Isolating Physical PER for Smart Rate Selection in 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 Introduction The core idea The core idea RCA modification RCA modification –Fragmentation implementation –RTS/CTS implementation Performance Performance Conclusion Conclusion

Introduction Rate selection algorithms in use in suffer from a fundamental problem Rate selection algorithms in use in suffer from a fundamental problem –Implicity or explicity make decisions bases on total Packet Error Rate (PER) False rate decision lowers the system throughput False rate decision lowers the system throughput

Introduction Total PER = PPER + CPER Total PER = PPER + CPER –PPER: Physical PER –CPER: contention PER It is often the case CPER dominates PPER 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 novel way to isolate the physical error Providing a modular way of isolating the contention error which any rate adaptation algorithm can be enhanced Providing a modular way of isolating the contention error which any rate adaptation algorithm can be enhanced

Fragmentation Only consider a packet is fragmented into 2 frames Only consider a packet is fragmented into 2 frames The fragment 2 is sent out only after an SIFS after the final ACK The fragment 2 is sent out only after an SIFS after the final ACK –Provided there are no hidden terminals

Fragmentation If the first fragment is lost, for nay reason, the second will not even be sent 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 Observation: the loss of a second fragment on the first transmission attempt –Physical error

RTS/CTS Once the RTS-CTS exchange is completed, the data packet will have the channel reserved, and will only encounter error for physical reasons 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 e An option in IEEE e An option in IEEE e The second data packet of such blocks could be used to directly measure PPER 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 Not to use these methods all the time in practice Only trigger their use often enough to gather good statistics on PPER 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 Only descriptors of second fragments are passed to the RCAs A mechanism is needed to trigger the fragmentation of selected packets 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 RTS/CTS protection is switched on by a per-packet decision Use Long Retry Count (LRC) to record the statistic of RTS/CST transmission Use Long Retry Count (LRC) to record the statistic of RTS/CST transmission

Performance Two main metrics Two main metrics –Weighted mean rate summaries –IP throughput All experiments are run in IEEE a in testbed All experiments are run in IEEE a 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 St[1-13] are stations, Sniff[1-2] are sniffers

Two implementation protocols SampleRate SampleRate –A MIT master thesis –Implemented on MadWifi AMRR 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

Conclusion The paper investigates two ways to isolate Physical PER 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 Higher channel rates were used when PPER was in fact low, even when contention is present

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