wi-fi range demystified tutorial - university of cyprus and coverage xirrus… · type of antenna...

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Wi-Fi Range DemystifiedTutorial

Across Down1. Originalbandoffrequencies9.Whenasignalchanges

direction11.Typeofantennaresembling

aflashlight12.Ratioofsignaltonoise14.Highestperformingaccess

device16.Totalareaoftheradio18.Contiguousfrequencies19.Maximumdistancebetween

tworadios21.Rateatwhicharepeating

eventoccurs

25.One-millioncyclespersecond

26.Amountofdatasentinagiventime

27. Pipediameter31.AnIsotropicantenna32.Pathforsignals33.OnlyWi-FiPowerPlay35.ReceiveSignalStrength

Indicator36.Antennapatternresembling

adonut37. Instituteofengineers38.Circuitrytointerpretand

execute

2.Whenthesignalisscattered3. Splitsonebandintomany4.Whenthesignalbecomes

bent5. dBistheabreviation6.Managesaddressingand

protocolinformation 7. Receive/sendradiosignal8. Occurswhensignallsbounce10.Oppositeoftransmitter13.Speedoflight15.Multipleantennastoimprove

rateandrange

17. Reductioninsignalpassingthroughobjects

20.109Hz22.Oppositeofreceiver23.Xirruslanguage24.Whenthesignalreflectsback25.Toblenddataintoacarrier

signal28.Boostpower,voltageor

currentofasignal29.Shpericalradiationpattern30.Conveysdatabetweenpoints34.Anotherwordfor

amplification

2 ©2008Xirrus,Inc.AllRightsReserved.

Wi-Fi Range Demystified

ContentsIntroduction............................................................................... 3

DefiningRangeandCoverage...................................................... 3

RangeBasics............................................................................. 4

AntennaDesign.......................................................................... 5

RangeandCoverage................................................................... 6

RangeLimitingFactors................................................................ 6

Multipath................................................................................... 7

Attenuation................................................................................ 8

HiddenNode.............................................................................. 8

Signal-to-NoiseRatio(SNR)......................................................... 9

RangeVersusCapacity.............................................................. 10

802.11nTechnology................................................................. 11

Recommendations.................................................................... 12

LeadingArchitecture................................................................. 13

AboutXirrus............................................................................. 13

©2008Xirrus,Inc.AllRightsReserved. 3

IntroductionDistancelimitationsanddataratesarefullyunderstoodwithwiredEthernetnetworksduetotheutilizationofspecifictransmitter/receiverstandardsandacontrolledmedia,thewire.However,thedistancelimitationsanddatarateswithWi-Finetworksaremoredifficulttocalculateduetovaryingdatarates,capacity,interference,etc.ThisdocumentwillwalkyouthroughsomebasicWi-Fiprinciplessuchasantennadesign,gain,pathloss,frameformat,multipath,etc.toenableyoutodeployahighperformingWi-Finetworkforyourorganization.

Defining Range and Coverage

Basic Wi-Fi Radio Link

Baseband Radio

Radio Module Radio Wave (Travels at the speed of light, c)

BasebandRadio

Radio Module

Radio frequency signal is modulated and encoded with data. Wavelength is represented by λ.

WavelengthLow NoiseAmplifier

Power Amp

Low NoiseAmplifier

Power Amp

Wi-Fi Range Demystified Tutorial

BeforetheRFsignalleavestheantenna,adigitalsignalprocessor converts the data stream into complexsymbolsthatcarryitovertheairasitistransmitted.From there thesignalgoes intoa radio transceiverthat translates those symbols to a specific carrierfrequency.InthecaseofWi-Fi,itiseitherthe2.4GHz(for802.11b/g)or5GHz(802.11a)frequencyranges.Thosesignalsthenpassthroughapoweramplifieronthewayoutofthetransmitter’santenna.Theantennaontheotherendofthesignalwillpassthatreceivedsignaldowntoalownoiseamplifierofthereceiver.Thiscompletestheradioconnectionbetweenthetwoendsofaradiolink.

Betweenthetwoantennas,informationwilltravelonthe radio waves at the speed of light as itmovesfromtransmittertoreceiver.Radiofrequencysignalismodulatedandencodedwithdataandawavelength.

