Lec 12 Wi-Fi Indoor Wireless Communication

Wi-Fi Indoor Wireless Communication Lecture Slides

1. SpreadSpectrum
Wi-Fi
Engr. Mian Shahzad Iqbal
Lecturer
Department of Telecommunication
Engineering
WirelessFidelity
CSMA
Ad-hocNetworks

2. OutlineforToday
•WelearnedhowtosetupaWiFinetwork.
•Thistimewewilllearnabouttheprotocolsthat
enablethesenetworks.

3. ‘Wifi’–Whatitstandsfor?
“Wi-fi/Wireless-Fidelity,Otherwiseknownas802.11Wireless
Network,isanopen-standard,openspectrum,open-source,open-
hardware, modeofwirelessinter-connectivityforparticipating
devices.”
..

4. ..
Whatawirelessnetworkismadeupof:-
Basestations,Accesspoints,orGateways
WirelessNetworkcards
-Radios which send and receive signals from other radios or access points,
usually PCMCIA* cards which fit into Laptop expansion slots, orPCI Bus in case
of Desktop computers. (There are other, simpler options using USB).
PDAslike PALM, and Pocket PC having a compact flash slot can also connect.
*PCMCIA–Personal Computer Memory Card International Association.
-The base station sends and receives radio signals to and from the Wi-Fi
radio inyour laptop or PC, enabling you to share your Internet connection with
other userson the network. Access points and gateways have a wide range of
features andperformance capabilities, but they all provide this basic network
connection service.

5. ..
TypicalCommunityWifiConstituents
High gainParabolic grid antennasto beam the
signal to over 30kmfrom tower to tower..
Typically5.7-5.8 GHz,2'Diameter Parabolic
GridAntenna,26 dBigain, 6 degree beam
width,N-Female connector
Customer Premises equipment(CPE)to access the
signal from the towers..
Typically23dBm Radio+15dBi Antenna = 38dBm
Sector antennasto beam thesignal from the towers
to the community users
Typically2.4-2.5 GHz, 90 degree sector antenna, 17
dBi gain
Other componentsthat need to be installed in order to put
the above systems together

6. 3GEvolution(source:Nokia)

7. ISMBand
ISMstandsforindustrial,scientific,andmedical.
ISMbandsaresetasideforequipmentthatis
relatedtoindustrialorscientificprocessesoris
usedbymedicalequipment.Perhapsthemost
familiarISM-banddeviceisthemicrowaveoven,
whichoperatesinthe2.4-GHzISMband.The
ISMbandsarelicense-free,providedthat
devicesarelow-power.Youdon'tneedalicense
tosetupandoperateawirelessnetwork.

8. BasicTechnologyConcepts
WiFi b-a-g
IEEEStandardsforWirelessLANSpreadSpectrum
RadioTechnology(802.11)
•802.11b-2.4GHz@11mbps
•802.11a-5GHz@54mbps
•802.11g-2.4GHz@54mpbs
•802.11e-QoSservices
•802.11i-802.1xsecurity

9. BasicTechnologyConcepts
WiFi b-a-g
Cordless phone
Microwave oven
Bluetooth
Hiperlan
devices
Cordless phone
Microwave oven
Bluetooth
Interferenc
e sources
WorldwideUSWorldwide
availability
54Mbps54Mbps11Mbps
Max data
rate
2.4GHz5GHz2.4GHz
Frequency
band
802.11g802.11a802.11b
The Rules of Thumb of Radio
Higher data rates usually imply shorter transmission range
Higher power output increases range, but increases power consumption (less battery life)
The higher the frequency, the higher the data rate (but smallerrange).

10. BasicTechnologyConcepts
WiFi b-a-g
?
5.5Mbps
11Mbps
36Mbps
54Mbps
802.11g
estimates
12Mbps11Mbps
125 ft
1Mbps
350 ft
2Mbps
250 ft
6Mbps5.5Mbps
150 ft
36Mbps11Mbps
100 ft
54Mbps11Mbps
50 ft
802.11a
@40Mw
802.11b
@100Mw

11. Spreadspectrumin802.11
•Itisarequirementimposedbythe
regulatoryauthoritiesfordevicesinISM
bandinordertoreduceinterference.
•Thereisalsolimitationsontransmitted
power.
•Wediscusstwomethodsspecifiedin
802.11,FHSS(FrequencyHopping
SpreadSpectrum)andDSSS(Direct
SequenceSpreadSpectrum)

12. DSSSin802.11
•Direct-SequenceSpreadSpectrum
(DSSS)
•Usedby802.11b
•Symboltransmissionrate=1Mbps
•Multipathspreadofupto1/1Mbps=1 µsdoes
notcauseISI. Forindoorapplicationsthis
ensuresthatthesystemdoesnotsufferfromISI.
•Chiprate=11Mcps
•Resolutionisontheorderof1/11Mcps=90ns.

