multi-class qos in 802.11 networks using gdmc
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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
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
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
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)
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
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
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
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 !
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
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
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
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 !
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
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
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
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
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
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
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
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
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
November 30, 2007 Multi-Class QoS in 802.11 using GDMC22
Questions?
Thanks!
Contact:banjum@ncsu.edu
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