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CarrierSenseMultipleAccesswithCollisionAvoidance
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CSMA/CDvs.CSMA/CA(1)
CSMA/CD(CollisionDetection)istheMACmethodusedinawiredLAN(Ethernet).WiredLANstationscan(whereaswirelessstationscannot)detectcollisions.
BasicCSMA/CDoperation:
1) Waitforfreemedium2) Transmitframe3)
Ifcollision,stoptransmissionimmediately
4) Retransmitafterrandomtime(backoff)
CSMA/CDrule:Backoffaftercollision
1.BasicCSMA/CAoperation
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CSMA/CDvs.CSMA/CA(2)
CSMA/CA(CollisionAvoidance)istheMACmethodusedinawirelessLAN.Wirelessstationscannotdetectcollisions(i.e.thewholepacketswillbe
transmittedanyway).BasicCSMA/CAoperation:
1) Waitforfreemedium2) Waitarandomtime(backoff)3) Transmitframe4) Ifcollision,thestationsdonotnoticeit5) Collision=>erroneousframe=>noACKreturned
CSMA/CArule:Backoffbeforecollision
1.BasicCSMA/CAoperation
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Basicwirelessmediumaccess
.HereisInfrastructurebasicserviceset(BSS).
.AllstationsandAPhaveequalpriority
transmissionindownlink(fromtheAP)anduplink(fromastation)issimilar.
CSMA:Onepacketatatime
AP
wiredLAN
1.BasicCSMA/CAoperation
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Wirelessmediumaccess(1)
DIFS SIFS
ACK
(B=>A)
Transmittedframe(A=>B)
Whenaframeisreceivedwithoutbiterrors,thereceivingstation(B)sendsanAcknowledgement(ACK)framebacktothetransmitting
station(A).
Ifthereceivedframeiserroneous,noACKwillbesent
CyclicRedundancyCheck(CRC)isusedforerrordetection
1.BasicCSMA/CAoperation
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Wirelessmediumaccess(2)
DIFS SIFS DIFS
ACK
(B=>A)
Transmittedframe(A=>B)
Duringthetransmissionsequence(Frame+SIFS+ACK)themedium(radiochannel)isreserved.Thenextframecanbetransmittedat
earliestafterthenextDIFSperiod.
Nextframe(fromanystation)
Earliestallowedtransmissiontimeofnextframe
1.BasicCSMA/CAoperation
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Wirelessmediumaccess(3)
DIFS SIFS DIFS
ACK
(B=>A)
Transmittedframe(A=>B)
Therearetwomechanismsforreservingthechannel:
PhysicalcarriersensingVirtualcarriersensingusingtheNetworkAllocationVector(NAV).
Nextframe
1.BasicCSMA/CAoperation
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Wirelessmediumaccess(5)
ThetwomostimportantinterframespacingtimesareSIFSandDIFS:
SIFS(ShortInterframeSpace)=10s(16s)
DIFS(DCFInterframeSpace)=50s(34s)
Whentwostationstrytoaccessthemediumatthesametime,theonethathastowaitforthetimeSIFSwinsovertheonethathastowaitforthetimeDIFS.Inotherwords,SIFShashigherpriorityoverDIFS.
802.11b 802.11g
1.BasicCSMA/CAoperation
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Wirelessmediumaccess(6)
DIFS SIFS DIFS
ACK
Transmittedframe
NAV
Virtualcarriersensing
ItmeansthataNAVvalueissetinallstationsthatwereabletoreceiveatransmittedframeandwereabletoreadtheNAVvalueinthisframe.
NAVvalueisgivenhere Nextframe
TransmissionisnotallowedaslongasNAVisnon-zero
1.BasicCSMA/CAoperation
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Wirelessmediumaccess(7)
DIFS SIFS
Transmittedframe
NAV
VirtualcarriersensingusingNAVisimportantinsituationswherethechannelshouldbereservedforalongertime(RTS/CTSusage,
fragmentation,etc.).
ACKACK
Longtransaction
DIFS
1.BasicCSMA/CAoperation
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NAVvalueiscarriedinMACheader
MPDU(MACProtocolDataUnit)
MACpayloadAddr1 Addr2 Addr3 Addr4(optional)
FCS
Durationfield:15bitscontaintheNAVvalueinnumberofmicroseconds.Thelast(sixteenth)bitiszero.
AllstationsmustmonitortheheadersofallframestheyreceiveandstoretheNAVvalueinacounter.Thecounterdecrementsin
stepsofonemicrosecond.Whenthecounterreacheszero,thechannelisavailableagain.
1.BasicCSMA/CAoperation
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Wirelessmediumaccess(8)
DIFS SIFS t>DIFS
ACK
(B=>A)
Transmittedframe(A=>B)
Whenastationwantstosendaframeandthechannelhasbeenidleforatime>DIFS(countedfromthemomentthestationfirstprobed
thechannel)=>cansendimmediately.
Nextframe(fromanystation)
ChannelwasidleatleastDIFSseconds
1.BasicCSMA/CAoperation
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Wirelessmediumaccess(9)
DIFS SIFS DIFS
ACK
(B=>A)
Transmittedframe(A=>B)
Whenastationwantstosendaframeandthechannelisbusy=>thestationmustwaitabackofftimebeforeitisallowedtotransmitthe
frame.Reason?Nexttwoslides
Nextframe
Channelwasbusywhenstationwantedtosendframe
Backoff
1.BasicCSMA/CAoperation
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Nobackoff=>collisioniscertain
Supposethatseveralstations(BandCinthefigure)arewaitingtoaccessthewirelessmedium.