13. Frequency-hoppingspread
spectrum
•Frequency-hoppingspreadspectrum
(FHSS)isamethodoftransmittingradio
signalsbyrapidlyswitchingacarrier
amongmanyfrequencychannels,usinga
pseudorandomsequenceknowntoboth
transmitterandreceiver.

14. 802.11b
•The802.11bstandarddefinesatotalof14
frequencychannels.
•FCCallowschannels1through11withinthe
U.S. MostofEuropecanusechannels1
through13.InJapan,only1choice:channel14.
•Channelrepresentsacenterfrequency. Only5
MHzseparationbetweencenterfrequenciesof
channels.
21 3 4 5 6 7 8 91011
2.412 2.417 2.422 2.427 2.432 2.437 2.442 2.447 2.452 2.457 2.462
Channel
Center
Frequency
(GHz)
5 MHz

15. 802.11b(Cont’d)
•Any802.11bsignaloccupiesapproximately30
MHz.
•Thus,802.11bsignaloverlapswithseveral
adjacentchannelfrequencies.
•Onlythreechannels(channels1,6,and11for
theU.S.)thatcanbeusedwithoutcausing
interferencebetweenaccesspoints.
•Anygivenareacanthereforesupportatmost3
accesspoints(operatingondifferentchannels)
atonce. Equivalently,itcanatmostsupport
threelocalad-hocconnections.

16. 802.11b(Cont’d)
2
1
3
Access
Point
NeighboringAP’susedifferentchannelstoreduce
interference.“Reusecluster”sizeisequalto3.

17. 802.11b(Cont’d)
•Ideally,802.11bsupportswirelessconnections
betweenanaccesspointandawirelessdevice
atfourpossibledatarates:1Mbps,2Mbps,5.5
Mbps,and11Mbps.
•Specifically,asterminaltravelsfartherfromits
AP,theconnectionwillremainintactbut
connectionspeeddecreases(fallsback).

18. 802.11b(Cont’d)
11 Mbps
5.5 Mbps
2 Mbps

19. 802.11bSpreadSpectrum
•Whena802.11bradioisoperatingat1Mbps
andwishestotransmitabit1,ithastodosoin
0.000001seconds.
•Theway802.11bdoesisthisbyactually
transmittingafixedsequenceof11shorterbits
(01001000111)torepresentasinglebit“1”.
These11“shorterbits”(whichrepresentone
informationbit)aresentin1/11thetime,i.e.,
0.0000000909seconds.
•Theseshorterbitsarecalled“chips.”

20. 802.11bSpreadSpectrum
(Cont’d)
•Whentheradiowishestotransmita0
informationbit,itusesthe0.000001secondsto
transmitadifferentfixedsequenceofchips,
01001000111.
•Thechipsequenceusedfor“1”isthe
complementofthechipsequenceusedfor
sendinga“0”.

21. 802.11bSpreadSpectrum
•Assumetheoriginalsignal(theinformation
streamof1’sand0’s)occupiesafrequency
bandwidthofWHz.
•WhenweuseNchipstotransmit1bit,the
bandwidthoftheresultingsignalnowoccupies
NWHz.
•Thenewsignalhasalargerspectrum,i.e.,the
informationsignalofbandwidthWhasbeen
spreadtoabandwidthofNW. Forthisreason,
thisprocessiscalledspreadspectrum.

22. SpreadSpectrum
W Hz
Frequencyrepresentationoftransmittedsignal,
beforeandafterspreading.
NW Hz
Before After
Both signals contain the same information. The second signal uses
less power/Hz (height is less). This helps meet FCC mandates in
unlicensed bands.

23. 802.11b(Cont’d)
•Theaboveprocedureisusedtoget1Mbps.
•Whataboutthehigherdatarates?
•Thisisachievedbyusingmorecomplex
modulationschemesand/orchangingthechip
sequence.
•Recallmodulationschemeistheschemeused
toencodeabitstreamintohigh-frequencysine
waves,i.e.,radiowaves.

