution– Point Coordination Function (PCF)– A round-robin polling Inefficient
• 802.11e Solution– Hybrid coordination functions– Require changes to original DCF
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC7
Our Goal
• Maintain original DCF mechanism
• Provide multi-class QoS• Remain as scalable as the DCF
• Enable strict service differentiation– For high traffic load
• Increased network utilization– For relaxed network conditions
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC8
Observations
1. Use of Multiple Contention Windows
Different CW for different traffic classes Service differentiation
Lesson:Use CW – one for each traffic class !
2. Sequential Decrease of CWcurLarge CWcur recent collisions
Lesson:Do not reset CWcur on success !
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC9
Existing Approaches
• Improving CW Management– Using Network History
•Better Utilize Network Resources
– Change in Backoff procedures•Modify doubling and resetting
– CW Range based Differentiation•Each traffic class has its own CW• Independent backoff time values
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC10
Example Schemes
• Predictive DCF– Backoff time based on network
history
• Sliding Contention Window (SCW)– For each traffic class ‘c’
• Keep CWc,LB and CWc,UB
• Adjust these using network history
• Gentle DCF (and Probabilistic DCF)– MIMD procedure for CW adjustment
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC11
Shortcomings• Maintaining Network History
– Continuous monitoring of channel– Virtual carrier sense forgone– Energy efficiency compromised
• Use of additional parameters– Loss ratio α– Medium Occupancy Ratio B(T)– Parameters foreign to DCF
• Despite these shortcomings:– SCW and similar schemes allow service
differentiation
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC12
Observations
1. Use of Multiple Contention Windows
Different CW for different traffic classes Service differentiation
Lesson:Use CW – one for each traffic class !
2. Sequential Decrease of CWcurLarge CWcur recent collisions
Lesson:Do not reset CWcur on success !
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC13
The GDMC Scheme
• One Contention Window for each class ‘c’
• Maintain: CWmin,c CWmax,c CWcur,c
• Backoff time [c] = U~[CWmin,c : CWcur,c]
CWmin,c1 CWcur,c1 CWmax,c1
CW[c1]
CWmin,c2 CWcur,c2 CWmax,c2
CW[c2]
c1: higher priority
c2: lower priority
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC14
GDMC: CW Management
CWmin,c CWmax,c
ContentionWindow (CW)
CWmin,c CWmax,c
ContentionWindow (CW)
Failed Attempt to Transmit
CWcur,c
CWcur,c
After Successful Transmission
CWcur,c is doubled
CWcur,c is halved
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC15
Simulation Setup
• OMNET++ Simulator• 2 Mb/s WLAN in BSS mode• 4-way access mechanism
– RTS/CTS/DATA/ACK– No hidden node problem
• Sources are CBR• Three traffic classes
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC16
Throughput: High Priority
Simulation Time in seconds
Th
roughput
Rati
o
No wait time in GDMC for gathering historyGDMC performs better than SCW
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC17
Throughput: Medium Priority
Th
roughput
Rati
o
Simulation Time in seconds
Once again, GDMC performs better than SCW and others
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC18
Throughput: Low Priority
Th
roughput
Rati
o
Simulation Time in seconds
DCF outperforms all other schemes – as expected
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC19
Delay Characteristics
Dela
y in
mill
iseco
nds
Simulation Time in seconds
Network history not collected GDMC exhibits lowest delay
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC20
Multiple Traffic Classes
Number of Nodes (in each traffic class)
Th
roughput
Rati
o
Throughput is visually distinct
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC21
Conclusions
• GDMC uses:– Independent CW for each traffic
class– MIMD procedure for each class
• Throughput improvement:– About 30% for high priority– About 20% for medium priority
• Operation of GDMC:– Under standard DCF– Scalable to large number of nodes– Support for many distinct traffic
classes
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November 30, 2007 Multi-Class QoS in 802.11 using GDMC22
Questions?
Thanks!
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