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QoS Support in 802.11: Contention-Free MAC Perspective

QoS Support in 802.11: Contention-Free MAC Perspective. Sunghyun Choi Philips Research-USA Briarcliff Manor, New York. QoS Support in 802.11 WLAN. Approach: a contention-free (CF) MAC on top of DCF, e.g., PCF, MediaPlex, Whitecap Problems/Limitations of PCF Lesson for .11e MAC

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QoS Support in 802.11: Contention-Free MAC Perspective

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  1. QoS Support in 802.11: Contention-Free MAC Perspective Sunghyun Choi Philips Research-USA Briarcliff Manor, New York Sunghyun Choi, Philips Research

  2. QoS Support in 802.11 WLAN • Approach: a contention-free (CF) MAC on top of DCF, e.g., PCF, MediaPlex, Whitecap • Problems/Limitations of PCF • Lesson for .11e MAC • Protecting CF access of a BSS from • DCF-running STAs in the BSS • Overlapping BSSs running in the same channel Sunghyun Choi, Philips Research

  3. Problems Identified • Beacon collisions: CF-Period performance degraded significantly • Contention from overlapping BSSs: Contention-free access not guaranteed during CF-Period • ACK transmission problem: RTS/CTS not working properly Sunghyun Choi, Philips Research

  4. Papers Submitted/Presented • 00/107: “Beacon collisions in IEEE 802.11 WLAN” • 00/108: “Protecting QoS-enabled BSSs in situations of overlapping BSSs in IEEE 802.11 WLAN” • 00/109: “ACK transmission problem in 802.11 WLAN” Sunghyun Choi, Philips Research

  5. Beacon Collision Problem • CP and CFP over time • CFP starts with a beacon transmission • Inconsistent descriptions found in 802.11-1999 Sunghyun Choi, Philips Research

  6. Why Beacon Collision Happens? • Due to non-zero processing delays: • RxTX turn-around delay, PLCP delays, RF delays TBTT Collision Margin Channel idle for at least DIFS interval STA 1 Data t1 t2 TBTT Channel idle for at least PIFS interval AP Beacon t3 Sunghyun Choi, Philips Research

  7. Why Beacon Collision Bad? • Collision extended to CFP TBTT DIFS STA 1 Data Collision duration TBTT SIFS PIFS AP Beacon Data + Poll TBTT DIFS RTS STA 2 Collision Duration Sunghyun Choi, Philips Research

  8. Extremely Bad Case • Whole CFP in Collision TBTT DIFS STA 1 Data Collision duration TBTT PIFS PIFS PIFS SIFS PIFS CF-End Poll2 Poll3 Poll1 AP Beacon Sunghyun Choi, Philips Research

  9. AP Channel Sensing Proposed Solutions • Three Components S1, S2, and S3 TBTT PIFS PIFS Combined solutions S1 and S2 AP Beacon Data + Poll S2 S1 TBTT PIFS PIFS Solution S3 AP BNCI BCI + Data + Poll S3 BNCI: Beacon with No Critical Information BCI: Beacon with Critical Information Sunghyun Choi, Philips Research

  10. Protecting QoS-Enabled BSS • Overlapping BSSs: • Contention-free access not guaranteed any more • Required Features of .11e MAC • Avoiding overlapping BSSs • Avoiding contention from overlapped BSSs • Communications among APs • RTS/CTS during CFP • Handling contention from overlapped BSSs • Error control & packet scheduling Sunghyun Choi, Philips Research

  11. Example: Overlapping BSSs • STA2,1 can hear AP1, so beacon heard STA1,2 AP1 AP2 STA2,1 STA2,1 STA1,1 Sunghyun Choi, Philips Research

  12. Example2: Overlapping BSSs • STA2,1 cannot hear AP1, so beacon not heard STA1,2 AP1 AP2 STA2,1 STA1,1 Sunghyun Choi, Philips Research

  13. RTS/CTS during CFP • Data from AP to STA AP1 RTS Data to STA1,1 STA1,1 CTS Set up the NAV STA2,1 Sunghyun Choi, Philips Research

  14. RTS/CTS during CFP (cont.) • Data from STA to STA per being polled AP1 Poll STA1,1 RTS Data to STA1,2 STA1,2 CTS Set up the NAV STA2,1 Sunghyun Choi, Philips Research

  15. When to Use RTS/CTS? • Feedback from STAs to AP on overlapping BSS existence • AP detection of overlapping BSSs • Threshold frame size; c.f., RtsThreshold • Threshold frame loss: also handling bursty errors efficiently Sunghyun Choi, Philips Research

  16. Limitations of RTS/CTS ? • RTS/CTS exchange failure not detected • STA2,1 receives when STA1,1 sends CTS • ACK transmission when non-zero NAV • STA2,1 sends an ACK while STA1,1 receiving • BSS2 in CFP based on PCF • STA2,1 will respond to AP2while STA1,1 receiving Sunghyun Choi, Philips Research

  17. ACK Transmission Problem • Example: IBSS STA3 STA0 STA2 STA1 Sunghyun Choi, Philips Research

  18. RTS/CTS Failure • STA2 sends ACK when non-zero NAV RTS DATA to STA1 STA0 T0 T2 Detect a collision CTS STA1 T1 ACK Set up the NAV STA2 T1’ T4 DATA to STA2 STA3 T3 Sunghyun Choi, Philips Research

  19. Proposed Solutions • Change in DCF • ACK not transmitted when non-zero NAV • Exceptions: • Responding to a DCF frame that causes stretching to the CFP • Responding to a CF-Poll of a non-CF-pollable STA during CFP • Different from CF-ACKs Sunghyun Choi, Philips Research

  20. Conclusion • Problems on contention-free MAC approach for QoS support • Proposal to protecting CF access from • STAs in same BSS • STAs in overlapping BSSs • 2 modifications in PCF or .11e MAC • 1 modification in DCF Sunghyun Choi, Philips Research

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