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IEEE 802.11 EDCF: a QoS Solution for WLAN Javier del Prado 1 , Sunghyun Choi 2 and Sai Shankar 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. Outline. IEEE 802.11 WLAN

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IEEE 802.11 EDCF: a QoS Solution for WLAN Javier del Prado 1 , Sunghyun Choi 2 and Sai Shankar 1

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  1. IEEE 802.11 EDCF: a QoS Solution for WLAN Javier del Prado1, Sunghyun Choi2 and Sai Shankar1 1Philips Research USA - Briarcliff Manor, NY 2Seoul National University – Seoul, Korea Email: {javier.delprado,sai.shankar}@philips.com, sunghyun.choi@ieee.org

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

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

  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. The Enhanced DCF for QoS: EDCF EDCF Bursting

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

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

  13. Performance Evaluation

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

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

  17. DCF EDCF Simulation Results • A) Throughput

  18. DCF EDCF Simulation Results • B) Data Dropped

  19. DCF EDCF Simulation Results • C) Delay

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

  21. Throughput Data Dropped Simulation Results

  22. Video Delay Voice Delay Simulation Results

  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

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