Whenthechannelbecomesidle,thesestationsstartsendingtheirpacketsatthesametime=>collision!
StationA
StationB
StationC
DIFS
Collision!
ACK
1.BasicCSMA/CAoperation
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Backoff=>collisionprobabilityisreduced
Contendingstationsgeneraterandombackoffvaluesbn.Backoffcounterscountdownwards,startingfrombn.Whenacounterreacheszero,thestationisallowedtosenditsframe.Allothercountersstopcountinguntilthechannelbecomesidleagain.
StationA
StationB
StationC
DIFS
bnislarge
bnissmall
Backoff
RemainingbackofftimeACK
1.BasicCSMA/CAoperation
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Contentionwindow(CW)for802.11b
Iftransmissionofaframewasunsuccessfulandtheframeisallowedtoberetransmitted,beforeeachretransmissiontheContentionWindow(CW)fromwhichbnischosenisincreased.
DIFS CW=25-1=31slots(slot=20
s)Initialattempt
DIFS
CW=26-1=63slots1stretransm.
DIFS CW=2
10
-1=1023slots5th(andfurther)retransmissions
:
CW 802.11b
1.BasicCSMA/CAoperation
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Contentionwindow(CW)for802.11g
Inthecaseof802.11goperation,theinitialCWlengthis15slots.Theslotdurationis9s.Thebackoffoperationof802.11gissubstantiallyfasterthanthatof802.11b.
DIFS CW=24-1=15slots(slot=9
s)Initialattempt
DIFS
CW=25-1=31slots1stretransm.
DIFS CW=2
10
-1=1023slots6th(andfurther)retransmissions
:
CW 802.11g
1.BasicCSMA/CAoperation
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Wirelessmediumaccess(2)
StationA
StationB
StationC
StationD
DIFS DIFS
Defer
3)StationDalsowishestosendapacket
4)However,stationBiswinnerandstartssendingpacket3
4
ACK
2.Wirelessmediumaccessexample
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Wirelessmediumaccess(3)
StationA
StationB
StationC
StationD
DIFS
5)StationDstartssendingpacket.Nowthereisnocompetition.
DIFS
5
ACK
2.Wirelessmediumaccessexample
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3.UsageofRTS&CTS
TheRTS/CTS(Request/ClearToSend)schemeisusedasacountermeasureagainstthehiddennodeproblem:
AP
WS1
WS2
Hiddennodeproblem:WS1andWS2canheartheAPbutnoteachother
=>
IfWS1sendsapacket,WS2doesnotnoticethis(andviceversa)=>collision!
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ReservationofmediumusingNAV
RTS
SIFS
DIFS
NAV=CTS+Data+ACK+3xSIFS
CTS
Dataframe
ACK
SIFS
SIFS
WS1
AP
NAV=Data+ACK+3xSIFS
NAVinRTS
NAVinCTS
3.UsageofRTS&CTS
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DangerofcollisiononlyduringRTS
WS2doesnotheartheRTSframe(andassociatedNAV),butcanheartheCTSframe(andassociatedNAV).
RTS
NAV=CTS+Data+ACK+3xSIFS
CTSDataframe
ACKWS1AP
NAV=Data+ACK+3xSIFS
NAVinRTS
NAVinCTS
Dangerofcollision
3.UsageofRTS&CTS
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AdvantageofRTS&CTS(1)
UsageofRTS/CTSoffersanadvantageifthedataframeisverylongcomparedtotheRTSframe:
RTSCTS
DataframeACK
WS1AP
Shortinterval:collisionnotlikely
Dataframe
ACK
WS1
AP
Longinterval:collisionlikely
(RTS/CTSnotused)
(RTS/CTSused)
3.UsageofRTS&CTS
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AdvantageofRTS&CTS(2)
Alongcollisiondangerinterval(previousslide)shouldbeavoidedforthefollowingreasons:
Largerprobabilityofcollision
Greaterwasteofcapacityifacollisionoccursandtheframehastoberetransmitted.
Athresholdparameter(dot11RTSThreshold)canbesetinthewirelessstation.FramesshorterthanthisvaluewillbetransmittedwithoutusingRTS/CTS.
3.UsageofRTS&CTS
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4.Fragmentation
FragmentationmakesuseoftheRTS/CTSschemeandtheNAV
mechanism:
RTS
SIFS
DIFS
RTS
CTS
Frag0
ACK0
SIFS
SIFS
WS1
AP
CTS
NAVinWS
NAVinAP
Frag1
ACK1
SIFS
SIFS
Frag0
ACK0
Athresholdparameter(dot11FragmentationThreshold)canbesetinthewirelessstation.Frameslongerthanthisvaluewillbetransmittedusingfragmentation.
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Sequencecontrolfield
MPDU(MACProtocolDataUnit)
MACpayloadAddr1 Addr2 Addr3 Addr4(optional)
FCS
Fragmentnumber(for
identifyingfragments)
Framesequencenumber(for
identifyingframes)
4.Fragmentation
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