24. 802.11a
•802.11aspecificationoperatesatradio
frequenciesbetween5.15and5.825GHz,i.e.
802.11autilizes300MHzbandwidth
•TheFCChasdividedtotal300MHzinthisband
intothreedistinct100MHzbands:low,middle,
andhigh,eachwithdifferentlegalmaximum
power.
Band Channel Max Power
High band 5.725-5.825 GHz 9-12 1000mW
Middle band5.25-5.35 GHz 5-8 250mW
Low band 5.15-5.25 GHz 1-4 50mW

25. 802.11a(Cont’d)
•Becauseofhighpoweroutput,highbandused
forbuilding-to-buildingproducts. Lowertwo
bandssuitableforin-buildingwirelessproducts.
•In802.11a,radiosignalsaregeneratedusinga
methodcalledOrthogonalFrequencyDivision
Multiplexing(OFDM).
•OFDMisdefinedoverthelowertwobands(low
andmiddle).

26. 802.11a(Cont’d)
•Thelowandmiddlebandshaveatotalof200
MHzoffrequency.
•This200MHzsupports8non -overlapping
channels.
•Eachchannelissplitin52bands,each
approximately300kHzwide.
•Eachofthesesmallerbandsiscalledasub
carrierinOFDMterminology.
•InOFDM,atransmittercanselectsomenumber
ofsubcarrierstotransmitasignalover.

27. 802.11a(Cont’d)
•Dependingonthenumberofsubcarriers
chosen,thetransmittercanachieve
transmissionratesof6,9,12,18,24,36,48,or
54Mbps.
•Sincethereareeightnon-overlappingchannels,
802.11acansupport8differentaccess-pointto
wirelessdevicelinksinagivenlocation. Or
equivalently,itcansupportatmost8adhoc
connectionssimultaneously.
•Thisisanimprovementover802.11b,where
only3couldbesupported.

28. 802.11a(Cont’d)
1
2
3
Access
Point
4
5
6
7
8
NeighboringAP’susedifferentchannelstoreduce
interference.“Reusecluster”sizeisequalto8.

29. 802.11a(Cont’d)
•Thevariousdataratesaresupportedin802.11a
byvaryingthenumberofsubcarriers,the
modulationscheme,etc.
•802.11a(like11b)hasaratefallback
mechanism,i.e.,asthedistancebetweenthe
transmitterandreceiverincreases,the
supporteddataratedecreases.

30. 802.11a(Cont’d)
54 Mbps
48 Mbps
36 Mbps
24 Mbps
12 Mbps
2 Mbps
5.5 Mbps
11 Mbps
802.11a 802.11b

31. 802.11g
•802.11goffersthroughputof802.11awith
backwardcompatibilityof802.11b.
•802.11goperatesover3non-overlapping
channels.
•802.11goperatesin2.4GHzbandbutit
deliversdataratesfrom6Mbpsto54Mbps.
•802.11galsousesOFDMbutsupportsspread -
spectrumcapabilitiesifanyonecomponentof
thesystemhasolderequipment,i.e.,802.11b
equipment.

32. 802.11g
•Onceagain,802.11g’s"backwardcompatibility"
with802.11bmeansthatwhenamobile802.11b
devicejoinsan802.11gaccesspoint,all
connectionsonthataccesspointslowdownto
802.11bspeeds.
•Soboth11aand11gofferthesamedatarates.
Whichisbetter?

33. Comparing11aand11g
•802.11aoperatesinunderused5GHzband;
802.11goperatesinheavilyused2.4GHzband.
•11gsystemsexperienceinterferencefrom
other2.4GHzdevicessuchascordlessphones,
microwaveovens,satellites,etc.
•Both802.11aand802.11goffersupto54Mbps
speedsinthelab.

34. Comparing11aand11g
(Cont’d)
•Inthefield,802.11adeliversabout20Mbps.
•802.11b's11Mpbstheoreticalspeedismore
often4Mbpsinpractice.
•Therealisticdataratesquotedfor802.11gthus
farrangefrom6Mbpsto20Mbps.
•11ghastocontendwithmoreinterferenceinthe
2.4GHzrangeascomparedto11ainthe5GHz
band.

