Multi-Class QoS in 802.11 Networks Using GDMC

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


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


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)


DCF: CW Management

CWmin CWmax (31) (1023)


CWmin CWmax (31) (1023)


Failed Attempt to Transmit

CWcur

CWcur

After Successful Transmission

CWcur is doubled

CWcur is reset to CWmin


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


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


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 !


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


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


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


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 !


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


GDMC: CW Management

CWmin,c CWmax,c


CWmin,c CWmax,c


Failed Attempt to Transmit

CWcur,c

CWcur,c

After Successful Transmission

CWcur,c is doubled

CWcur,c is halved


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


Throughput: High Priority

Simulation Time in seconds

Th

roughput

Rati

o

No wait time in GDMC for gathering historyGDMC performs better than SCW


Throughput: Medium Priority

Th

roughput

Rati

o


Once again, GDMC performs better than SCW and others


Throughput: Low Priority

Th

roughput

Rati

o


DCF outperforms all other schemes – as expected


Delay Characteristics

Dela

y in

mill

iseco

nds


Network history not collected GDMC exhibits lowest delay


Multiple Traffic Classes

Number of Nodes (in each traffic class)

Th

roughput

Rati

o

Throughput is visually distinct


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


Questions?

Thanks!

Contact:[email protected]

mailto:[email protected]

Multi-Class QoS in 802.11 Networks Using GDMC

Documents

Transcript of Multi-Class QoS in 802.11 Networks Using GDMC