收入费用利润 - Asia Future Internet Forum.ppt
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1、Dynamic Rate Adaptation in IEEE 802.11 WLANs,SongYiLinICT August 10, 2008,References,1 On the Performance Characteristics of WLANs: Revisited (SIGMETRICS 2005) 2 CARA: Collision-Aware Rate Adaptation forIEEE 802.11 WLANs (INFOCOM 2006) 3 Robust Rate Adaptation for 802.11 Wireless Networks (MOBICOM 2
2、006) 4 IEEE 802.11 Rate Adaptation: A Practical Approach (MSWiM 2004) 5 Link Adaptation Strategy for IEEE 802.11 WLAN via Received Signal Strength Measurement (ICC 2003),Outline,Introduction of Rate Adaptation in IEEE 802.11 WLANsExisting ChallengesDevelopment of Rate Adaptation AlgorithmsPresentati
3、ve old onesNew Kids in this areaMy Opinion,Outline,Introduction of Rate Adaptation in IEEE 802.11 WLANsExisting ChallengesDevelopment of Rate Adaptation AlgorithmsPresentative old onesNew Kids in this areaMy Opinion,802.11 MAC (with RTS/CTS on),2,RTS: 20 bytes in mac,CTS: 14 bytes in mac,What is “Ra
4、te Adaptation” ?,The 802.11 a/b/g/n standards allow the use of multiple transmission rates802.11b, 4 rate options (1,2,5.5,11Mbps)802.11a, 8 rate options (6,9,12,18,24,36,48,54 Mbps)802.11g, 12 rate options (11a set + 11b set)The method to select the transmission rate in real time is called “Rate Ad
5、aptation”Rate adaptation is important yet unspecified by the 802.11 standards,Why do we need “Rate Adaptation” ?,AccessPoint,MN,Why do we need “Rate Adaptation” ?,AccessPoint,MN,Why do we need “Rate Adaptation” ?,AccessPoint,MN,Distance Effects :attenuationfading interference,SRN,Why do we need “Rat
6、e Adaptation” ?,5,BPSK 1MbpsQPSK 2MbpsBPSK/QPSK/CCKDifferent modulation schemes,Best throughput,How to adjust the rate ?,Rate adaptation plays a critical role to the throughput performanceIdeally, the transmission rate should be adjusted according to the ,throughput decreases,Rate too high,Loss rati
7、o increases,Rate too low,under-utilize the capacity,channel condition,How to estimate channel condition ?,SNR of the channelSNR of receiver no feedback in 802.11fluctuation of SNRGauging how well the currently chosen rate performs (Statistics: transmission loss/success)not timelyaffected by random c
8、ollisions (unnecessary downshift),easy to implement,receiver,sender,Modified frame,Symmetric link,Outline,Introduction of Rate Adaptation in IEEE 802.11 WLANsExisting ChallengesDevelopment of Rate Adaptation AlgorithmsPresentative old onesNew Kids in this areaMy Opinion,Channel Dynamics,Wireless cha
9、nnel exhibits rich channel dynamics in practical scenariosRandom channel errorMobility-induced changeCollisions induced byHidden-terminalsMultiple contending clientsRandom collisioncongestion,Channel Dynamics,When should the transmission rate be updated?too quick: perform bad when channel conditions
10、 fluctuate acutely too slow: not in time (base on a relative long history)The algorithm should be adaptive,Equidistant Distribution,AccessPoint,MN2,MN1,MN4,MN3,Node Contention Effects:Collisions induced by: random backoff hidden terminal,Equidistant Distribution- random collisions,1,means:unnecessar
11、y downshift,Equidistant Distribution- hidden terminal,3,means:unnecessary downshift,Hidden terminal broadcast packets at a mild rate of 0.379Mbps continuously while other nodes begin with 11Mbps,Non-equidistant Distribution,AccessPoint,MN3,Nodes Diversity Effects:collisionshidden terminalchannel div