Wedefinerangeas“themaximumdistanceatwhichtworadioscanoperateandmaintainaconnection.”Thereforewecanusesimplegeometrytodeterminethe coverage area of an Access Point using theformulatodeterminetheareaofacircle(π)r2wherethe radius (r) is the range of theWi-Fi signal. ThecoverageareaofanAccessPointisoftenreferredtoasacellandthesetermswillbeusedinterchangeablethroughoutguide.

Forexample,theexpectedcoverageareaofaWi-Fidevicewitha300’operatingrange(r),wouldyielda280,000squarefootcoveragearea.


Link Formulas

RR LGPathLossGP TT −+−+=PR

Expected Free Space Signal Strength at the Receiver taking Transmit Power, Antenna Gain, Receiver Gain, Distance, and Frequency into account.

2

= GGPP RTTR 4 dπλ

Signal Strength (RSSI)

Gain and Transmit Power

~ 2dPP T

R

In Free Space, Power varies inversely with the square of the distance between two points.

RF Power Dissipation

Path Loss

Expected Signal Loss between a Transmitter and a Receiver using an appropriate Path Loss Exponent, n, for the environment. (See Path Loss Exponent chart).

4πλ

dPathLossdB n log1020 log +=

Link Budget

Expected Signal available in an interference-free environment for a given Transmit Power, Antenna Gain, Path Loss, and Receiver Loss. (See SNR).

PR—Power at the Receiver

PT—Power at the Transmitter

GT—Antenna Gain of the Transmitter

GR—Antenna Gain of the Receiver

λ—Wavelength (speed of light/frequency)

π—Ratio of a Circle’s Circumference to its Diameter, approximately 3.14

d—Distance in Meters

L R—Receiver Loss including Insertion Loss, Noise Figure, etc.

Signal Priorto Gain

Signal AfterGain

Ampl

itude

Range BasicsIt is important tounderstand that range isa functionofdatarateorsimplyput,thehigherthedatarate,theshortertherange.InordertounderstandwhatgoesintodeterminingtherangeofanAccessPoint,afewtermsneed tobedefinedandabasicunderstandingof themathematicsthatgoesintodeterminingthedistancebywhicharadiosignalwilltravelneedstobeprovided.

Inanopenenvironment,orwhatisreferredtoasFreeSpace, Power varies inversely with the square of thedistance between two points (the receiver and thetransmitter). The stronger the Transmit Power, thehigher thesignalstrengthorAmplitude.AntennaGainalsoincreasesAmplitudeandwillbefurtherdiscussedinasubsequentsectionofthischapter.

WhileGainandPowerincreasethedistanceawirelesssignalcantravel, theexpectedsignal loss (PathLoss)betweenthetransmitterandareceiverreducesit.PathLoss is the reduction in signal strength that a signalexperiencesasittravelsthroughtheairorthroughobjects

between the transmitter and receiver. The relativestrength of that signal at the receiver is measuredas the Received Signal Strength Indicator (RSSI).RSSIisnormallyexpressedindBmorasanumericalpercentage.Forclarificationpurposes,adB(Decibel)isameasureoftheratiobetweentwoquantitieswhiledBmisaDecibelwithrespecttomilliwattsofpower.Anoverall LinkBudget canbedefinedby taking intoaccountallthegainsandlossesofasignalasitmovesfromatransmittertoareceiver.

Range and Coverage

Range (Distance)

Coverage Area

r

2rarea π=Range—Maximum Distance between two radios for which a connection can be maintained.

Coverage—The total area in which all radios can maintain a connection to the AP.

Example: If the range of a cell is 300ft then the coverage area = 3.14 * (300)2 = 282,600sqft.


AntennasplayakeyroleindeterminingtheamountofrangeandcoverageareaofaWi-Finetworksolet’sstartwithashortdiscussiononthephysicallayeroftheWi-Ficonnectionwithanoverviewonantennas.

An Isotropic antenna has a radiation pattern of aperfectsphere. Imagineadevice thathasapowerdensityequalinalldirections.TheIsotropicpatternforms the basis fromwhich all other antennasaremeasured.Oneof thesimplestantennadesigns isthe dipole, like the simple whip antenna on mostcars.WhereanIsotropicantennapatternisspherical,thedipolehasaradiationpatternofatorrid(likeadonut).Thelargestamountofenergyisbeingradiatedperpendiculartotheantenna,inmostcasesthisisalongthehorizontalplane.Youcanvisualizethisbythinkingofapebbledroppedintoacalmpond,thewavepattersextendfromthecenterinalldirectionsalongatwodimensionalplane.