35. Comparing11aand11g
(Cont’d)
•Highernumberofchannelsin11aallowsmore
flexibilityinavoidinginterference.
•Rangewilldependonantennagain,transmit
powerappliedtotheantenna,thereceive
sensitivityoftheradiocardandtheobstacles
betweenpathends.
•802.11ahasrange150-300ftinpractical
scenarios. 11ghasrangecomparableto11b
(approximately1000ft).
•11arangeissmallerthan11band11g. Thisis
because11aoperatesatamuchhigher
frequencyband.

36. Comparing11aand11g
(Cont’d)
•Generally,802.11aisthemostexpensiveofthe
threeoptions.
•802.11bisthecheapestandmostpopular
WLANoption.
•802.11gismoreexpensivethan11bbut
cheaperthan11a.
•Becauseofitssmallerrange,11arequiresmore
AccessPointstoaregion,therebyincreasing
cost.

37. Whatdoesatypical802.11
Packetlooklike?
•Typical802.11packet:
•Preambleisusedtosynchronizethereceiver,so
itcantellwhenthepacketstarts. Itcontains96
bits.
•PLCP(PhysicalLayerConvergenceProcedure)
indicateshowmanybytesindataportion,what
isthedatarateofthetransmission,etc. This
portioncontainsabout192bits.
PreamblePLCP Header Data CRC

38. 802.11Packet(Cont’d)
•Dataistheactualdatatransmittedbythe
source. Thiscontainssource/destination
addresses,theinformationconveyedbetween
thetwo,whetherWEPisonornot,etc. The
amountofdatabitscanvary. ~200bitsto
~18000bits.
•CRCisthecyclicredundancycheck,whichis
wayofcheckingiftherewasanerrorinthe
receivedsequenceofbits. Thisisusually32
bitslong.
PreamblePLCP Header Data CRC

39. HowareMultipleTransmitters
Supported?
•Recallthemethodforsupportingmultiple
transmitteriscalledthemultipleaccessmethod.
•In802.11systems,onlyoneuserisallowedto
communicatewithareceiveratatime(cannot
useanotherfrequencychannelsupporta
secondorthirdadditionaluser).
•Thewaytheoneuserisselecteddependson
thecarriersensemultipleaccesswithcollision
avoidance(CSMA/CA)randomaccessmethod.

40. CSMA
•TohelpillustratetheoperationofCSMA,wewill
useananalogyofadinnertableconversation.
•Let’srepresentourwirelessmediumasadinner
table,andletseveralpeopleengagedinpolite
conversationatthetablerepresentthewireless
nodes.

41. CSMA(Cont’d)
•Thetermmultipleaccesscoverswhatwe
alreadydiscussedabove:Whenonewireless
devicetransmits,allotherdevicesusingthe
wirelessmediumhearthetransmission,justas
whenonepersonatthetabletalks,everyone
presentisabletohearhimorher.
•Nowlet'simaginethatyouareatthetableand
youhavesomethingyouwouldliketosay.
•Atthemoment,however,Iamtalking.

42. CSMA(Cont’d)
•Sincethisisapoliteconversation,ratherthan
immediatelyspeakupandinterrupt,youwould
waituntilIfinishedtalkingbeforemakingyour
statement.
•ThisisthesameconceptdescribedintheCSMA
protocolascarriersense.
•Beforeastationtransmits,it"listens"tothe
mediumtodetermineifanotherstationis
transmitting.Ifthemediumisquiet,thestation
recognizesthatthisisanappropriatetimeto
transmit.

43. CSMA/CA
•Carrier-sensemultipleaccessgivesusagood
startinregulatingourconversation,butthereis
onescenariowestillneedtoaddress.
•Let’sgobacktoourdinnertableanalogyand
imaginethatthereisamomentarysilenceinthe
conversation.
•YouandIbothhavesomethingwewouldliketo
add,andweboth"sensethecarrier"basedon
thesilence,sowebeginspeakingat
approximatelythesametime.In802.11
terminology,acollisionoccurswhenweboth
spokeatonce.

44. CSMA/CA(Cont’d)
•Thecollisionwillresultinanunexplained
messagetotheintendedreceivers(listeners).
•Whatweneedisapolitecontentionmethodto
getaccesstothemedium;thisisthecollision
avoidancepartofCSMA/CA.
•802.11hascomeupwithtwowaystodealwith
thiskindofcollision.
•Oneusesatwo-wayhandshakewheninitiating
atransmission.
•Theotherusesafour-wayhandshake.

