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A Multichain Backoff Mechanism for IEEE 802.11 WLANs

A Multichain Backoff Mechanism for IEEE 802.11 WLANs. Alkesh Patel & Hemant Patel ECE 695 – Leading Discussion By : Shiang- Rung Ye and Yu-Chee Tseng. Background. WLAN is emerging as a promising technology. MAC plays an important role on efficient and fair use of the wireless medium.

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A Multichain Backoff Mechanism for IEEE 802.11 WLANs

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  1. A Multichain Backoff Mechanism for IEEE 802.11 WLANs Alkesh Patel & Hemant Patel ECE 695 – Leading Discussion By : Shiang- Rung Ye and Yu-Chee Tseng

  2. Background WLAN is emerging as a promising technology. MAC plays an important role on efficient and fair use of the wireless medium. Multiple Access Scheme (CSMA) require station to sense carries on the wireless channel before transmitting

  3. Related Work • MILD increases the contention window by 1.5 times when collision occurs & decreases the contention window by 1 when transmission succeed • In high traffic load this increases collusion probability and decreases throughput • DCF of IEEE 802.11 is a variant of persistent CSMA • If medium is busy, transmission defer until the medium becomes idle • Physical Carrier Sense  Physical layer • Virtual Carriers Sense  MAC

  4. Concept/Idea • MCB – Multi Chain Backoff • Enables station to adapt to different congestion level • No restriction on number of contending stations • High throughput and fair channel access

  5. MCB Algorithm • During back off period station shall detect any collision event by other station • Collision flag fcol is used to record whether frame collision occurs on the wireless channel • fcol set to 1 if a station itself experiences a collision or medium is busy – Collision/Transmission occurring with other station • fcol set to 0 after successful transmission. Backoff counter reaches 0, the station transmit data.

  6. MCB Algorithm Figure 1: The transition diagram of MCB. The j- th backoff stage of chain i is denoted by (i, j) in the figure

  7. MCB Algorithm wi: The minimum contention window of chain i mi: The maximum back off stage of chain i ui: The transition probability from chain ito chain i+1 vi: The transition probability from chaini to chain i -1

  8. Performance Analysis • Saturation Throughput S • (i, j, k) The station is in stage j of chain i & has a backoff value k. • T  Probability that station will transmit in a randomly chosen backoff slot • Xi  Probability that a station will detect at least one collision during backoff period

  9. Performance Analysis • Expressing collision probability can be expressed by T • Now the saturation throughput can be obtained by • Ptr Probability that transmission occurs randomly • Ps  Probability that transmission succeeds in a backoff slot • Ts Time require for frame exchange • Tc Length of a colliding duration

  10. Performance Evaluation • Performance opposed to MILD, DCF, GDCF, EIED • Comparison of saturation throughput & fairness index • FI is bounded in the interval [0,1] • Algorithm is fair as its FI is close to 1 • Optimal value of u and v when they are the same

  11. Fig 3: The optimal u and v with frame size 1024

  12. Fig 4: The ratio of optimal u and v under different n and c

  13. Fig 5: Saturation throughput under different u and v with a frame size of 1024 bytes

  14. Fig 6: Saturation throughput under different u and v with a frame size of 128 bytes

  15. Fig 7: Saturation throughput versus frame sizes

  16. Fig 8: Throughput of MCB with u and v which are chosen for n=6 and n=46

  17. Fig 9: Fairness Index

  18. Fig 10: Saturation throughput of MCB and GDCF with frame size 128 bytes

  19. Fig 11: Saturation throughput of MCB and GDCF with frame size 1024 bytes

  20. Fig 12: Fairness index of MCB and GDCF with frame size 1024 bytes

  21. Fig 13: Saturation throughput of MCB, IEEE 802.11, and MILD

  22. Fig 14: Saturation throughput of MCB and EIED(x, y) with fixed y=2Fig 15: Throughput of MCB and EIED(x, y) with fixed x =2 Fig: 14 Fig: 15

  23. Fig 15: Throughput of MCB and EIED(x, y) with fixed x=2

  24. Fig 16: Fairness index of MCB and EIED (x, y) with fixed x = 2

  25. Conclusion • MCB algorithm explores the possibility of using multiple backoff chains • Considering collision event offers • To choose a proper chain for transmission • Capability of switching to different backoff chain • High throughput then any existing algorithms • Fair access to the wireless channel • How to apply Multichain concept to an error-prone wireless channel to resolve the issue would be the next step in future development

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