Gain (also known as Amplification) is critical toimproving the range of an antenna and thereforeplaysacriticalpartindetermining(orextending)therangeofaWi-Finetwork.GainreferstoanincreaseoftheAmplitudeorSignalStrengthandcomesintwoforms;activeandpassive.ActiveGain refers toanincreaseinpowerthatisappliedtotheantennawhere

passiveGain isachievedby focusing theenergyoftheantennainaparticulardirection.GainisusuallyexpressedasaratioindB’s.Itisthelogoftheratioofthepowers.Forexample,atypicaldipoleantennawillhaveabout2dbi,(i=isotropic)ofGain.

One of the advantages of a dipole, or any type ofdirectionalantenna,isgreaterantennaGain;thisisaresultoftheRFenergypatternbeingfocusedvs.anisotropicdesign.Other typesofantennasaremoredirectionalindesigntakingtheirradiatedenergyandsqueezingitintoaverynarrowpattern.

A good analogy here is to think of the isotropicantenna like a light bulb radiating energy equallyinall directions,and thedirectionalantenna likeaflash lightwith the light focused inonedirection–theenergyofthedirectionalantennaisconcentratedinaparticulardirection,enablingthebeamtotravelmuchfartherthananisotropicantenna.

AntennaGain is bi-directional so it will amplify thesignalasitisbeingtransmittedandasitisreceived.Soifadirectionalantennaisproviding6dbGainontransmit,itwillalsoincreasereceivedsensitivityanequal amount so the antenna design of the Wi-FiAccess Point plays a critical role in the amount ofrange(coverage)delivered.

Antenna Basics

Isotropic Dipole

0dBi 2.2dBi 6dBi 6dBi per sector

High Gain, Directional High Gain, DirectionalMulti-sector Array

Example: Light Bulb Example: Flashlight Example: Multiple Flashlights

Pisotropic

PdirectionalGain = Power Density (directional Antenna)

Power Density of Isotropic AntennaGl=

Antenna Design


Gain and antenna design can be used to increasetherangeofaWi-FiAccessPoint.Wealsomentionedthat physical and environmental factors will impactthatsignal,thissectionwillexamineseveralofthosefactors.

802.11b/guses the2.4GHz ISMband.Manyotherdevices also operate in the 2.4GHz ISM band andinterference causes data to be garbled forcingpackets to be re-transmitted. This causes reducedend-user throughput and increased latency of datatraversingtheWi-Finetwork.

Conversely,the5GHzbandfor802.11aisrelativelyclean from interfering devices. 802.11a is alsodeemedas theprimaryuserof thespectrum.Thisdisallowsothertypesofwirelessdatadevicesinthisband.Since the2.4GHzband ismore susceptibletointerference,itishighlyrecommendedtomigratetowardsan802.11aenvironment.

Devices in the 2.4GHz Band

Bluetooth Devices

HomeRF

Cordless Devices

Microwave Ovens

X10 Wireless Video Cameras

HAM Radio Operators

Range and CoverageThevastmajorityofAccessPointdeploymentstodayconsistofproductsthatuseomni-directionalantennas.Forthemostpart,thistypeofdeploymenthasservedthemarketwellforhomeuseandlightuseintheenterpriseandinpublic-accesstypesoflocationslikeairportsandcoffeeshops.ButwiththeincreaseinWi-FiusersandtheassociatednumberofAccessPointstosupportthem,theomni-directionalantennabecomesitsownworstenemyinthebattletoaddressimprovedrangeperformance.

NomatterwhattypeofAccessPointisused,theirrespectiveuseofanomni-directionantennathatblastsRFenergyinalldirectionsbecomesabarriertotheperformanceneededfortoday’sWi-Finetworks.Thisproblemconsistsofanumberof issuesthatall limithigh-performancedeployments:cellsize,channelreuse,hiddennodesandmulti-path.

Aswe’vediscussed,Omni-directionalantennastransmitandreceiveRFenergyinalldirections.DirectionalorsectoredantennasfocusRFenergyintoasingledirection,therebyintensifyingthestrengthofthesignal(Gain)that is transmittedand increase the receiversensitivity for trafficcoming fromtheclients.SincedirectionalantennasoffermoreGain,theyhavetheabilitytotransmitfurtherand“listenbetter”tothesignalsofwirelessstations(clients)thereforeincreasingtherangeandcoverageoftheAccessPointinagivendirection.

