TABLEOFCONTENTS:
1.Wi-FiProtocol:Networking,FrameFormats,Security,Attributes
2.OSIModelforWi-Fi
3.ArchitectureandNetwork
Topology
4.Security,ServicesandApplications
Wi-FiProtocol:Networking,FrameFormats,Security,
Attributes
Wi-FiArchitectureandNetworkTopology
Whentwoormoregadgets(stationsorSTA)areconnectedtoeachotherthroughWi-Fi,theyformabasicserviceset
(BSS)andtheareathatdevicesconnectedinaBSSarecoveringistermedasBasicServiceArea(BSA).ABSAcan
extendup-totensofmetersinsidewithBSSunitscancommunicateinafullyfledgedmanner.Interestingly,twoormore
BSScanbecollocatedwiththesameBSAandwillstillnotinterfereintoeachother’sworking.Thisispossiblethrough
ad-hocnetworking.Underad-hocnetworking,thestationsarespecificallyconnectedtotheonetheywantto
communicatewith.Ad-hocmodeofnetworkingistemporaryandstationsconnectedunderitcanbedisbanded
whenevertheuserwants.
WhenoneBSShastocommunicatewiththeother,aservicecalledAccessPointisaddedtotheBSS.Through
AccessPoint,awirelessdistributionsystem(WDS)getscreatedwhichconnectstwoormoreBSS.Interconnectionof
multipleBSS’sistermedasExtendedServiceSet(ESS).ItisthroughanESSthatusersofthewirelesssystemcan
connecttoawiredsystemlikeinternetoranEthernetnetwork.AidingESSinthisconnectionisportalwhichisplaced
onthebordersofthedistributionsystem.
Wi-FiFraming
InWi-Fitechnology,threetypesofframehavebeenprotocoled:managementframes,controlframesanddataframes.
TypeofFramePrimeResponsibilitySub-frames
ManagementFrame(MAC
protocolDataUnit)
1.Connectionanddisconnectionof
STAwithAP,
AssosiationRequest
AssosiationResponse
ReassosiationRequest
ReassosiationResponse
Proberequest
ProbeResponse
Beacon
AnnouncementTrafficIndication
Message(ATIM)
Disassosiation
Authentication
Deauthenication
Action
ControlFrame1.Assistwithdeliveryofdataframe.
2.Provideunicastframe
acknowledgements.
PowerSave(PS)Poll
RequesttoSend(RTS)
Cleartosend(CTS)
Acknowledgement(ACK)
Contention-Free(CF)-End(PCF
only)
CF-End+CF-ACK(PCFonly)
Black-ACK(HCF)
BlackAckRequest(HCF)
DataFrame1.Carryactualdatathatispassedfromhigher
layerprotocols.
Data
Data+CF-Ack(PCFonly)
Data+CF-Poll(PCFonly)
Data+CF-Ack+CF-Poll(PCFonly)
Nulldata(nodatatransmitted)
CF-Ack(nodatatransmitted)(PCF
only)
CF-Poll(nodatatransmitted)(PCF
only)
Data+CF-Ack+CF-Poll(PCFonly)
QosData(HCF)
QosNull(NoData)(HCF)
QosData+CF-Ack(HCF)
QosData+CF-Poll(HCF)
QosData+CF-Ack+CF-Poll(HCF)
QosCf-Poll(HCF)
QosCF-ACK+CF-Poll(HCF)
FrameformatinMAC:
Preamble:ItisfirstpartofthePLCPheaderandItindicatestothereceiverthatitisabouttoreceivedata.Thisaidsin
receiveridentifyingbeginningofsignalreceptionandsynchronizeframetransmission.Apreambleisoftwotypes:
LongPreamble:Compatiblewiththe802.11legacy,longpreambletakes192microsecondsfortransmission.Hence,
majorityoftheWi-Firoutersandadaptersarepre-configuredforlongpreamblereception.
