CMAP: Harnessing Exposed Terminals in Wireless Networks

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CMAP: Harnessing Exposed Terminals in Wireless Networks. Mythili Vutukuru Joint work with Kyle Jamieson and Hari Balakrishnan. u. v. x. y. z. The Problem. Which transmissions concurrently? Increase throughput by maximizing concurrency. X. u. v. y. z. x. Todays Solution: CSMA. - PowerPoint PPT Presentation

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  • CMAP: Harnessing Exposed Terminalsin Wireless NetworksMythili Vutukuru

    Joint work with Kyle Jamieson and Hari Balakrishnan

  • The ProblemWhich transmissions concurrently? Increase throughput by maximizing concurrency.

    X

  • Todays Solution: CSMAExposed terminal problem.Energy > carrier sense threshold

  • Key InsightExisting solutions: rules to predict which concurrent transmissions increase throughput.

    Instead, watch and discover which concurrent transmissions increase throughput.

  • CMAP: Conflict MapMap of conflicting transmissions based on empirical evidence.Built in distributed, online manner.

    Exposed terminals 2x gain on CSMA.u v & x y NO!

  • RoadmapCMAP DesignConflict mapsACK & Backoff PolicyImplementationEvaluation

  • What is a conflict?Conflict throughput lower when concurrent.Loss rate threshold to decide conflicts.uyxNo Conflict.Conflict.50% loss

  • Discovering ConflictsLoss rate of uv when x is concurrent >50% then infer conflict at v.Conflict entries timed out periodically.uyxWhen u transmits to me, x causes interference.pq

  • Populating the Conflict MapuyxWhen u transmits to me, x causes interference.Do not transmit to v when x anyone.Do not transmit to anyone when u v.Conflict mapz

  • Channel Access DecisionsNodes always overhear channel.Consult conflict map before transmission.Carrier sense always disabled!

  • RoadmapCMAP DesignConflict mapsACK & Backoff PolicyImplementationEvaluation

  • Windowed ACKsSliding window of packets at sender.X

  • Backoff PolicyCannot defer when hidden terminals.Exponential backoff.When loss rate in ACKs > threshold.uyxDo not transmit to v when x anyone.u must hear x.

  • RoadmapCMAP DesignConflict mapsACK & Backoff PolicyImplementationEvaluation

  • Implementation ChallengesAt receiver: Identify colliding senders.

    At sender: Identify ongoing transmissions.PHYMACTrailer

  • Implementation OptionsSoftware radios Partial Packet Recovery. [Jamieson and Balakrishnan, SIGCOMM 2007]Commodity hardware separate header and trailer packets.

    Header pktTrailer pkt

  • Prototype ImplementationMadWifi DriverAtheros 802.11 card Conflict MapsACKs & BackoffCSMA, ACKs & Backoff disabled.Click Kernel ModulePHYMAC

  • RoadmapCMAP DesignConflict mapsACK & Backoff PolicyImplementationEvaluation

  • Evaluation50-node 802.11a indoor testbed.Does CMAP improve throughput by increasing concurrency?

  • Two Senders In RangeSenders in range.1400-byte UDP @ 6 Mbps.50 unique sets of four nodes.CMAP, CSMA, no CS no acks.Exposed terminals.Interfering transmissions.

  • Two Senders In RangeCSMA better.No CS better.

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    1

    CSMA

    No CS, No Acks

    Aggregate Throughput (Mbits/s)

    CDF

    all2pairs-windowedACK

    all2pairs-windowedACK

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    11

    Ideal

    CMAP

    Ideal + ACKs

    Aggregate Throughput (Mbits/s)

    CDF

    all2pairs-idea&cmap

    all2pairs-idea&cmap

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    1

    Ideal

    CMAP

    Aggregate Throughput (Mbits/s)

    CDF

    all2pairs-ideal

    all2pairs-ideal

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    1

    CSMA

    No CS, No Acks

    Ideal

    Aggregate Throughput (Mbits/s)

    CDF

    all2pairs-cs only

    all2pairs-cs only

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