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Design, Implementation and Evaluation of an Efficient Opportunistic Retransmission Protocol

Design, Implementation and Evaluation of an Efficient Opportunistic Retransmission Protocol. Mei- Hsuan Lu Peter Steenkiste Tsuhan Chen MobiCom 09. Outline. Introduction Estimating link quality Protocol design Collision and fairness Multi-rate PRO Evaluation. Introduction.

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Design, Implementation and Evaluation of an Efficient Opportunistic Retransmission Protocol

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  1. Design, Implementation and Evaluation of an Efficient Opportunistic Retransmission Protocol Mei-Hsuan Lu Peter SteenkisteTsuhan Chen MobiCom 09

  2. Outline • Introduction • Estimating link quality • Protocol design • Collision and fairness • Multi-rate PRO • Evaluation

  3. Introduction • PRO - Protocol for Retransmitting Opportunistically • IEEE 802.11 WLAN • S<->D distance R S D

  4. Estimating link quality • Monitor success or failure of probe messages • Respond slowly to channel dynamics • Require extra bandwidth • Monitor SNR of packets at receiver • RSSI (received signal strength indicator) • Noisy • Thh

  5. Estimating link quality

  6. Protocol design • Relay qualification • Relay selection • Relay prioritization • Retransmission

  7. Protocol design • Relay qualification • Relay->destination ≠ destination->relay • Thh, on-line calibration • Relay selection (eligible relay) • Broadcast “I am qualified relay!” • Select the node has highest RSSI w.r.t destination • Add node nest highest … • Until the prob. of having a node hearing source > threshold Thr

  8. Protocol design • Relay prioritization • Higher RSSI w.r.t destination -> higher priority -> smaller CWmin(contention window size) • Retransmission • Lack of ACK -> retransmit • Retransmission fail -> double CW, contend for channel again • Terminate: an ACK heard or retry limit reached or a new packet arrived • Re-ACK : to avoid collision, send “null” data packet

  9. Collision and fairness • Collision • Limiting number of eligible relays • Fairness • More relays, more likely to gain access to channel • Mitigate unfairness: large initial CW, non uniform selection of time slot in CW

  10. Multi-rate PRO • Rate adaption – reduce packet error rate by lowering bitrates (no relay) • SampleRate : probe-based • CHARM : SNR-based • Combine PRO with CHARM • Transmission failed : eligible relay retransmit when its rate ≥ source rate (having better link quality) • Aggressive rate selection

  11. Evaluation • Emulation • Static • Overall • Per-relay • Mobile • fairness • Real world • Office building • Student lounge • 802.11g with multi-rate PRO

  12. Emulation - static • 3 environment scenarios • Freespace (outdoor) • Fading_k5 (small fading) • Fading_k0 (severe fading) • 5 mechanisms • 802.11 • 802.11 with CHARM • 802.11 with SampleRate • Mesh • Optimal PRO

  13. Overall

  14. Thr works well!

  15. Per-relay

  16. Emulation - mobile D S

  17. Emulation - fairness D1 D1 S1 S2 D2 S1 S2 D2 100m 100m 100m 50m

  18. Real world • Office building • Night • Student lounge • Day • Severe fading • Experiment • 10 laptops as nodes • Take turns as the source and send packet to other 9 nodes one by one • Nodes other than source and destination serve as relay

  19. High contention High fading

  20. Real world – 802.11g with multi-rate PRO High contention High fading

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