Thedrawbackofadirectionalhigh-gainantennaisthatitdoesnotcoverthesameareaasastandarddipoleantenna. Thesolutionhere is toarrange thedirectional antennas ina circular patternand createanarrayofantennas.Thisprovidesthe360degreesofcoverageofatraditionaldipoleantenna,yetofferstherangeadvantagesofadirectionalantenna.Somesectoredantennasystemshavemultipleradiosandmultipleantennasthatallowfor360degreesofincreasedcoverage.

Range Limiting Factors


Omni-directional antennas, and to a lesser extentdirectional antennas generate vast amounts of RFenergythatistransmittedintotheWi-Fienvironment.Multipath occurs when signals bounce offmultipleobjectsintheenvironmentandarereflectedbacktothereceiver.

Multipath Interference

The effect is that weaker “copies” of the originalsignal arrive slightly later than the primary signal.Thiscausesinter-symbolinterferenceandgetsworseasthe“delayspread”,ortimebetweenthereceptionoftheprimarysignalandsecondarysignalincreases.Theend result is corrupt packets thatmust be re-transmitted,loweringnetworkperformance.

Multipath Time Delay

Secondary Reflected Signal

Time

Primary Desired Signal

Any type of reflected signal that can be additive ordestructivetotheoriginalsignalisidentifiedasmultipathinterference.Asthesignalstrikesanobjective,itcanreact in several ways creating reflection, scattering,refraction,diffractionoralloftheabove.

Signal Changes

Reflection

Scattering

Refraction

Diffraction

Reflectionissimplywhenthesignalisreflectedback

towards the transmitter. Scattering occurs when

thesignalisscatteredbacktowardsthetransmitter

into multiple new signals. Refraction occurs when

the signal is bent as it passes through an object

and Diffraction happens when the signal changes

directionasitpassesaroundanobject.

Insomecases,astrongenoughsignalreceivedoutof

phasecanessentiallycreateanull,aspotwhereno

signalisavailable.Yetonlyafewfeetawayyoumay

haveastrongsignal.Thisiscalledamultipathnull.

Oneimportantnotehereisthebenefitofdirectional

antennas in limiting multipath interference. As

previouslydiscussed,omni-directionalAccessPoints

inherently create large amounts of performance-

robbing multipath. With a directional antenna, this

problem is greatly reduced because RF energy is

not blindly transmitted in all directions. RF signals

aretransmittedinthedirectionofthewirelessclient

withinagivensector

Multipath


Attenuation

Hidden Node

RFsignalstrengthisreducedasitpassesthroughvariousmaterials.ThiseffectisreferredtoasAttenuation.Asmore Attenuation is applied to a signal, its effectiverangewillbereduced.TheamountofAttenuationwillvarygreatlybasedonthecompositionofthematerialtheRFsignalispassingthrough.

Attenuation Effects

Attenuation

The signal strength is reduced as it passes through an object.

Material Typical Attenuation (Loss) @ 5GHz

Cubical Wall 2dB

Drywall or Sheetrock 3dB

Brick Concrete or Block Wall 15dB

Elevator Shaft 10dB

Glass or Window 3dB

Concrete Floor 11dB

Cubicalwallsoffer relatively lowattenuation, in the2dbrange,whileconcreteandbrickwallswillcausehigher attenuation levels reducing the range of anAccess Point. It is extremely important to considernot just the type of obstruction, but how manyobstructionstheRFsignalmustpassthroughwhendesigningaWi-Finetwork.

Adirectionalantennaoffersanadvantageoveromni-directionalantennaswhenitcomestoattenuationastheyarebetterabletopenetratedifferentmaterialsthantraditionaldipoleantennas.

ThebottomlinetorememberisthatantennaselectionandthephysicalenvironmentofthefacilityhavethebiggestimpactonrangeandcoverageperformanceofanAccessPoint.However,itisimportanttonotethesearenottheonlyfactorsinvolved;the802.11specification in and of itself creates issues thatimpacttheoverallperformanceoftheWi-Finetwork.