45. 2WayHandshake
•Nodewithpackettosendmonitorschannel.
•Ifchannelidleforspecifiedtimeintervalcalled
DIFS,thennodetransmits.
•Ifchannelbusy,then
–nodecontinuestomonitoruntilchannelidle
forDIFS.
–Atthispoint,terminalbacks-offforrandom
time(collisionavoidance)andattempts
transmittingafterwaitingthisrandomamount
oftime.

46. 2WayHandshake
•Ifthenodedoesnotback-offtherandomtime,
thenitwilldefinitelycollidewithanothernode
thathassomethingtosend.
•Reasonforrandomback-offtimeisthatifI
choosearandomtimeandyouchoosea
randomtime,theprobabilitythatwechoosethe
samerandomtimeisslim.
•Thiswaywebothback-offtransmittingandwill
thereforewillprobablynotinterferewitheach
otherwhenwearereadytotransmit.

47. 2WayHandshake(Cont’d)
•Firstwayofthe2wayhandshakewasforthe
transmittertosenditsinformationpackettothe
destinationnode,afterfollowingthecollision
avoidancemethoddescribedabove.
•Ifthepacketreachesthedestinationwithout
problems,thedestinationsendsashortpacket
overthewirelessmediumacknowledgingthe
correctreception.
•ThispacketistypicallycalledanACKpacket.
ACKisthesecondwayofthe2wayhandshake.

48. 4WayHandshake
•“Listenbeforeyoutalk”
•Ifmediumisbusy,nodebacks-offforarandom
amountoftimeafterwaitingDIFS,justas
before.
•Butnow,insteadofpacket,sendsashort
message:ReadytoSend(RTS). Thismessage
isbasicallyattemptingtoinformothersthat“I
havesomethingtosend.”

49. 4WayHandshake(Cont’d)
•RTScontainsdestinationaddressandduration
ofmessage.
•RTStellseveryoneelsetoback-offforthe
duration.
•IfRTSreachesthedestinationokay(noone
elsecollideswiththismessage),thedestination
sendsaCleartoSend(CTS)messageafter
waitingaprescribedamountoftime,called
SIFS.

50. 4WayHandshake(Cont’d)
•AftergettingtheCTS,theoriginaltransmitter
sendstheinformationpackettoitsdestination.
•Inthesesystems,thetransmittercannotdetect
collisions. ThereceiverusestheCRCto
determineifthepacketreachedcorrectly. Ifit
doesthen,itsendsoutanACKpacket.
•IftheinformationpacketnotACKed,thenthe
sourcestartsagainandtriestoretransmitthe
packet.

51. 4WayHandshake(Cont’d)
Access
Point Laptop
RTS
CTS
Data
ACK

52. WirelessLANNetworks

53. WLANArchitecture—AdHoc
Mode
•Ad-Hocmode:Peer-to-peersetupwhere
clientscanconnecttoeachotherdirectly.
Generallynotusedforbusinessnetworks.

54. AdHocStructure
•Mobilestationscommunicatetoeach
otherdirectly.
•It’ssetupforaspecialpurposeandfora
shortperiodoftime. Forexample,the
participantsofameetinginaconference
roommaycreateanadhocnetworkatthe
beginningofthemeetinganddissolveit
whenthemeetingends.

55. WLANArchitecture--Mesh
•Mesh:Everyclientin
thenetworkalsoacts
asanaccessorrelay
point,creatinga“self-
healing”and(in
theory)infinitely
extensiblenetwork.
–Notyetinwidespread
use,unlikelytobein
homes.

56. WLANArchitecture—Infrastructure
Mode
To Wired Network

57. Infrastructurenetwork
•ThereisanAccessPoint(AP),whichbecomesthe
hubofa“startopology.”
•AnycommunicationhastogothroughAP. Ifa
MobileStation(MS),likeacomputer,aPDA,ora
phone,wantstocommunicatewithanotherMS,it
needstosendtheinformationtoAPfirst,thenAP
sendsittothedestinationMS
•MultipleAPscanbeconnectedtogetherandhandle
alargenumberofclients.
•UsedbythemajorityofWLANsinhomesand
businesses.

58. ExtendedServiceArea

59. Q&A