UConn CSE 5300 - Performance issues & improvement on 802.11 MAC

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Performance issues & improvement on 802.11 MACAn Overlay MAC Layer for 802.11 NetworksMotivationProblem 1: different data ratesProblem 2: unpredictabilityProblem 3: forwarding on behalf of othersProblemsApproachesAdvantages of an overlay approachBigger picture: overlay networkBigger picture: Overlay networkEven bigger picture: virtualizationOverlay MAC Layer (OML) design goalsOverlay MAC Layer (OML): what can it control?Detour: strict priority schedulingDetour: weighted fair queuingOWL main idea: use TDMA-like scheduleQuestionsClock synchronization & slot sizeInterference regionWeighted Slot Allocation: decide winner w/o communicationEvaluation methodologySummary of resultsSimulation resultsSimulation results (cont.)SummaryLimitations1Performance issues & improvement on 802.11 MACreview of 802.11 MACperformance issuesimprovementsidle sensean overlay approach more …2An Overlay MAC Layer for 802.11 NetworksAnanth Rao Ion StoicaUC BerkeleyMobisys 20053MotivationInternet Gateway802.11 hardware provides initial ease of deployability for many applicationsmesh networkslong haul linkslarge Infrastructure Networksthese apps stretching 802.11 beyond its design goals (Wireless LANs)4Problem 1: different data ratesData Rate ThroughputCase IA 11 Mbps 3.09 MbpsB 11 Mbps 3.36 MbpsCase IIA 11 Mbps 0.76 MbpsB 1 Mbps 0.76 MbpsRBA5Problem 2: unpredictability21 3 4 56Problem 3: forwarding on behalf of othersEthernet1/21/61/61/61/31/91/91/9This problem cannot be solved by local scheduling or queue management algorithms like WFQ7802.11 provides no control over resource allocationDefault allocation policy ill-suited for multi-hop networksBad fish problemForwarders get same share as othersProblemsA B C D1M11MA B C DEF8Approachesworkarounds in routing/transport layereasy to deploycannot address some issueschange/replace MAC new protocols, new standardmore powerful, hard to deployoverlay MAC layer (OML)directly on top of 802.11 MACno need to change hardwaredirectly use interfaces exposed by 802.11 cards9Advantages of an overlay approacheasy to deployeasy to modifyimplemented in softwareeasy to modify for diverse requirementstighter integration between MAC and upper layersperformance benefitsutilize information from higher layers10Bigger picture: overlay network11Bigger picture: Overlay networkFocus at the application level12Even bigger picture: virtualizationVirtualization of resources: powerful abstraction in systems engineering:computing examples: virtual memory, virtual devicesvirtual machines: e.g., javaIBM VM os from 1960’s/70’sNetworking examples:connecting local heterogeneous networksIP over ATMoverlay networksVPN13Overlay MAC Layer (OML) design goalsefficientfair or differentiated allocationflexible and low costavoid modifying MAC14Overlay MAC Layer (OML): what can it control?no control in upper layercannot decide when getting a packetno control in MACcannot decide when packet is actually sentcan control only when to send packet to network cardpacket scheduling policy: FIFO, strict priority scheduling, weighted fair queuing15Detour: strict priority schedulingtransmit highest priority queued packet multiple classes, with different prioritiesclass may depend on marking or other header info, e.g. IP source/dest, port numbers, etc..real world example: reservations versus walk-ins 13452time1time3245arrivalsdeparturespacketservice5423116Detour: weighted fair queuingeach class gets weighted amount of service in each cycleequal weight: Round Robin scheduling17OWL main idea: use TDMA-like scheduledivide time into slotsweighted slot allocation (WSA): allocate slots to nodes according to weighted fair queuing policyassigns a weight to each nodeallocate slots proportion to nodes’ weights -> weighted allocationa slot is only assigned to one node in an interference region -> reduce packet loss18Questionsclock synchronization?slot length?interference region?weighted slot allocation how to choose weight? decide a winner w/o communication?19Clock synchronization & slot sizeLoose time synchronizationleader-based slot size: 10 packets of maximum sizelarger than clock synchronization errorlarger than packet transmission timeas small as possible20Interference regionideally node i applies WSA to all nodes that interfere with ihow to determine who interfere with me? assume a node can interfere with all nodes within k-hop distanceonly an approximation, not accuratehow to determine interference relationship is an active research!21Weighted Slot Allocation: decide winner w/o communicationeach node uses pseudo-random function to generate a random numberHi = H(ni, t) 1/wit: time slot, wi: weight of node ni can generate random number for all nodes in the collision domain (e.g., 2-hop neighborhood) the highest number wins22Evaluation methodologySimulation in QualnetImplementation in Atheros Madwifi driver + Click router23Summary of resultsOverhead: OML thruput comparable to native 802.11Reduced contention and retransmissionsFairness: Fairness index for OML network much higherA node’s share = # flows passing thru itLimitations: Impact of mobility; Interference from native 802.11 clients24Simulation resultsSimilar throughput to 802.11Control overhead is small25Simulation results (cont.)Improved fairness over standard 802.11Weight set to number of nodes in output queue26SummaryCoarse-grained scheduling on top of 802.11:alleviate inefficiencies of the MAC protocol in resolving contentionovercome the lack of flexibility of assigning priorities to sendersEnables experiment with new scheduling and bandwidth management algorithms27LimitationsInterference from other 802.11 clientsFace incrementally deployment issuesImpact of mobilityTakes some time for newly joined nodes to get its proportional share How to set weight? How to know of weights of nodes in interference region (weights can be


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