The802.11specificationoperatesundera“collisionavoidance”schemawhereasclientsmustwaitforthemediumtobefreebeforemakingatransmission.ThisbasicpremisecreatesasituationwheretwoclientswithinaWi-Ficell(coverageareaoftheAccessPoint)arewithinrangeoftheAccessPoint,butoutofrangeofeachother.Awirelessstationononeedgeofacellmaynothearastationontheothersideofthecell.Because of this, wireless stations will not be ableto hear when the other is transmitting; incorrectlyassumingtheairisidleandbegintotransmititsownpackets. This will cause the two transmissions tocolliderequiringbothstationstore-transmitgreatlyreducingtheeffectivebandwidthwithinthecell.

A protection mechanism exists with in the 802.11standard calledCTS-RTS that can help address thisissuerequiringeachclienttoaskforpermissionfromtheAccessPointbeforetransmitting.Buttheuseofthisprotocolcreatesoverheadonthenetworkandwillreduceoverallperformanceby30%.Anothermethod

commonlyusedtoeliminateahiddennodeissueistoreducetherangeoftheAccessPointbydecreasingthetransmitpower.ByreducinganAccessPoint’srange,itincreasestheprobabilitythatallclientswithinthecellwillheareachother;butgreatlyincreasesthenumberofAccessPointsneededwithinthedeployment.

The use of a directional antenna over an omni-directional antennawill also eliminate the issue ofHiddenNodesbecauseallwirelessstations(clients)in a given RF sector are associated to the sameAccess Point; so they are geometrically within thesamesector.Sincetheclientsoperateinthesamesector,thehiddennodeproblemiseliminatedasallstations are able to hear each other and correctlydeterminewhentheairisbusyoridle.Thiseliminatesthe performance-robbing issues found with legacyomni-directional Access Points and the use of theCTS-RTSprotocol.ItalsohastheaddedbenefitofnotincreasingthenumberofAccessPointsorreducingtheirrespectivecoverageareas.


Hidden Node Problem

Station 1 Station 2

Sectored Approach Eliminates Hidden Nodes

Note: Station 1 and Station 2 cannot hear each other’s transmissions in the omni-directional antenna example (above left) whereas stations can hear each other in the directional (sectored) antenna approach.

Signal-to-Noise Ratio (SNR)The rangeofanAccessPoint isa functionofdatarate.Thenotionthathigherdataratesdonotappearto“travel”asfarasthelowerdataratesisafunctionoftheSignaltoNoiseRatio(SNR)andnotbecausethe Access Point and the client can’t necessarily“hear”eachother.

SNR is the ratioof thedesiredsignal to thatofallothernoiseandinterferenceasseenbyareceiver.SNRisimportantasitdetermineswhichdataratescan be correctly decoded in a wireless link. It isexpressedindbasaratio.

Thereceivedsignal,thenoiselevel,(ornoisefloor)andwiththesetwovalues,theSNRcanbedetermined.Asdata rates increase from 6Mbps to 54Mbps, morecomplexmodulation and encodingmethods are usedthatrequireahigherSNRtoproperlydecodethesignal.

Using a 54 megabits per second signal requires25DBofSNR–whichmeansthesignalwillnotbeproperlydecodedatgreaterdistances–becauseasthesignalmovesfurtherfromthesource,agreateramountofpathlossoccurs(thesignalisattenuated).Lower data rate transmissions, can bemore easilydecodedandasaresultappearto“travel”farther.

Asanexample inanoutdoorenvironmentwith justfree space loss, a 6 Mbps signal can actually bedecoded7timesfurtherawaythana54.

Signal to Noise Ratio (SNR)

Desiredsignal

Noise

Ampl

itude

(Stre

ngth

)

Frequency

The higher the signal isabove the noise, the

better the signalto noise ratio

=SNR(db) Signal(dBm) – Noise(dBm)

SNRSignalNoise=

Note: Higher data rates cannot be transmitted at the same Power as the less-complicated encoding of lower data rates.

