Multi-Class QoS in 802.11 Networks Using GDMC

1. 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

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 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 Multi-Class QoS in 802.11 using GDMC

4. DCF: Contention Window Contention Window (CW) CWmin CWcur CWmax (31) (1023) Backoff time • 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 Multi-Class QoS in 802.11 using GDMC

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

8. Observations • Use of Multiple Contention Windows Different CW for different traffic classes  Service differentiation Lesson: Use CW – one for each traffic class ! • Sequential Decrease of CWcur Large CWcur  recent collisions Lesson: Do not reset CWcur on success ! 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 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 CWc,LB and CWc,UB • Adjust these using network history • Gentle DCF (and Probabilistic DCF) • MIMD procedure for CW adjustment 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 Multi-Class QoS in 802.11 using GDMC

12. Observations • Use of Multiple Contention Windows Different CW for different traffic classes  Service differentiation Lesson: Use CW – one for each traffic class ! • Sequential Decrease of CWcur Large CWcur  recent collisions Lesson: Do not reset CWcur on success ! Multi-Class QoS in 802.11 using GDMC

13. The GDMC Scheme CW[c1] CWmin,c1 CWcur,c1 CWmax,c1 CWmin,c2 CWcur,c2 CWmax,c2 CW[c2] • One Contention Window for each class ‘c’ • Maintain: CWmin,c CWmax,c CWcur,c • Backoff time [c] = U~[CWmin,c : CWcur,c] c1: higher priority c2: lower priority Multi-Class QoS in 802.11 using GDMC

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

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

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

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

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

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

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