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Improving Wireless LAN Performance via Adaptive Local Error Control

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Improving Wireless LAN Performance via Adaptive Local Error Control. Presented by Yuanfang Cai. Outline. Local error control introduction Evaluations Simple local error control MAC & LLC design and implementation Experimental approach Results Adaptive local error control

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Improving Wireless LAN Performance via Adaptive Local Error Control

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  1. Improving Wireless LAN Performance via Adaptive Local Error Control Presented by Yuanfang Cai

  2. Outline • Local error control introduction • Evaluations • Simple local error control • MAC & LLC design and implementation • Experimental approach • Results • Adaptive local error control • MAC & LLC design and implementation • Experimental approach • Results • Summary

  3. Local versus End-to-end Error Control • Attractions: • Understand local characteristics • More efficient • Easier to deploy • Problems: • Confusing higher layer protocols • Undesirable interaction • Wasted Effort

  4. Design Tradeoffs for Local Error Control • Hardware error control • Simple • Can not differentiate flows • “Pure” link-layer approaches • Per-packet basis • Flow-aware • “Protocol-aware” link-layer protocols • Requires gateways to understand a wide variety of protocols. • “Gateway-style”/”indirect” error control • Might have to understand multiple protocols • Routing changes

  5. Simple local error control • MAC design • Master/slave transactions • INVITE and JOIN • POLL-DATA and DATA-ACK • LLC design • Entirely lost, partially lost, corrupted • Stop-and-wait retransmission

  6. Simple local error control—MAC and LLC design

  7. Simple local error control—Implementation and Performance • Intel 80486 and Pentium laptops using 915 MHz PCMCIA card WaveLAN units • NetBSD Unix • 43% throughput loss

  8. Simple local error control--Experimental Approach Single Hop Ethernet + wireless WAN extension Client: 75 MHz Pentium Toshiba Satellite pro 400CDT Wireless Host Basestation 25 MHz 80486 DEC pc-4255SL

  9. Evaluation—Pure local error control • Pattern-based evaluation • Packet killer • Basic robust evaluation • TCP without local error control • TCP with local error control • Broader scenarios • Ethernet + wireless • WAN extension • Competing TCP streams

  10. TCP without local error control

  11. TCP without local error control

  12. TCP with local error control

  13. TCP with local error control

  14. Ethernet + wireless

  15. WAN extension

  16. Competing TCP streams

  17. Simple local error control--Analysis • Steady state conditions (Assume that TCP is stable) • Lost packets always indicate congestion. • Avoid packet reordering • Don’t have long delay • Dynamic error environment • Upgrade • Degrade

  18. Simple local error control--Analysis • Persistence of local error control • Perpetual retransmission • Give up after a few transmissions • The higher error environment, the more persistent the retransmission need to be. • Packet Delay by persistent local retransmission

  19. Simple local error control--Analysis

  20. Simple local error control--Analysis

  21. Simple local error control--Analysis

  22. Simple local error control--Analysis • 3% overlap • End-to-end retransmission timeouts should be substantially longer than the single-hop round-trip time • TCP features that allow persistent retransmission with a small efficiency loss • Delay variation • Cautious minimum timeout • Slow-start probing

  23. Adaptive local error control • LLC Design • Add FEC and packet shrinking • Packet truncation • Rare for short packets • Bit corruption • Have only a few bit errors • Packet Shrinking • Forward Error Correction (FEC) • Reed-Solomon codes • Observe the quality of the link • Tell slaves using POLL-DATA • Employ adaptive policies

  24. Adaptive local error control • LLC Implementation • Implement packet shrinking through packet segmentation and reassembly • Data transmission: • Add to the packet sequence number: • starting byte offset, • a byte count • a packet complete bit • Acknowledgement: • A package sequence number • A cumulative length indicating correctly received bytes • Rare for short packets • Emulates the effects of Forward Error Correction (FEC)

  25. Adaptive local error control • Static Policies • BOLD—Without coding or shrinking • LIGHT—5% coding overhead • Robust—Sends minimally-sized packets with nearly 1/3 of each devoted to coding overhead. • Adaptive policies • BIMODAL • BOLD in good conditions • ROBUST in poor conditions • BI-CODE—BIMODAL that only adjust coding overhead • BI-SIZE—BIMODAL that only adjust coding overhead • FLEX—adapts the packet size and degree of FEC redundancy independently

  26. Adaptive local error control

  27. Adaptive local error control

  28. Adaptive local error control

  29. Summary • “Pure” link-layer local error control mechanism can greatly increase the efficiency of data transfer in wireless LAN’s. • Flow-aware instead of Protocol-aware • Simple adaptive policies outperformed static policies across a range of error environments.

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