Modulation Wireless Minimumand Encoding Data Rate Required802.11a (Mbps) SNR (dB)

BPSK 1/2 6 8

BPSK 3/4 9 9

QPSK 1/2 12 11

QPSK 3/4 18 13

16-QAM 1/2 24 16

16-QAM 3/4 36 20

64-QAM 2/3 48 24

64-QAM 3/4 54 25


Range Versus CapacityManyoftheinitialWi-FideploymentsfocusedonprovidingthemaximumamountofcoveragesotherangeofanAccessPointwasacriticalfactorinthepurchaseprocess.Whilestillimportant,rangebecomeslessofanissueasmoreclientsconnecttothenetworkgiventheinheritbandwidthlimitationsoftoday’sAccessPoints.Itisimportanttorealizethatgreaterrangeisnotalwaysapositivething.Sometimesitworksagainstyoudependingonyourapplicationandthenumberofusersonthenetwork.AsyouincreasetherangeofanAccessPoint,thecoverageareaincreasesandyouwillnowbecoveringmoreuserswithasingledevice.Asmoreusersjointhenetwork,theywillallbevyingforthefiniteamountofbandwidthavailablewithinthatcell.

Asthesizeofthecell increases,clientsattheedgeofthecellwillbeusing lowerdataratesandthereforeconsumemoretimeonthewirelessmediumloweringtheperformanceofalloftheclients.

HereareafewsimplerulestorememberwhendeployingaWi-Finetworkusingomni-directionalantennas:

Asomni-directionalcoverageincreases,the�� numberofpotentialcoveredusersincreasesaswellastheamountofcapacityrequired

Asthenumberofusersincreases,theavailable��

capacityoreffectivebandwidthperuserdecreases

Thelargertheomni-directionalcellsize,the��

greaterthechancethattwostationswillnotheareachother’stransmissionscreatingaHiddenNodeproblem

Amulti-sector,directionalantennaapproachcan��

helpprovidebothrangeandcapacityandcanalsohelpmitigatehiddennodes

Range Versus Capacity

54

802.

11 D

ata

Rate

(Mbp

s)

Distance (ft)

60

54

48

42

36

30

24

18

12

6

025 50 75 100 125 150 175 200 225 250 275 300

5448

24

18

129

62 1

48

36

Station 1 Station 2

Hidden Node Problem

Data Rate Supported at Given RangeMaximum Users Supported at 2Mbps TCP per User

More coverage = more users

DirectionalantennasalsohelptoalleviatetheproblemsofcellcapacitybyfocusingtheRFenergyinspecificsectors. Inthisway,greaterdistancecanbeattainedalongthedefinedsectorwithoutalsoassociatingwithstationsoutsidethesector.

Ontheoppositeendofthescaleistheideaofshrinkingcellsizestoincreasecapacity.First,shrinkingthecellsizeofanAccessPointbyloweringthetransmitpowerdoesnotlowerthetransmitpowerofthewirelessstations(clients)thatareassociatedtoit.Second,theclient’stransmitpowerandreceiversettingsarenotunderthecontroloftheAccessPointanddonotchange.BydecreasingthetransmitpoweroftheAccessPoint,theoverallcellsizeshrinksonlyslightlysotherealsizeoftheWi-FicellisnotjustthetransmissionrangeoftheAccessPoint.Infact,therealsizeofaWi-FicellisthetransmissionrangeoftheAccessPointandthetransmissionrangeofallthewirelessclientsinthatcell.

Shrinkingcellsalsocreatesadditionalinterferenceissuesandlimitsrange.WhentwoAccessPointsareplacedneareachotheronthesamechannel,theymaynotheareachother,buttheclientsthatassociatetothoseAccessPointsmaywellinterferewithstationsontheadjacentcellandviceversa.InterferingstationsareshowninbetweenthetwoAccessPoints.


802.11n TechnologyWorkonthe802.11nstandardstartedinJanuaryof2004whentheIEEEannouncedthatithadformedanewTaskGroup(TGn)todevelopthenextamendmenttothe802.11standardforwirelesslocalareanetworks.ThegoaloftheIEEEwhenworkbeganon802.11nwastoobtainTCPthroughputsinexcessof100Mbps.

In order achieve this goal, changes to the physicalandMAClayersof802.11standardwouldneedtobemade.While802.11nbuildsuponprevious802.11standards,updatestotheMAClayerweresubstantialaswellasintroducingMIMO(MultipleInputMultipleOutput)signalprocessing.

MIMO uses multiple transmitter and receiverantennas and takes advantage of multipathreflections to improvesignalcoherence thatgreatlyincreases receiver sensitivity. This extra sensitivitycanbeusedforgreater rangeorhigherdatarates.Thenewlyenhancedsignalistheprocessedsumofindividual antennas. Signal processing eliminatesnulls and fading that any one antenna would see.MIMOSignalProcessingissophisticatedenoughtodiscernmultiplespatialstreams.

