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IEEE 802.11 Rate Adaptation : A Practical Approcah

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ContentsIntroductionRelated work

Low and High Communication Latency SystemThe Adaptive ARF AlgorithmThe Adaptive Multi Rate Retry AlgorithmConclusion

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802.11 ComparasionThis journal published on 2004 (CARA is on 2006)

Protocol Year Modulation Throughput Frequency

802.11 1997 DSSS, FHSS 1 / 2 Mbps 2.4 GHz802.11b 1999 DSSS Up to 11 Mbps 2.4 GHz802.11a 1999 OFDM Up to 54 Mbps 5 GHz802.11g 2003 OFDM, DSSS Up to 54 Mbps 2.4 GHz

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Volatile nature of wireless mediumDistance Effects :

Attenuation

FadingInterference

AccessPoint Station

Throughput decrease

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Parameter that can be OptimizedPower Consumption

Since there is finite battery life

ThroughputRate to high will increase Bit Error RateRate to low means under utilized capacity

Paper will cover only how to maximize throughput

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ContentsIntroductionRelated work

Low and High Communication Latency SystemThe Adaptive ARF AlgorithmThe Adaptive Multi Rate Retry AlgorithmConclusion

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ARF Auto Rate Fallback Designed for Wavelan II (802.11,DSSS)Use packet transmission situation to estimate thechannel condition:

If two consecutive ACK frames are not received correctly,lower the rate and a timer is started . When the number of successfully received ACKs reaches

10 or the timer goes off , a probe frame is sent at the nexthigher rate.However, if an ACK is NOT received for this frame, therate is lowered back and the timer is restarted.

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ARF Weakness :

Constantly up-shift try when channel condition is stable

If the channel conditions change very quickly, it cannotadapt effectively

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RBARRTS and CTS are modified.Uses RTS reception to estimate SNR (Signal To Noise)

then pick SNR thresholdPick Rate based on SNR thresholdsSend choice back to sender in CTS.

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RBAR Weakness

Requires incompatible changes to the IEEE standard

SNR is not easy to get (most WLAN doesn’t have it) RTS/CTS is required

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ContentsIntroductionRelated work

Low and High Communication Latency SystemThe Adaptive ARF AlgorithmThe Adaptive Multi Rate Retry AlgorithmConclusion

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Communication LatencyThe algorithm must be able to choose rate betweenend of a successful Transmission and the start of thefollowing TransmissionTime to choose :

Tsuccess equals DIFST failure equal DIFS + ACKTimeout

Since Tsuccess less than T failure the time used will be T success It means that rate to be usedmust be decided within28 µs (802.11g)

Standard T success

802.11a 34 µs802.11b 50 µs

802.11g 28 µs

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Low Latency Allow implement per packet adaptationIt means, wait for feedback before sending next packet

Example : Wave LAN 802.11bBecause dedicated CPU can answer real-timerequirements

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High LatencyCannot handle per packet adaptationExample:

AR5212CPU might be able to handle real timeBut the OS Can’t – non real-time OS

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ContentsIntroductionRelated work

Low and High Communication Latency SystemThe Adaptive ARF AlgorithmThe Adaptive Multi Rate Retry AlgorithmConclusion

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AARFImprovement based on ARF.To fix one existing problem of ARF (Constantly up-

shift try when channel condition is stable)

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ARF and AARF Comparasion

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Simulation Environmentns-2 network simulator with extensions from the CMUMonarch project and the RBAR implementation

It does not directly support infrastructure networkUse workaround using 2-node ad hoc network

AccessPoint StationStation

60 Seconds

5 m

50000 packet 0.46ms apart

50000 packet 0.46ms apart

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ContentsIntroductionRelated work

Low and High Communication Latency SystemThe Adaptive ARF AlgorithmThe Adaptive Multi Rate Retry AlgorithmConclusion

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The AR 5212 chipsetTransmission :Transmission starts with r 0. If fails, it will try to send with rate r 0 , c0 - 1 timesIf keeps falling, it will try to use r 1, c1 times, then r 2 , c2times and finally r 3 , c3 timesIf transmission failed c 0 , c1 , c2 , c3 times thetransmission is abandoned.

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The MadWifi algorithmThe short term variations uses c 0 = 4, c1= 2, c2 = 2, c3 =2. While long term variations are handled by changingthe value of the r 0/c 0 , r1/c 1, r2/c 2 , r3/c 3 at regular fixedintervals.

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The AMRR algorithmUse c0 = 1, c1= 1, c2 = 1, c3 = 1 to ensure short-term variations of the wireless medium are quickly actedRate r

3 will be the lowest rate

Rate r 1and r 2 are determined by rate r 0

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Performance Evaluation (Implementation Issues)

MadWifi driver initialize the transmission descriptorsin FIFO only once When a PER (Packet Error Rate) is low, it will increasethe rate.However, next transmission is still use the old rate.Therefore, it will be increased again.

At some point, rate will be to high and most likely fail which will make the rate decreased quickly.This paper modified the driver to parse thetransmission FIFO each time a rate change happens.

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Experimental Results (real world use case)Set up using approximate as closely as possible real- world use cases

802.11b/g Access PointProxim Orinoco Gold pcmcia card based on AR5212 (setup in another office approximately 10 meters)Modified 2.6.5 Linux kernel and a RedHat 8.0

Start 600 second continuous 30 Mbps UDP streamExecuted the experiment 5 times

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ContentsIntroductionRelated work

Low and High Communication Latency SystemThe Adaptive ARF AlgorithmThe Adaptive Multi Rate Retry AlgorithmConclusion

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ConclusionSimulation shows that this algorithm performs similar with impractical RBARIn infrastructure networks AMRR algorithm improveachievable performanceSuffer from random collisions and hidden terminals

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Reference[1] http://en.wikipedia.org/wiki/IEEE_802.11#802.11-1997_.28802.11_legacy.29[2] IEEE 802.11 Rate Adaptation: A Practical Approach[3] Dynamic Rate Adaptation in IEEE 802.11 WLANs Yilin Song, ICT of China Academy of Science