1. IEEE 802.11 EDCF: a QoS Solution for WLAN Javier del Prado 1, Sunghyun Choi 2 and Sai Shankar 1 1 Philips Research USA - Briarcliff Manor, NY 2 Seoul National University – Seoul, Korea Email: {javier.delprado,sai.shankar}@philips.com, sunghyun.choi@ieee.org

2. 2 Outline IEEE 802.11 WLAN IEEE 802.11e EDCF EDCF Bursting Performance Evaluation Conclusions

3. 3 IEEE 802.11 WLAN IEEE 802.11 MAC  Can be considered a wireless version of Ethernet  Best-Effort Traffic  Referred as legacy MAC Currently the IEEE 802.11 Working Group is defining a supplement to support Quality of Service (QoS): IEEE 802.11e MAC  Multimedia services

4. 4 IEEE 802.11 WLAN MAC MAC is based on logical functions:  Distributed Coordination Function (DCF) – Based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)  Point Coordination Function (PCF) – Poll and Response mechanism MAC works with a single FIFO queue

5. 5 The DCF of the 802.11 MAC Distributed MAC based on local assessment of the channel: is the medium busy?

6. 6 Backoff Process Each station maintains its Contention Window (CW) value to select the Backoff Count (BO) The BO is a pseudorandom integer drawn from [0,CW] The CW is determined as follows:  Originally is assigned CWmin  After unsuccessful transmission: CW := 2 · (CW + 1) – 1  Upper Bound of CWmax  CW is reset to CWmin after successful transmission

7. 7 The Enhanced DCF for QoS: EDCF EDCF Bursting

8. 8 The Enhanced DCF (EDCF) Extension of the legacy DCF MAC for QoS Defined in 802.11e Draft Ability to differentiate frames with different priorities  Each frame from higher layers carries its user priority (UP) value: supported up to 8 UPs Provides differentiated channel accesses to frames with different UPs

9. 9 EDCF AIFS[UP] and CWmin[UP] instead of DIFS, and CWmin

10. 10 EDCF Contention Parameters AIFS[UP]  PIFS to protect Acknowledgement (ACK) transmission AIFS[UP] and CWmin[UP] announced by AP in beacon frames The smaller AIFS[UP], CWmin[UP] the shorter the channel access delay for UP

11. 11 EDCF Multiple Queues Multiple FIFO queues in the MAC: up to 8 queues Every queue is an independent contention entity with its own contention parameters

12. 12 EDCF Bursting EDCF TXOP limit announced in beacon frames by AP Legacy 802.11 allows single frame transmission EDCF Bursting: allows transmission of multiple frames within a time limit called Transmission Opportunity (TXOP)

13. 13 Performance Evaluation

14. 14 Simulation Parameters IEEE 802.11b PHY layer: 11 Mbps physical rate 3 types of traffic

15. 15 Simulation Parameters EDCF parameters per traffic type May not be optimal parameters. These should be adapted dynamically by the AP Data traffic type parameters are equivalent to legacy DCF

16. 16 DCF vs. EDCF 4 voice stations 2 video stations 4 data stations Every Station sends a single traffic type

17. 17 Simulation Results A) Throughput DCF EDCF

18. 18 EDCF Simulation Results B) Data Dropped DCF

19. 19 Simulation Results C) Delay DCFEDCF

20. 20 EDCF Bursting 4 voice stations 4 video stations EDCF TXOP limit = 3.5 ms  2 video frames at 11 Mbps

21. 21 Simulation Results Throughput Data Dropped

22. 22 Simulation Results Video Delay Voice Delay

23. 23 Conclusions Comparison between DCF and EDCF  EDCF can provide differentiated access among different user priorities Evaluated EDCF Bursting  Increases throughput performance at the cost of larger delays for voice  Reduces contention overhead Admission control unit and traffic policer are needed  More than two video stations can not be accommodated