1. Multi-Class QoS in 802.11 Networks Using GDMC IEEE Globecom 2007 – Washington, DC Friday, November 30, 2007 Bushra Anjum North Carolina State University Authors: Bushra Anjum and Zartash Afzal Uzmi School of Science and Engineering, LUMS, Pakistan

2. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 2 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

3. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 3 802.11 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

4. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 4 DCF: Contention Window CW cur may vary from CW min to CW max Backoff time is random from CW Single CW for all traffic in DCF –No support for multiple traffic classes CWminCWcurCWmax (31) (1023) Backoff time Contention Window (CW)

5. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 5 DCF: CW Management CWminCWmax (31) (1023) Contention Window (CW) CWminCWmax (31) (1023) Contention Window (CW) Failed Attempt to Transmit CWcur After Successful Transmission CWcur is doubled CWcur is reset to CWmin

6. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 6 802.11 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

7. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 7 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

8. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 8 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 CWcur Large CWcur  recent collisions Lesson: Do not reset CWcur on success !

9. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 9 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

10. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 10 Example Schemes Predictive DCF –Backoff time based on network history Sliding Contention Window (SCW) –For each traffic class ‘c’ Keep CW c,LB and CW c,UB Adjust these using network history Gentle DCF (and Probabilistic DCF) –MIMD procedure for CW adjustment

11. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 11 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

12. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 12 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 CWcur Large CWcur  recent collisions Lesson: Do not reset CWcur on success !

13. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 13 The GDMC Scheme One Contention Window for each class ‘c’ Maintain: CW min,c CW max,c CW cur,c Backoff time [c] = U~[CW min,c : CW cur,c ] CWmin,c1CWcur,c1CWmax,c1 CW[c1] CWmin,c2CWcur,c2CWmax,c2 CW[c2] c1: higher priority c2: lower priority

14. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 14 GDMC: CW Management CWmin,cCWmax,c Contention Window (CW) CWmin,cCWmax,c Contention Window (CW) Failed Attempt to Transmit CWcur,c After Successful Transmission CWcur,c is doubled CWcur,c is halved

15. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 15 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

16. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 16 Throughput: High Priority Simulation Time in seconds Throughput Ratio No wait time in GDMC for gathering history GDMC performs better than SCW

17. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 17 Throughput: Medium Priority Throughput Ratio Simulation Time in seconds Once again, GDMC performs better than SCW and others

18. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 18 Throughput: Low Priority Throughput Ratio Simulation Time in seconds DCF outperforms all other schemes – as expected

19. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 19 Delay Characteristics Delay in milliseconds Simulation Time in seconds Network history not collected  GDMC exhibits lowest delay

20. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 20 Multiple Traffic Classes Number of Nodes (in each traffic class) Throughput Ratio Throughput is visually distinct

21. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 21 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

22. November 30, 2007 Multi-Class QoS in 802.11 using GDMC 22 Questions? Thanks! Contact: banjum@ncsu.edu