computer science 37 lecture 30
TRANSCRIPT
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Lecture30
WirelessNetworks
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ReasonstoGoWireless
1)2)
3)
ChallengesinGoingWireless
1)
2)
3)
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Medium:TheRadioSpectrum
Wirelesscommunicationsusetheelectromagneticspectrum,whichis
regulatedbygovernmentinstitutionssuchastheFederalCommunications
Commission(FCC).
Regulationsspecifywhatbandsoffrequencycanbeusedfordifferent
applications.Forinstance:FMradiohas88-108MHz(200KHz)andAM
radiohas540-1600KHz(10KHzbandwidth).Regulationsalsospecifythe
transmissionpowerthatcanbeusedineachband.
ThereareportionsofthespectrumthatareUNLICENSED,however. The
mostpopularwirelessnetworksoftodayoperateinunlicensedbands.
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Omnidirectional Antenna
Radiatesinall
azimuthdirections.
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DirectionalAntenna
Radiatesinacone.
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DesignAlternatives
Point-to-pointchannels: Informationflowsinbeamsthat
connectcommunicatingparties.
signalreceived
signalnotreceived
Theantennasonthetransmitterandreceiver
needtobeproperlyalignedforsignalstogo
through.Ontheflip-side,directionalantennas
havegreatpowerefficiencyandrange.
Directionalantennasareagoodchoiceforsystemswithfixedinfrastructure.Theyintroduceadditionaldifficultiesininfrastructure-lesssystemsorwhen
transmittersandreceiverscanmovearound,butofferreducedpower
consumption.
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Broadcastchannels: Informationradiatesinallpossible
directionsfromthetransmitter.
DesignAlternatives
signalreceived
signalreceived
Theresnoneedtoalignantennas
onthetransmitterandreceiver.If
signalsradiateinalldirections,a
receiverwillthetransmitters
independentlyoftheirrelative
positions.Becausepowerradiates
allaround,omnidirectional
antennascantreachasfaras
directionalantennas.Notealsothat
quiteabitofpowercanbewasted.
Thisisagoodchoiceformobilesystems.Attheexpenseof
increasedpowerconsumption,
coveragereaches360 o.
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ConceptsinRadioCommunications
Interference
B
Coverage=f(PowerTX)
EA
D
CMultipath
Noise
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MultipleAccesstotheRadioSpectrum
frequencyspectrumA B
FrequencyDivisionMultipleAccess (FDMA):eachpairofnodesusesa
distinctsubrange ofthetotalfrequencybandfortheapplication.
SpatialDivisionMultipleAccess (SDMA):eachpairofnodes
communicatesthroughatightbeamthattakesaportionofspace.
T0 T1 T2 T3 T4 T5 T6 T7 T8
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MultipleAccesstotheRadioSpectrum
TimeDivisionMultipleAccess (TDMA):eachpairofnodesuses
adifferenttimeslottocommunicate.Duringitstime,thepaircanusetheentirefrequencybandallocatedfortheapplication.
time
onetimeunit onetimeunit onetimeunit
CodeDivisionMultipleAccess (CDMA):canbeseenasa
combinationofFDMAandTDMA.
T0 T0 T0 T1T1T1 T2 T2 T3T3 T4 T4
FrequencyHoppingSpreadSpectrum (FHSS):transmittersuse
eachfrequencybandforarandomtimethenmovetoanother
randomlychosen TXandRXmustagreetoahoppingsequence.
timeT0
timeT1
timeT2
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RememberEthernet?
WhenProblemsGetWorse:
Assumethatallwirelessdevicesusethesamechannel.Arbitration ofaccesstothemedium(MediumAccessControl,orMAC,aprotocolintheDataLinklayer)issimilartoEthernetsCSMA/CD.
Mostradiosinwirelessnetworkingcanttransmitandreceiveat thesametime,sowecantdetectcollisions.Instead,welldoCSMA/CA(collisionavoidance).
Collisionsarebadbecausetheyreducetheeffectivebandwidthandalsobecausetheycausewasteofpower.
Evenwhentwotransmissionsdonotcollide,theymaystillinterferewith
eachothercausingbiterrorrates torise.
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IEEE802.11DCF (CSMA/CA)
start
NAV=0
SenseMedium
MediumIdle
TransmitFrame
Collision?
RandomBackoff
Time
NO
NO
NO
YES
YES
YES
NAVstartswiththeDurationfieldvalueofthelasttransmissionsensedonthemediumandcountsdowntozero.
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TheHiddenNode Problem StationCcansensestationsAandB. StationsAandCcantsenseeachother.
Problem: coordinatetransmissionsfromAandCsoastoavoidcollisions.
A B C
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TheHiddenNode Problem StationCcansensestationsAandB. StationsAandCcantsenseeachother.
Problem: coordinatetransmissionsfromAandCsoastoavoidcollisions.
A B C
Solution: RTS/CTS/DATA/ACK handshake AsendsRTStoB,BsendsCTStoA,ChearsCTSandstaysquiet,AsendsDATAtoB,BrepliestoAwithanACK.
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TheExposedNodeProblem
A B C D
Anexposednodeisonethatisinrangeofthetransmitter,butoutsiderangeofthereceiver.
Problem: exposednodesreducebandwidth.
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TypesofWirelessNetworks
BS BS BS
wiredbackbone
FixedInfrastructure
AdHoc
Easytodeploy.
Goodinchangingenvironments.Allowsfornodemobility.
Canbedesignedforself-configurability.
Canbedesignedforscalability.
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DataLinkLayer:MediumAccessControl(Coordinatedaccesstoasharedresource)
Powerisascarceresource(soistheRFspectrum).
Collisionsleadtowastedpower(ANDwastedspectrum).
Needtoimposesomekindofaccessdisciplinesoastoavoidcollisions.
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TheMACLayerChallenge
Maximizethroughput:
Minimizecollisions.
Avoidexposednodes.
Aninterestingoption:schedulemediumaccess.
Relatedchallenges:
Clocksynchronization. Distributedcoordinationfordeterminingschedule.
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TheNetworkLayerChallenge
Howdowebuildroutesdynamically?Pro-activealgorithms.
Reactivealgorithms.
WilltheroutingprotocolscaleuptoLARGEnetworks?
Canroutingadapttochangesinnetworktraffic,propagationconditions,etc.?
Packetforwardingcostspower.Canwedoroutinginawaythatbalancespowerconsumption?
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SecurityIssues
Asoftoday,thenetworkcanbevulnerableatmultiplelevels:
PHY:radiojamming.
MAC:DoS viafakerequestsorschedules.
NET:fakerouteadvertisements(blackholeattack). Afunnybutscarynotion:cavemanattacks.
Confidentiality
Authenticity
Integrity
Freshness
Scalability
Availability
Accessibility
Self-organization
Non-repudiation
Flexibility
Desirableproperties: