QoS Schemes for IEEE 802.11 Wireless LAN – An Evaluation

1. QoS Schemes for IEEE 802.11 Wireless LAN – An Evaluation by Anders Lindgren, Andreas Almquist and Olov Schelen Presented by Tony Sung, 10th Feburary 2004

2. Outline • Introduction – Why QoS? • Existing IEEE 802.11 MAC Algorithms • DCF and PCF • Proposed QoS Mechanisms • Enhanced DCF • Blackburst • Distributed Fair Queuing • Performance Comparison • Conclusion

3. Introduction – Why QoS? • Shared Medium • Medium Utilization can be Low • Collision is Possible • To Support Real-time / Multimedia Traffic • Require Service Differentiation -> QoS • Prioritization • Resource Sharing

4. Existing IEEE 802.11 MAC Algorithms Distributed Coordination Function (DCF) Mobile Stations try to Compete for Accessing the Medium Point Coordination Function (PCF) Access Point polls the Stations and Grant Access • Currently Two Methods are used to provide Medium Access:

5. Existing IEEE 802.11 MAC Algorithms ACK ACK Sense Medium( for DIFS ) DIFS DIFS Backoff( for a random time in [0, CW) ) Backoff Timeout Increase CWND & Backoff Has Data to Send Start TX Has Data to Send Start TX Backoff Timeout Backoff Timer Suspended Resume Has Data to Send Has Data to Send Start TX Increase CWND & Backoff Start TX • Distributed Coordination Function (DCF) • Based on CSMA/CA Algorithm • Sense the Medium before Sending • with Contention Window and Backoff Unknown Delay, Unknown Bandwidth, Low Medium Utilization

6. Existing IEEE 802.11 MAC Algorithms Contention Free Period (CFP) DIFS PIFS Backoff Contention-Free Period Start Send Beacon Frame Poll 1 Poll 2 Declare End of CFP Station 1:ACK & Data Station 2:ACK & Data • Point Coordinate Function (PCF) • Extends and Coexists with DCF • Controlled by Point Coordinator (i.e. Access Point) • Keeps a List of Stations to be Polled ACK Higher Utilization, But Delay and Bandwidth may still be an Unknown in High Load situation.

7. Proposed QoS Mechanisms • IEEE 802.11e Enhanced DCF • Stations wait for the Channel to become Idle for a pre-defined Time called Inter-frame Spacing (IFS) before sending • Shorter IFS will gain Higher Priority • When congested, Backoff time is determined by size of Congestion Window (CW) • Smaller CW will gain Higher Priority

8. Proposed QoS Mechanisms Backoff AIFS 1 Backoff Has Data to Send AIFS 2 Backoff Has Data to Send Start TX • IEEE 802.11e Enhanced DCF • Defines a new IFS called Arbitration IFS • Provide Packet Prioritization • Classifies Packets into 8 Different Traffic Classes, Each with different IFS and CW • Packet Bursting

9. Proposed QoS Mechanisms Winner PIFS High Priority Stations Start Black Bursting Winner TX Detects! Detects! Detects! • Blackburst • Reduce Delay Jitter of High Priority Traffics • Send out Black Burst by Jamming the Channel • Station that has Waited Longer sends Longer Normal Traffic

10. Proposed QoS Mechanisms • Distributed Fair Scheduling • Prioritization Completely Sacrifice Performance of Low Priority Traffic • DFS Provides Proportional Sharing Between Flows according to Assigned Weight • Utilizes the Backoff Mechanism of DCF

11. Proposed QoS Mechanisms • Distributed Fair Scheduling • Calculate Backoff Interval as follow: Smaller Packets have Higher Chance to be Sent Scale the Backoff Interval to a Reasonable Length Weight is Added Here Larger Weight means Smaller Backoff Interval, hence Higher Chance for Sending Random Variable to Provide Randomness of the Backoff Interval

12. Performance Comparison • Objective • Compare • Throughput, Medium Utilization, Collisions, Delay • Of • PCF ○ , EDCF △ , DFS ▓ , BB ● • Types of Traffic • High Priority (H-P) • 300 bytes (Normal Dist.) • 25ms Inter-packet Interval (96kb/s) • Low Priority (L-P) • 800 bytes (Normal Dist.) • 50ms Inter-packet Interval (128kb/s)

13. Performance Comparison • Average Throughput • All Schemes Achieved Similar Throughput for H-P Traffic • BB is best for # H-P Nodes < 13 • H-P Traffic Loss Performance 1st in DFS, while maintaining Finite Throughput for L-P Traffic • L-P Traffic Starves Rapidly

14. Performance Comparison • Medium Utilization • BB has Highest Peak, but Drops at Higher # H-P Nodes ( > 13) • EDCF and DFS has Substantially Low Utilization • Reasons in the next slides…

15. Performance Comparison • Overhead • High when # of H-P Nodes > 13 • =>Low Utilization

16. Performance Comparison • Collisions • EDCF Collides Easily • =>Low Utilization

17. Performance Comparison Many H-P Nodes,One CFP cannot Accommodate Many H-P Nodes,Packet Bursting Causes Low Delay for Bursting Packets, and Very High Delay for Waiting Packets Medium # of H-P Nodes Worst Case has Delay < 50ms Delay is Proportional to Packet Size, Span Out a Large Range Most Cases has Low Delay • Delay

18. Conclusion • Blackburst and EDCF • Starve L-P Traffic • Blackburst • Delay is minimal, Best for Real-time • Good at Avoiding Collision • EDCF • Starving can be Reduced if using same AIFS for all Traffic in EDCF -> Close to DFS Impl. • Already in IEEE 802.11e • DFS • Can be an Alternative if Starving L-P Traffic is Unfavorable • PCF • Polling Overhead is High

19. Thank You Questions are Welcomed

20. Appendix

21. Appendix