Analysis of IEEE 802'11e for QoS Support in Wireless LANs

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Analysis of IEEE 802.11e for QoS Support in
Wireless LANs

• S. Mangold, S. Choi, G. Hiertz, B. Walke

Fontas Fafoutis fontas_at_csd.uoc.gr
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Contents

• Introduction
• Legacy IEEE 802.11
• IEEE 802.11e
• Evaluation
• Conclusion

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Introduction

• IEEE 802.11 is widely and rapidly spread
• Why?
• Simplicity, Robustness, Fairness
• Quality of Service (QoS)
• 802.11a/b/g support best effort service
• 802.11e enhances 802.11 legacy MAC protocol to
  support Service Differentiation

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Contents

• Introduction
• Legacy IEEE 802.11
• IEEE 802.11e
• Evaluation
• Conclusion

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IEEE 802.11 MAC Protocol

• Distributed Coordination Function (DCF)
• Carrier Sense Multiple Access (CSMA)
• Listen-before-talk
• Collision Avoidance
• Backoff procedure
• Optional RTS/CTS
• Hidden node problem
• Point Coordination Function (PCF)
• Contention Period (CP) and Contention-Free Period
  (CFP) alternate periodically over time
• Polling during CFP

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IEEE 802.11 DCF
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IEEE 802.11 PCF
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PCF for QoS support?

• Beacon delays
• AP informs when the next CFP will start but there
  are no guarantees that the channel will be idle
  then
• Unknown transmission duration
• Reduces QoS provided to other stations during CFP

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Contents

• Introduction
• Legacy IEEE 802.11
• IEEE 802.11e
• Evaluation
• Conclusion

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IEEE 802.11e

• Hybrid Coordination Function (HCF)
• Enhanced Distributed Channel Access (EDCA)
• HCF Controlled Channel Access (HCCA)
• Basic Improvements
• Each station cannot obtain medium access for more
  than transmission opportunity (TXOP)
• A frame can be initiated only if it can be
  completed before the upcoming PCF-equivalent
  beacon
• A station can transmit to other stations without
  involving communication with the AP using direct
  link protocol (DLP)

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Enhanced Distributed Channel Access (EDCA)

• 4 access categories (ACs)
• AC_VO (voice)
• AC_VI (video)
• AC_BE (best effort)
• AC_BK (background)
• Different Parameters per AC dynamically defined
  by AP via beacon frames
• Arbitration Interframe Space (AIFSAC) instead
  of DIFS
• Minimum Contention Window (CWminAC)
• Maximum Contention Window (CWmaxAC)
• Transmission Opportunity (TXOPlimitAC)
• When a collision occurs the higher priority
  station will not back off

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Access Categories
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Different Priorities
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HCF Controlled Channel Access (HCCA)

• Contention-Free Period
• Stations transmit only upon being polled
• Contention Period
• Stations use EDCA rules
• QoS CF-Poll (after PIFS, no backoff)
• Maximum duration (TXOPlimitAC) specified
  accordingly in order to avoid delays

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802.11e Periodic Superframe
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Efficiency Improvements

• Block Acknowledgements
• Optional
• Stations are allowed to transmit multiple data
  units consecutively without individual ACK frames
• At the end of the block data units are
  acknowledged together
• Reduced overhead
• Direct Link Protocol (DLP)
• Stations can communicate with other stations
  without communicating via the AP
• At best case scenario -gt Half channel capacity

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Contents

• Introduction
• Legacy IEEE 802.11
• IEEE 802.11e
• Evaluation
• Conclusion

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ECDA - Achievable Throughput

• 3 stations
• No RTS/CTS
• No hidden nodes
• 1 stream per class per station
• 24Mbps transmission rate

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ECDA - Achievable Throughput

• 16 stations
• 4 x 1 Mbps per station
• High collision probabilities
• Undesirable results
• However, AP can set new values for EDCA parameters

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QoS Guarantees

• EDCA delays can increase unpredictably
• HCCA delays remain below a threshold
• However, in overlapping scenarios delays can
  still be unpredictable

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Conclucion

• IEEE 802.11a/b/g cannot support QoS
• IEEE 802.11e extends legacy MAC protocol to
  provide relative QoS suport
• Experiments suggest that on isolated scenarios
  802.11e can be easily configured to provide
  desirable results

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Thank you

• Questions?