Wireless Networks - Princeton University Computer Science · Wireless Networks Reading: Secon ......
Transcript of Wireless Networks - Princeton University Computer Science · Wireless Networks Reading: Secon ......
WirelessNetworksReading:Sec5on2.8
COS461:ComputerNetworksSpring2011
MikeFreedmanhEp://www.cs.princeton.edu/courses/archive/spring11/cos461/
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WidespreadDeployment
• Worldwidecellularsubscribers– 1993:34million– 2005:morethan2billion
– 2009:morethan4billion
>landlinesubscribers
• Wirelesslocalareanetworks– Wirelessadaptersbuiltinto
mostlaptops,andevenPDAs
– Morethan220,000knownWiFiloca5onsin134countries
– Probablymany,manymore(e.g.,homenetworks,corporatenetworks,…)
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WirelessLinks:HighBitErrorRate
• Decreasingsignalstrength– Dispersesasittravelsgreaterdistance– AEenuatesasitpassesthroughmaEer
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WirelessLinks:HighBitErrorRate
• Interferencefromothersources– Radiosourcesinsamefrequencyband
– E.g.,2.4GHzwirelessphoneinterfereswith802.11bwirelessLAN
– Electromagne5cnoise(e.g.,microwaveoven)
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WirelessLinks:HighBitErrorRate
• Mul5‐pathpropaga5on– Electromagne5cwavesreflectoffobjects
– Takingmanypathsofdifferentlengths– Causingblurringofsignalatthereceiver
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receiver transmitter
DealingWithBitErrors
• Wirelessvs.wiredlinks– Wired:mostlossisduetoconges5on– Wireless:higher,5me‐varyingbit‐errorrate
• Dealingwithhighbit‐errorrates– Sendercouldincreasetransmissionpower
• Requiresmoreenergy(badforbaEery‐poweredhosts)• Createsmoreinterferencewithothersenders
– Strongererrordetec5onandrecovery• Morepowerfulerrordetec5on/correc5oncodes
• Link‐layerretransmissionofcorruptedframes
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WirelessLinks:BroadcastLimita5ons
• Wiredbroadcastlinks– E.g.,Ethernetbridging,inwiredLANs– Allnodesreceivetransmissionsfromallothernodes
• Wirelessbroadcast:hiddenterminalproblem
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A B
C • AandBheareachother• BandCheareachother• But,AandCdonot
So,AandCareunawareoftheirinterferenceatB
WirelessLinks:BroadcastLimita5ons
• Wiredbroadcastlinks– E.g.,Ethernetbridging,inwiredLANs– Allnodesreceivetransmissionsfromallothernodes
• Wirelessbroadcast:fadingoverdistance
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A B C
A’s signal strength
space
C’s signal strength
• AandBheareachother• BandCheareachother• But,AandCdonot
So,AandCareunawareoftheirinterferenceatB
ExampleWirelessLinkTechnologies
• Datanetworks– 802.15.1(Bluetooth):2.1Mbps–10m– 802.11b(WiFi):5‐11Mbps–100m– 802.11aandg(WiFi):54Mbps–100m– 802.11n(WiFi):200Mbps–100m
– 802.16(WiMax):70Mbps–10km
• Cellularnetworks,outdoors– 2G:56Kbps– 3G:384Kbps– 3Genhanced:4Mbps
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WirelessNetwork:WirelessLink11
network infrastructure
Wireless link
• Typically used to connect mobile(s) to base station
• Also used as backbone link • Multiple access protocol
coordinates link access
WirelessNetwork:WirelessHosts12
network infrastructure
Wireless host
• Laptop, PDA, IP phone • Run applications
• May be stationary (non-mobile) or mobile
WirelessNetwork:BaseSta5on13
network infrastructure
Base station
• Typically connected to wired network
• Relay responsible for sending packets between wired network and wireless host(s) in its “area”
• E.g., cell towers, 802.11 access points
WirelessNetwork:Infrastructure14
network infrastructure
Network infrastructure
• Larger network with which a wireless host wants to communicate
• Typically a wired network
• Provides traditional network services
• May not always exist
Scenario#1:InfrastructureMode15
network infrastructure
Infrastructure mode
• Base station connects mobiles into wired network
• Network provides services (addressing, routing, DNS)
• Handoff: mobile changes base station providing connection to wired network
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Ad hoc mode
• No base stations • Nodes can only transmit to other
nodes within link coverage • Nodes self-organize and route
among themselves
Scenario#2:Ad‐HocNetworks
Infrastructurevs.AdHoc
• Infrastructuremode– Wirelesshostsareassociatedwithabasesta5on– Tradi5onalservicesprovidedbytheconnectednetwork– E.g.,addressassignment,rou5ng,andDNSresolu5on
• Adhocnetworks– Wirelesshostshavenoinfrastructuretoconnectto– Hoststhemselvesmustprovidenetworkservices
• Similarinspirittothedifferencebetween– Client‐servercommunica5on– Peer‐to‐peercommunica5on
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Bluetoothpiconets
• Upto7“slave”devicesand225“parked”devices• Operatesonunlicensedwirelessspectrum
– Howtopreventinterference?
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PHY:SpreadSpectrum–FrequencyHopping
• Nodesrapidlyjumpbetweenfrequencies
• Senderandreceivercoordinatedinjumps– Howcoordinate?Pseudorandomnumbergenerator,withsharedinputknowntosender/receiver
• IfrandomlycollidewithothertransmiEed,onlyforshortperiodbeforejumpagain
• Bluetooth– 79frequencies,oneachfrequencyforjust625us– EachchannelalsousesTDMA,witheachframetaking1/3/5consecu5veslots.
