Computer Science 37 Lecture 30

8/4/2019 Computer Science 37 Lecture 30

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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:

Documents

Computer Science 37 Lecture 30