Isolating physical per for smart rate selection in 802 11
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Isolating Physical PER for Smart Rate Selection in 802.11. Malik Ahmad Yar Khan and Darryl Veitch ARC Special Centre for Ultra-Broadband Information Networks (CUBIN) The University of Melbourne, Australia IEEE Infocom 2009 (acceptance rate = 19.65%). Outline. Introduction The core idea

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Isolating Physical PER for Smart Rate Selection in 802.11

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Isolating Physical PER for Smart Rate Selection in 802.11

Malik Ahmad Yar Khan and Darryl VeitchARC Special Centre for Ultra-Broadband Information Networks (CUBIN)The University of Melbourne, AustraliaIEEE Infocom 2009(acceptance rate = 19.65%)


  • Introduction

  • The core idea

  • RCA modification

    • Fragmentation implementation

    • RTS/CTS implementation

  • Performance

  • Conclusion


  • Rate selection algorithms in use in 802.11 suffer from a fundamental problem

    • Implicity or explicity make decisions bases on total Packet Error Rate (PER)

  • False rate decision lowers the system throughput


  • Total PER = PPER + CPER

    • PPER: Physical PER

    • CPER: contention PER

  • It is often the case CPER dominates PPER

    • Poor latency, loss and throughput

PER under a ideal environment

Goal of the paper

  • Providing a novel way to isolate the physical error

  • Providing a modular way of isolating the contention error which any rate adaptation algorithm can be enhanced


  • Only consider a packet is fragmented into 2 frames

  • The fragment 2 is sent out only after an SIFS after the final ACK

    • Provided there are no hidden terminals


  • If the first fragment is lost, for nay reason, the second will not even be sent

  • Observation: the loss of a second fragment on the first transmission attempt

    • Physical error


  • Once the RTS-CTS exchange is completed, the data packet will have the channel reserved, and will only encounter error for physical reasons

Block reservation in 802.11e

  • An option in IEEE 802.11e

  • The second data packet of such blocks could be used to directly measure PPER

Main concept

  • Not to use these methods all the time in practice

  • Only trigger their use often enough to gather good statistics on PPER

Scatterplots of PER with no cross traffic

Scatterplots of PER with CT and fragmentation

Scatterplots of PER with CT and fragmentation (RTS/CTS on)

Rate dependence of IP throughput

Fragmentation implementation

  • Only descriptors of second fragments are passed to the RCAs

  • A mechanism is needed to trigger the fragmentation of selected packets

    • Maintains a simple ON/OFF variable

RTS/CTS implementation

  • RTS/CTS protection is switched on by a per-packet decision

  • Use Long Retry Count (LRC) to record the statistic of RTS/CST transmission


  • Two main metrics

    • Weighted mean rate summaries

    • IP throughput

  • All experiments are run in IEEE 802.11a in testbed

    • Full rate set:{6,9,12,18,24,36,48,54}

    • Packet size: 1500 bytes

    • 10 stations if no further explanation

The wireless testbed

  • St[1-13] are stations, Sniff[1-2] are sniffers

Two implementation protocols

  • SampleRate

    • A MIT master thesis

    • Implemented on MadWifi

  • AMRR

    • A multi-rate algorithm also implemented on MadWifi

Single user with different RCAs

TPER for single user case for AMRR

SampleRate with different packet sizes (1472B)

SampleRate with different packet sizes (716B)

UDP upstream system throughput

TCP upstream system throughput


  • The paper investigates two ways to isolate Physical PER

    • RTS/CTS

    • MAC level packet fragmentation

  • Higher channel rates were used when PPER was in fact low, even when contention is present

Thank you!!

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