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Throughput Enhancement Through Dynamic Fragmentation in Wireless LANs

Throughput Enhancement Through Dynamic Fragmentation in Wireless LANs. Byung-Seo Kim, Yuguang Fang, Tan F. Wong, and Younggoo Kwon IEEE Transactions on Vehicular Technology Vol. 54, No. 4, July 2005. Outline. Introduction Proposed protocol Fragmentation Scheme Fragment burst NAV update

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Throughput Enhancement Through Dynamic Fragmentation in Wireless LANs

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  1. Throughput Enhancement Through Dynamic Fragmentation in Wireless LANs Byung-Seo Kim, Yuguang Fang, Tan F. Wong, and Younggoo Kwon IEEE Transactions on Vehicular Technology Vol. 54, No. 4, July 2005

  2. Outline • Introduction • Proposed protocol • Fragmentation Scheme • Fragment burst • NAV update • Performance evaluation • Conclusion

  3. Introduction • Wireless communication link in a WLAN is time varying • Rate-adaptive MAC protocols have been proposed in the past for WLANs • Channel condition • RTS/CTS handshake

  4. Introduction (cont.) • Receiver based rate determination • RTS • CTS (selected rate information) • DATA • Overhearing nodes update their NAV • ACK

  5. Introduction (cont.) • Fragmentation in IEEE 802.11 MAC with rate adaption scheme > aFragmentThreshold Add PLCP Header and Preamble

  6. Introduction (cont.) • These proposed protocols only allowed static size of fragments • Higher overhead of transmitting each fragment • Channel could not be used effectively

  7. Proposed protocol • A dynamic fragmentation scheme to enhance throughput • Durations of all fragments, except the last one, should be set the same in any data rate • A Rate-Based fragmentation thresholding scheme is employed • A new fragment is generated only when the rate is decided for the next fragment transmission • Dynamic Fragmentation

  8. Fragmentation Scheme • In order to generate fragments with the same time duration, the different aFragmentationThresholds should be used in different data rate R

  9. Fragmentation Scheme (cont.) • The additional overhead of ThresholdB is

  10. Fragmentation Scheme (cont.)

  11. Fragment Burst Modified format of DSSS PLCP Header Current Rate (4) Next Rate (4)

  12. Fragment Burst (cont.) RTS (base rate) S R CTS (base rate) next rate is included DATA is transmitted with selected rate In aFragmentationThreshold size ACK (selected rate) next rate is included DATA is transmitted with selected rate In aFragmentationThreshold size ˙ ˙ ˙ RTS Fragment 1 Fragment 2 ˙˙˙ CTS ACK Next rate Next rate

  13. NAV update • Because the durations of all the fragments are the same, except the last fragment • MoreFragments = 1 • NAV update

  14. Failure policy • When the transmission of a fragment fails • The size of the retransmitted fragment may not be the same as before • Channel condition may have changed • Sender only decreases the remaining MSDU size when receives the ACK from the receiver

  15. Performance Evaluation • Single-hop environment • No hidden node • Transmission rage : 300 m • Rate selection

  16. Performance Evaluation (cont.) • 3 different configuration • RFT-DF • Rate-based • Dynamic • RFT-CF • Rate-based • Conventional • SFT-CF • Single fragmentation threshold • Conventional

  17. Throughput vs. number of nodes 4 m/s

  18. Packets per MSDU vs. number of nodesTime overhead vs. number of nodes

  19. PER vs. number of nodesMAC Service Time vs. number of nodes

  20. MSDU dropping rate vs. number of nodes

  21. Throughput vs. Node speed 40 nodes

  22. Throughput vs. Max. MSDU Size 4 m/s 40 nodes

  23. Throughput vs. Predictor Efficiency 4 m/s 40 nodes

  24. Conclusion • This paper proposes a new rate-adaptive MAC protocol with dynamic fragmentation • Nodes with good channels can transmit more data • Constant duration in physical layer simplifies the process of NAV update • Simulations show the throughput gain from the conventional scheme

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