– Onlymastercanstartinoddslot,slaveonlyinresponse
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802.11LANArchitecture22
• Access Point (AP) – Base station that
communicates with the wireless hosts
• Basic Service Set (BSS) – Coverage of one AP – AP acts as the master – Identified by an “network
name” known as an SSID BSS 1
BSS 2
Internet
hub, switch or router AP
AP
SSID: Service Set Identifier
ChannelsandAssocia5on
• Mul5plechannelsatdifferentfrequencies– NetworkadministratorchoosesfrequencyforAP– InterferenceifchannelissameasneighboringAP
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• BeaconframesfromAPs• Associaterequestfromhost• Associa5onresponsefromAP
ChannelsandAssocia5on
• Mul5plechannelsatdifferentfrequencies– NetworkadministratorchoosesfrequencyforAP– InterferenceifchannelissameasneighboringAP
• Accesspointssendperiodicbeaconframes– ContainingAP’sname(SSID)andMACaddress
– Hostscanschannels,listeningforbeaconframes– Hostselectsanaccesspointtoassociatewith
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• BeaconframesfromAPs• Associaterequestfromhost• Associa5onresponsefromAP
MobilityWithintheSameSubnet• H1remainsinsameIPsubnet
– IPaddressofthehostcanremainsame– Ongoingdatatransferscancon5nueuninterrupted
• H1recognizestheneedtochange– H1detectsaweakeningsignal– Startsscanningforstrongerone
• ChangesAPswithsameSSID– H1disassociatesfromone
– Andassociateswithother
• Switchlearnsnewloca5on– Self‐learningmechanism
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hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
router
WirelessLANaddressingandbridging26
Func5on Addr1(Receiver)
Addr2(Transmi?er)
Addr3 Addr4
Intra‐BSS Dest Source
ToAP BSSID Source Dest
FromAP Dest BSSID Source
BridgedAPs Reciever TransmiEer Dest Source
CSMA:CarrierSense,Mul5pleAccess
• Mul5pleaccess:channelissharedmedium– Sta5on:wirelesshostoraccesspoint– Mul5plesta5onsmaywanttotransmitatsame5me
• Carriersense:sensechannelbeforesending– Sta5ondoesn’tsendwhenchannelisbusy– Topreventcollisionswithongoingtransfers– But,detec5ngongoingtransfersisn’talwayspossible
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A B
C A B C
A’s signal strength
space
C’s signal strength
CA:CollisionAvoidance,NotDetec5on
• Collisiondetec5oninwiredEthernet– Sta5onlistenswhiletransmiong– Detectscollisionwithothertransmission– Abortstransmissionandtriessendingagain
• Problem#1:cannotdetectallcollisions– Hiddenterminalproblem– Fading
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CA:CollisionAvoidance,NotDetec5on
• Collisiondetec5oninwiredEthernet– Sta5onlistenswhiletransmiong– Detectscollisionwithothertransmission– Abortstransmissionandtriessendingagain
• Problem#1:cannotdetectallcollisions– Hiddenterminalproblem– Fading
• Problem#2:listeningwhilesending– Strengthofreceivedsignalismuchsmaller– Expensivetobuildhardwarethatdetectscollisions
• So,802.11doescollisionavoidance,notdetec5on
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HiddenTerminalProblem
• AandCcan’tseeeachother,bothsendtoB
• Occursb/c802.11reliesonphysicalcarriersensing,whichissuscep5bletohiddenterminalproblem
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CBA
Virtualcarriersensing
• Firstexchangecontrolframesbeforetransmiongdata– Senderissues“RequesttoSend”(RTS),incl.lengthofdata– Receiverrespondswith“CleartoSend”(CTS)
• IfsenderseesCTS,transmitsdata(ofspecifiedlength)
• IfothernodeseesCTS,willidleforspecifiedperiod
• IfothernodeseesRTSbutnotCTS,freetosend
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HiddenTerminalProblem
• AandCcan’tseeeachother,bothsendtoB
• RTS/CTScanhelp– BothAandCwouldsendRTSthatBwouldseefirst– BonlyrespondswithoneCTS(say,echo’ingA’sRTS)– CdetectsthatCTSdoesn’tmatchandwon’tsend
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CBA
ExposedTerminalProblem
• BsendingtoA,CwantstosendtoD• AsCreceivesB’spackets,carriersensewouldpreventitfromsendingtoD,eventhoughwouldn’tinterfere
• RTS/CTScanhelp– ChearsRTSfromB,butnotCTSfromA– Cknowsit’stransmissionwillnotinterferewithA
– CissafetotransmittoD
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CBA D
ImpactonHigher‐LayerProtocols
• Wirelessandmobilitychangepathproper5es– Wireless:higherpacketloss,notfromconges5on– Mobility:transientdisrup5ons,andchangesinRTT
• Logically,impactshouldbeminimal…– Best‐effortservicemodelremainsunchanged– TCPandUDPcan(anddo)runoverwireless,mobile
• But,performancedefinitelyisaffected– TCPtreatspacketlossasasignofconges5on– TCPtriestoes5matetheRTTtodriveretransmissions– TCPdoesnotperformwellunderout‐of‐orderpackets
• Internetnotdesignedwiththeseissuesinmind
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Conclusions
• Wireless– AlreadyamajorwaypeopleconnecttotheInternet– Graduallybecomingmorethanjustanaccessnetwork
• Mobility(notdiscussed)– Today’suserstoleratedisrup5onsastheymove– …andapplica5onstrytohidetheeffects– Tomorrow’susersexpectseamlessmobility
• Challengesthedesignofnetworkprotocols– Wirelessbreakstheabstrac5onofalink,andtheassump5onthatpacketlossimpliesconges5on
– Mobilitybreaksassocia5onofaddressandloca5on– Higher‐layerprotocolsdon’tperformaswell
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