CS 268: Computer NetworkingWireless in the Real WorldWireless ChallengesOverviewCharacterizing Current DeploymentsAP Stats, Degrees: PlacelabDegree Distribution: Place LabUnmanaged DevicesGrowing Interference in Unlicensed BandsWhat do we expect?Key QuestionsWhat we seeExperimental Setup802.11 Receiver PathTiming Recovery InterferenceInterference ManagementImpact of frequency separationTransmission Power ControlTransmission Power Control in PracticeDetails of Power ControlRate AdaptationSimple rate adaptation schemeChallenges in rate adaptationPower and Rate Selection AlgorithmsPower Control/Rate Control summarySlide 26Community Wireless NetworkRoofnetRoofnet DesignRoofnet Node MapSlide 31Typical Rooftop ViewA Roofnet Self-Installation KitSoftware and Auto-ConfigurationSlide 35EvaluationSlide 37Slide 38Slide 39Slide 40Slide 41Slide 42Roofnet SummaryRoofnet Link Level MeasurementsLossy Links are CommonDelivery Probabilities are Uniformly DistributedDelivery vs. SNRIs it Bursty Interference?Two Different Roofnet LinksIs it Multipath Interference?A Plausible ExplanationKey ImplicationsCS 268: Computer NetworkingL-11 Wireless in the Real WorldWireless in the Real World•Real world deployment patterns•Mesh networks and deployments•Assigned reading•Modeling Wireless Links•Architecture and Evaluation of an Unplanned 802.11b Mesh Network23Wireless Challenges•Force us to rethink many assumptions•Need to share airwaves rather than wire•Don’t know what hosts are involved•Host may not be using same link technology•Mobility•Other characteristics of wireless•Noisy lots of losses•Slow•Interaction of multiple transmitters at receiver• Collisions, capture, interference•Multipath interference4Overview•802.11•Deployment patterns•Reaction to interference•Interference mitigation•Mesh networks•Architecture•MeasurementsCharacterizing Current Deployments•Datasets•Place Lab: 28,000 APs•MAC, ESSID, GPS•Selected US cities•www.placelab.org•Wifimaps: 300,000 APs•MAC, ESSID, Channel, GPS (derived)•wifimaps.com•Pittsburgh Wardrive: 667 APs•MAC, ESSID, Channel, Supported Rates, GPS56AP Stats, Degrees: PlacelabPortland 8683 54San Diego 7934 76San Francisco3037 85Boston 2551 39#APs Max.degree(Placelab: 28000 APs, MAC, ESSID, GPS)1 2150 m7Degree Distribution: Place Lab8Unmanaged Devices•Most users don’t change default channel•Channel selection must be automated6 5111 211 1410 4Channel %ageWifiMaps.com(300,000 APs, MAC, ESSID, Channel)Growing Interference in Unlicensed Bands •Anecdotal evidence of problems, but how severe?•Characterize how 802.11 operates under interference in practice9Other 802.11What do we expect?•Throughput to decrease linearly with interference•There to be lots of options for 802.11 devices to tolerate interference•Bit-rate adaptation•Power control•FEC•Packet size variation•Spread-spectrum processing•Transmission and reception diversity10Interferer power(log-scale)Throughput (linear)TheoryKey Questions•How damaging can a low-power and/or narrow-band interferer be?•How can today’s hardware tolerate interference well?•What 802.11 options work well, and why?11What we see•Effects of interference more severe in practice•Caused by hardware limitations of commodity cards, which theory doesn’t model12PracticeInterferer power(log-scale)Throughput (linear)Theory13Experimental Setup802.11ClientAccessPointUDP flow802.11 Interferer802.11 Receiver Path•Extend SINR model to capture these vulnerabilities•Interested in worst-case natural or adversarial interference•Have developed range of “attacks” that trigger these vulnerabilities14MACPHYTimingRecoveryPreamble Detector/Header CRC-16 CheckerAGCBarker CorrelatorDescramblerADC6-bit samplesTo RF AmplifiersRF SignalReceiverData(includes beacons)DemodulatorPHYMACAnalog signalAmplifier controlSYNC SFD CRCPayloadPHY headerTiming Recovery Interference•Interferer sends continuous SYNC pattern•Interferes with packet acquisition (PHY reception errors)150.1110100100010000_∞ -20 -12 -2 0 8 12 15 20Interferer Power (dBm)Throughput (kbps)020040060080010001200Latency(microseconds)Throughput LatencyWeak interfererModerate interfererLog-scaleInterference Management•Interference will get worse•Density/device diversity is increasing•Unlicensed spectrum is not keeping up•Spectrum management•“Channel hopping” 802.11 effective at mitigating some performance problems [Sigcomm07]•Coordinated spectrum use – based on RF sensor network•Transmission power control•Enable spatial reuse of spectrum by controlling transmit power•Must also adapt carrier sense behavior to take advantage1617Impact of frequency separation•Even small frequency separation (i.e., adjacent 802.11 channel) helps0.1110100100010000_∞ -20 -12 0 8 12 15 20Interferer Power (dBm)Throughput (kbps)10MHz separation15MHz separationSame channel(poor performance)5MHz separation(good performance)Transmission Power Control•Choose transmit power levels to maximize physical spatial reuse•Tune MAC to ensure nodes transmit simultaneously when possible•Spatial reuse = network capacity / link capacity18AP1AP2Client1Client2AP1AP2Client1Client2Spatial Reuse = 1 Spatial Reuse = 2Concurrent transmissions increase spatial reuseTransmission Power Control in Practice•For simple scenario easy to compute optimal transmit power•May or may not enable simultaneous transmit•Protocol builds on iterative pair-wise optimization•Adjusting transmit power requires adjusting carrier sense thresholds•Echos, Alpha or eliminate carrier sense•Altrusitic Echos – eliminates starvation in Echos19AP1AP2Client1Client2d11d22d12d21Details of Power Control•Hard to do per-packet with many NICs•Some even might have to re-init (many ms)•May have to balance power with rate•Reasonable goal: lowest power for max rate•But finding ths empirically is hard! Many {power, rate} combinations, and not always easy to predict how each will perform•Alternate goal: lowest power for max needed rate•But this interacts with other people because you use more channel time to send the same data. Uh-oh.•Nice example of the difficulty of local vs. global optimization20Rate Adaptation•General idea:•Observe channel conditions like SNR (signal-to-noise ratio), bit errors, packet errors•Pick a transmission rate that will get best goodput•There are channel conditions when reducing the
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