ShortPreamble:Notcompatiblewiththe802.11legacyyet,shortpreambletakes96microsecondsfortransmission.It
isincorporatedinthenewstandardswhichareatdevelopmentalstages.
Preambleisdependentonthephysicallayerandconsistsoftwoparts:
Synch:thisis80bitlongsequenceimplementedbyphysicallayertochoosethedestinationandsynchronizedata
transmissionandreceptionfrequency.
StartFrameDelimiter:Itis16bitdigitalcodewhichaidsthereceiverindecidingframetiming.
PLCPHeader:Aforementioned,PLCPlayerconsistsoffunctionstocodeordecodethelogicalinformation(data
packets)transmittedorreceived.ThePLCPheaderconsistsofthreeparts:
PLCP_PDULengthWord:Containsinformationabouttotallybytescontainedindatapacket,hencehelpingthereceiver
todetermineendoftheframe.
PLCPSignalingField:Detailswiththedataratei.e.rateatwhichmessageistransmitted.
HeaderErrorCheck:Thisisa16bitfieldwhichappliesCRCcheckasanerrordetectiontechnique.
MACHeader:MACheaderdetailsaboutframecontrol,duration,addressing,sequencecontroletc.Let’sunderstandall
thefieldsinastandardMACheader.
FrameControl:Itis16bitfieldunderwhichfollowingarespecified:
·Theversionoftheprotocol(a,b,g,netc.)
·Typeofframe:management(00),data(10)orcontrol(01).
·Sub-typeoftheframesent.
·ToDSandFromDSindicatesignaltransmissionfromBSStoDSandDStoBSS,respectively.
·MoreFrag:Incasesoflargemessagetransmission,packetfragmentationtakesplace.This
fragmentationisindicatedbymorefragfield.
·Retry:Someframesmightrequirere-transmissionfromtimetotimeandthroughretryfield,the
receiverisabletofilteroffduplicateframesfromthosewhicharesentonpurpose.
·PowerManagement:Thisfieldindicateswhetherthetransmitterwouldbeinactivestateorpower
savingstateaftertransmissionofmessage.
·MoreData:Whenastationisinpowersavemode,ahighbitonthisfieldindicatesitthatmore
framesarereadytobetransmittedtoitfromAP.
·WEPField:Thisfieldindicatesthesecuritymeasuresontheframe,andgoeshighwhendatais
encryptedorencoded.
FrameDuration:A16bitlongfield,fielddurationisusedinmultipleways:(a)toindicatethedurationoftheframesent
usingnetworkallocationvector;(b)carryingtheIDofthestationthathastransmittedthedata(usedwhencontrolframes
aretransmitted)
Addressfields:ThequadrupleofaddressfieldsalongwithToDSandFromDSfieldofframecontrol,formthe
followingcombinationofdatatransmission:
ToDSFrom
DS
Address1Address2Address3Address4Meaning
01Destination
Address
Source
Address
BSSIDN/ADataFrame
fromstation
tostation
withinaBSS
01Destination
Address
BSSIDSource
address
N/ADataFrame
Exitingthe
DSS
10BSSIDSourceDestinationN/ADataframe
addressAddressdesignedfor
DS
11Receiver
Address
Transmitter
address
Destination
Address
Source
Address
WDSframe
being
distributed
fromAPto
AP
DS:
Distributed
System
AP:
Access
point
WDS:
Wireless
Distribution
system
4.SequenceControlMode:asthenamesuggests,thisfieldisresponsibleformanagingthesequencewithwhich
framesaretransmittedandreceived.Itmakessurethattherearenodisturbancesintheframesequencebyproviding
numbertoeachsequenceofinformation.
PostMACheaderfields
FollowingtheMACheaderaretheframebodyandCRCfield.Framebodycontainstheinformationasindicatedbythe
frametypeandsub-typefieldandCRCfieldgivesacyclingredundancychecktotheMACheaderandtheframebody,
thusminimizingerrorsinthetransmissionoftheframes.
|
|