Spatial multiplexing transmits completely separatedata streams on different antennas (in the samechannel) that are recombined to produce new802.11ndatarates.Higherdataratesareachievedby splitting the original data stream into separatedatastreams.Eachseparatestream is transmittedonadifferentantenna(inthesamechannel).MIMOsignal processing at the receiver can detect andrecovereachstream.Streamsarethenrecombinedwhichyieldshigherdatarates.

With changes to the 802.11 MAC and the use ofMIMO signal processing, data rates can reach over200MbpsandthusachievetheoriginalIEEEobjectivefor 802.11n. In addition to the increases in rawthroughput,wirelessnetworksusing802.11nwillalsoseeanincreaseinrangeandcoveragebutagain,therangeandvs.capacityargumentsstillholdtrue.

It is important tonote the introductionof802.11nwill force organizations to review their entire wiredinfrastructure as the increases in bandwidth from802.11nwillcreatebottlenecksinotherpartsofthenetwork.Equallyimportant,MIMOsignalprocessingwillprovideimprovementsinrangeevenifitisonlypresentononehalfofthelink,i.e.theclientortheAccessPoint.

802.11 Comparison Table

MIMO Signal Processing

Frequency Across Subcarriers

Attenuation

Antenna 1 Signal

MIMO Processed Signal

Antenna 2 Signal

Antenna 3 Signal

Receiver

Spatial Multiplexing

M

Rx 1

Rx 2

Rx M

M

TransmitterData Stream Data Stream

Receiver

Tx 1

Tx 2

Tx N


RecommendationsToday’straditionalWi-Finetworkscannotkeeppacewiththetrajectoryofneededwirelesscapacityandcoverageforvoice,videoanddataapplicationsrunningoverWi-Fi.BycombiningaWirelessLANController/Switch,upto16IntegratedAccessPoints,GigabitEthernetSwitch,Wi-FiFirewallandMulti-SectorAntennaSystemintooneeasy-to-managedevice,Xirrusdeliverstheonlylongrange,highperformanceWi-Fiplatformintheindustrythatcaneffectivelyextendwirednetworkcapabilitiestowireless–withoutcompromise.

SettingupalargescaleWi-FinetworkisverydifferentthanitswiredcousinEthernet.Giventhefluidnatureofthewirelessmedium,coverageandperformancecanvarywidelydependingupontheenvironment.MostAccessPointsuseanomni-directionalantennatopropagatetheirsignalswhichlimittheirabilitytoadequatelydeliverincreasedrange.

TogetthebestrangeoutofyourWi-Fisystemareasfollows:

Higherdataantennasareobviouslybetterwith theexceptionbeingyoudon’tget the rangeofcoverage��

exceptwithanarrayofantennaslikeintheWi-FiArray.

SpendasmuchtimeevaluatingtheWi-Fisolutionsinyournotebooks,PDA’s,andotherclientstolookfor��

thebestreceivers,antennas,andradios.

Use802.11atoreducechannelinterferenceandtoallowformorecapacity.��

RememberthatrepositioningyourAPsmaymakedramaticchangeswhenyouhavemultipathissuesinside��

complicatedenvironments.

WhenmountingyourWi-Figear,thehigherthelocationandthemoreobstructionandsubstancesyouget��

outofthepathoftheradiowavesthebetteroffyouare.Mountingithigherisbetter.

Whenyou’replanningyoursolutionbecarefulnottocreatecellsthataresolargethatitcoverstoomany��

users,especially if they’reuserswithhighdemand.Rangeandcapacitymatters;becarefulwhenyou’redesigningyoursystem;usesimultaneouschannels;usingaXirrusArrayarchitecturethatcandelivertherangeandcapacityneededforfutureneeds.


Leading ArchitectureXirrus planned for the success ofWi-Fi by develop-ing an award-winning Wi-Fi architecture powerfulenoughtohandlehigh-bandwidthapplicationstodayand modular enough to be upgraded for futureenhancements.

WiththeWi-FiArray,Xirrusdeliverstheonly ‘PowerPlay’architecture inWi-Finetworkingwith themostbandwidth and coverage per cable drop in theindustry. Xirrus Wi-Fi Arrays deliver up to 8x thebandwidthofasingleaccesspointandarecompact,easy-to-install, ceiling-mounted devices. No othercurrent-generation Wi-Fi technology can deliver thebandwidth or throughput of Xirrus Arrays becausetheyarelimitedto2radiosproducingonly108Mbpsofsharedbandwidth.

