Improving Loss Resilience with Multi-Radio Diversity in Wireless NetworksThe problemBit-errors tend to clusterCurrent solutions are inefficient for recovering lossesSlide 5Multi-Radio Diversity (MRD) – UplinkMulti-Radio Diversity (MRD) – DownlinkAre losses independent among receivers?Slide 9Challenges in developing MRDBit-by-bit frame combiningBlock-based frame combiningFailure decreases with NB and burst sizeFlawed retransmission schemesRequest-for-acknowledgment (RFA) for efficient feedbackMRD-aware rate adaptationExperimental setupMRD improves throughputMRD maintains high bit-rateDelay AnalysisRelated workSummaryImproving Loss Resilience with Multi-Radio Diversity in Wireless NetworksAllen Miu, Hari BalakrishnanMIT Computer Science and Artificial Intelligence LaboratoryC. Emre KoksalComputer and Communication Sciences, EPFL2The problemNew wireless applications demand high performanceBut: wireless channels are loss-proneInterferenceNoise AttenuationMulti-pathMobility, etc… Inconsistent and poor performance3Bit-errors tend to clusterBit error positionCount# of corrupt frame samples = 185584Current solutions are inefficient for recovering lossesCoding (e.g., FEC)Hard on highly variable channelsRetransmissionWasteful: outage durations can be long (tens to hundreds of milliseconds)Bit-rate adaptationHard on variable channelsSlows down other clients5May use only one pathUses only one communication pathToday’s wireless LAN (e.g., 802.11)AP1Internet6May use only one pathAllow multiple APs to simultaneously receive transmissions from a single transmitterToday’s wireless LAN (e.g., 802.11)AP1AP2Multi-Radio Diversity (MRD) – UplinkInternetMRDC10%20%Loss independence simultaneous loss = 2%7Multi-Radio Diversity (MRD) –DownlinkInternetAP1AP2MRDCAllow multiple client radios to simultaneously receive transmissions from a single transmitter8Are losses independent among receivers?Broadcast 802.11 experiment at fixed bit-rate: 6 simultaneous receivers and 1 transmitterCompute loss rates for the 15 receiver-pair (R1, R2) combinationsFrame loss rate FLR(R1), FLR(R2) vs. simultaneous frame loss rate FLR(R1 ∩ R2)9Individual FLR > Simultaneous FLRFLR(R1 ∩ R2)FLRy = xR1R2R1*R210Challenges in developing MRDHow to correct simultaneous frame errors?Frame combiningHow to handle retransmissions in MRD?Request-for-acknowledgment protocolHow to adapt bit rates in MRD?MRD-aware rate adaptation112. Select bit combination at unmatched bit locations, check CRCPatterns CRC OkBit-by-bit frame combining1100 0000R11101 1010R20001 10101. Locate bits withunmatched value1100 0000 --1100 0010 X1100 1000 X1100 1010 OCorrected frameTX: 1100 1010 Problem: Exponential # of CRC checks in # of unmatched bits.Combine failure11012Block-based frame combiningObservation: bit errors occur in burstsDivide frame into NB blocks (e.g., NB = 6)Attempt recombination with all possible block patterns until CRC passes# of checks upper bounded by 2NBFailure rate increases with NB131.0Failure decreases with NB and burst sizeBurst error length parameterProbability of failureNB = 2NB = 4NB = 6NB = 16…0.80.60.40.20010 20 30 40 50Frame size = 1500B14Flawed retransmission schemesConventional link-layer ACKs do not workFinal status known only to MRDCTwo levels of ACKs are redundantCannot disable link-layer ACKs15Request-for-acknowledgment (RFA) for efficient feedbacklinkIPlinkMRDClinkMRD MRDDATAACK DATA RFAMRD-ACKDATAIP16MRD-aware rate adaptationStandard rate adaptation does not workReacts only to link-layer losses from 1 receiverUses sub-optimal bit-ratesMRD-aware rate adaptationReacts to losses at the MRD-layerImplication: First use multiple paths, then adapt bit rates.17Experimental setup~20 mR1R2L• 802.11a/b/g implementation in Linux (MADWiFi)• L transmits 100,000 1,500B UDP packets w/ 7 retries • 802.11a @ auto bit rate (6, 9, 12, 18, 24, 36, 48, 54)• L is in motion at walking speed, > 1 minute per trial• Variants: R1, R2, MRD (5 trials each)18MRD improves throughput2.3x Improvement R1 R2 MRDThroughput (Mbps)8.25 Mbps18.7 MbpsEach color shows a different trial19MRD maintains high bit-rateFrame recovery data(% of total losses at R1)via R2 42.3%frame combining 7.3%Total 49.6%Selected bit rate (Mbps)Fraction of transmitted frames609 12 18 24 36 48 5620Delay AnalysisOne way delay (s)Fraction of delivered packets10-310-4010-210-11User space implementation caused high delay21Related workPhysical layer spatial diversity techniquesAntenna diversity802.16 MIMO/802.11nMacro-diversity in CDMA networksRetransmission with memory [Sindhu ’77]Opportunistic forwarding [Biswas ‘05]Bit rate selection (AARF, RBAR, MiSer, OAR, Sample Rate)22SummaryDesign of Multi-Radio Diversity WLANBlock-based frame combiningRequest-for-acknowledgment protocolMRD-aware rate adaptationAnalysis of block-based frame combiningExperimental evaluationMRD reduces losses by 50% and improves throughput by up to