12、ersity link capture,MN1,MN2,MN4,fairness,Non-equidistant Distribution- fairness,The poor-channel flows would consume more time and system resources,Different kinds of fairness:throughput fairnesstime-share fairnessIn single-rate networks: equivalentIn multi-rate networks: time-share fairness is the
13、one to be concerned,Outline,Introduction of Rate Adaptation in IEEE 802.11 WLANsExisting ChallengesDevelopment of Rate Adaptation AlgorithmsPresentative old onesNew Kids in this areaMy Opinion,Rate Adaptation Algorithms List,1997 ARF1998199920002001 RBAR2002 OAR 2003 LA 2004 AARF MultiRateRetry AMRR
14、 2005 ONOE LD-ARF SampleRate2006 CARA RRAA2007 AORA CHARMTBC ,Classification of existing algorithms,Open-loop,Local channel estimation,Closed-loop,Packet transmissionsituation,RTSCTS,Beaconprobe response,Information channel estimation depends on:,LACHARMAORA (idle slots),ARFAARFMMRAMRRONOELD-ARF(NAC
15、K)SampleRateCARARRAA,RBAROARCARARRAA,CHARM(noise),Classification of existing algorithms,Estimation (channel conditions),Which layer to use,Action (how to adjust),Which messages to use,How to estimate,sequentialrateadjustment,bestrateadjustment,PHY,MAC,hybrid,data,signal,probe,Noprobe,mapping,calcula
16、te,deterministic,statistical,Rate Adaptation Processing:,RBAROAR,ARF/AARFMMR/AMMRLD-ARFSamleRateCARA、RRAAAORA、ONOE,LA、CHARM,RBAROARCARALD-ARFRRAA CHARM,ARF/AARFMMR/AMMRLD-ARFSampleRateAORACHARM,RRAA,RBAROARLACHARM,ARFAARFLD-ARFMRRONOECARA,AMRRSampleRateRRAAAORACHARM,ARFAARFMRRAMRRONOELD-ARFCARARRAAA
17、ORA,RBARLASampleRateCHARM,Trend of rate adaptation algorithms,open-loop & statistics based: ARFclose-loop & SNR based (rts/cts): RBAR/OARstatistics & SNR based & adaptive: LAstatistics based & adaptive: AARFstatistics based & adaptive & estimate transimission time for different rates: SampleRatestat
18、istics based & collisions avoid/detect: CARA、RRAA,easy to implementcan not react on the real time channel situationsuffer from random collisions,not compliant with current 802.11 networks,The SNR is obtained based on” Symmetric link”, which is not accurate,improve the upshift performanceStill suffer
19、 from random collision,One probe every 10 framesCan react quickly to mobilityToo sensitive to probe failure,Differentiate the reasons for packet lossIncreasing load at some level,Each algorithm has its own Achilles heel,Outline,Introduction of Rate Adaptation in IEEE 802.11 WLANsExisting ChallengesD
20、evelopment of Rate Adaptation AlgorithmsPresentative old onesNew Kids in this areaMy Opinion,Presentative old ones,ARFRBAROAR LA AARF SampleRate,ARF- How does it work ?,Use packet transmission situation to estimate the channel condition: If two consecutive ACK frames are not received correctly, the
21、second retry and subsequent transmissions are done at a lower 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 next higher rate. However, if an ACK is NOT received for this frame, the rate is lowered back and the
22、 timer is restarted.,ARF- Does it work well ?,Advantages:Compliant with 802.11All things can be done by the senderEasy to implementDisadvantages:Suffer from random collisions and hidden terminalsConstantly upshift try when channel condition is stableRate can only be adjusted step by step,RBAR- How d
23、oes it work ?,Receivers control senders transmission rate:RTS and CTS are modified to contain info on size and rate.Uses analysis of RTS reception to estimate SNR and send choice back to sender in CTS.Receiver picks rate based on pre-defined SNR thresholds.,RBAR- Does it work well ?,Advantages:Rate
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