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2012 1/6 NSDI’08 Harnessing Exposed Terminals in Wireless Networks

2012 1/6 NSDI’08 Harnessing Exposed Terminals in Wireless Networks Mythili Vutukuru , Kyle Jamieson, and Hari Balakrishnan MIT Computer Science and Artificial Intelligence Laboratory. Introduction. A well-known way to maximize throughput: maximize the number of concurrent transmission

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2012 1/6 NSDI’08 Harnessing Exposed Terminals in Wireless Networks

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  1. 2012 1/6 NSDI’08 Harnessing Exposed Terminals in Wireless Networks MythiliVutukuru, Kyle Jamieson, and HariBalakrishnan MIT Computer Science and Artificial Intelligence Laboratory

  2. Introduction • A well-known way to maximize throughput: maximize the number of concurrent transmission • A problem in Carrier Sense Multiple Access (CSMA): Exposed Terminal • To improve throughput in a wireless network by harnessing exposed terminals, this paper proposesCMAP – ina distributed and lightweight way.

  3. Introduction

  4. Exposed Terminal AP1 AP2

  5. Overview of CMAP Key Insight: • Existing solutions: rules to predict which concurrent transmissions increase throughput (CSMA). • Instead: watch and discover which concurrent transmissions increase throughput (CMAP).

  6. CMAP Design Channel access: • The CMAP uses a distributed data structure called the conflict map • Use empirical observations of packet losses to populate the conflict map

  7. CMAP Design

  8. CMAP Design

  9. CMAP Design

  10. CMAP Design Windowed retransmission protocol • Link Layer ACK : stop-and wait retransmission protocol • CMAP: The ACKs sent by receivers are cumulative and contain a bitmap indicating which packets in the window have been received.

  11. CMAP Design Backoff policy: Hidden interference : receivers report the loss rate over a window of packets in every cumulative ACK, and senders back off when this loss rate exceeds a threshold. • loss rate < threshold : contention window(CW) = 0 • Loss rate > threshold : contention window(CW) = (CWstart,CWmax)

  12. CMAP Design • Handling Multiple Bitrates • annotate the interferer lists and defer tables with the bit-rates • The extensions to handle multiple power levels are similar. • Beyond Unicast Transmissions • Broadcast - treated as a collection of unicast transmissions • opportunistic routing - annotate the packet reception rates

  13. Implementation

  14. Evaluation • CMAP、CSMAenable、 CSMA disable

  15. Exposed terminals

  16. Senders in-range

  17. Senders out of range

  18. Access Point Topology • divide the testbedinto six “regions” • designate one node in each region as an AP • each AP is out of the communication range of every other AP • Clients : the set of nodes in that region that have a potential transmission link to that AP

  19. Access Point Topology

  20. Conclusions • presented the design, prototype implementation, and experimental evaluation of CMAP • CMAP uses empirical observations of packet loss to build a distributed data structure – conflict maps • CMAP successfully avoiding conflicting concurrent transmissions and increase the aggregate throughput • CMAP improves aggregate throughput by up to 47% and median per-sender throughput by 1.8× over 802.11

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