Xirrus Wi-Fi Array

Redundant Gigabit Ethernet Uplinks

Multiple Wi-Fi Radios Produce864Mbps of Bandwidth

High Gain Directional Antennas Increase Range

SectoredAntenna

SectoredAntenna

Wi-FiRadio

Wi-FiRadio

Wi-FiRadio

Wi-FiRadio

Wi-FiRadio

Wi-FiRadio

Wi-FiRadio

Wi-FiRadio

Wi-FiRadioWi-Fi

Radio

Wi-FiRadio

Wi-FiRadio

Wi-FiRadio

Wi-FiRadio

Wi-FiRadio

SectoredAntenna

SectoredAntenna

SectoredAntenna

SectoredAntenna

SectoredAntenna

SectoredAntenna

SectoredAntenna

SectoredAntenna

SectoredAntenna

Wi-Fi Controller

50% Sector Overlap

EthernetSwitch

SectoredAntenna

No other current-generation Wi-Fi technology can deliver the bandwidth or throughput of Xirrus Wi-Fi Arrays.

By integrating these key components: theWi-Fi controller, Gigabit Ethernet Switch, Gigabit uplinks,multipleaccesspoints, sectoredantennasystem,Wi-Fi stateful firewall andWi-Fi threatsensor intoasingledevice,XirrusArraysareabletoprovideacentrally-managedplatformthatdeliversunparalleledrange,clientcapacityandperformance,alongwithbetterRFmanagementandroamingforvoice,videoanddataapplications—allinasingledevicethatisfullyupgradeableto802.11n.

About XirrusXirrus,Inc.isaprivatelyheldfirmheadquarteredinWestlakeVillage,California.FoundedbythesameteamthatcreatedXircom(acquiredbyIntelin2001),XirrushasdevelopedthenextgenerationinenterprisewirelessLANarchitecturescenteredaroundtheaward-winningArray.

Backed by leading venture capital firms U.S. Venture Partners and August Capital, Xirrus brings a provenmanagement teamandpatentedapproach todelivering theperformance,scalabilityandsecurityneeded todeployatruewirelessextensionofthewiredEthernetnetworkcapableofdeliveringTriplePlay(voice,video,data)enablement.

Xirrus,Inc.

[email protected]

2101CorporateCenterDriveThousandOaks,CA91320,USA1.800.947.7871TollFreeintheUSA+1.805.262.1600Sales+1.805.262.1601Fax

Copyright©2008,Xirrus,Inc.AllRightsReserved.XirrusandtheXirruslogoaretrademarksofXirrus,Inc.Allothertrademarksbelongtotheirrespectiveowners.Protectedbypatent#USD526,973S.Otherpatentspending.

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2

Wi-Fi Range Demystified Crossword Puzzle—Answer Key

Across Down1. Originalbandoffrequencies9.Whenasignalchangesdirection11.Typeofantennaresembling

aflashlight12.Ratioofsignaltonoise14.Highestperformingaccessdevice16.Totalareaoftheradio18.Contiguousfrequencies19.Maximumdistancebetween

tworadios21.Rateatwhicharepeatingevent

occurs25.One-millioncyclespersecond26.Amountofdatasentina

giventime27. Pipediameter31.AnIsotropicantenna32.Pathforsignals33.OnlyWi-FiPowerPlay35.ReceiveSignalStrengthIndicator36.Antennapatternresembling

adonut37. Instituteofengineers38.Circuitrytointerpretandexecute

2.Whenthesignalisscattered3. Splitsonebandintomany4.Whenthesignalbecomesbent5. dBistheabreviation6.Managesaddressingandprotocol

information 7. Receive/sendradiosignal8. Occurswhensignallsbounce10.Oppositeoftransmitter13.Speedoflight15.Multipleantennastoimproverate

andrange17. Reductioninsignalpassing

throughobjects20.109Hz22.Oppositeofreceiver23.Xirruslanguage24.Whenthesignalreflectsback25.Toblenddataintoacarriersignal28.Boostpower,voltageorcurrent

ofasignal29.Shpericalradiationpattern30.Conveysdatabetweenpoints34.Anotherwordforamplification

wi-fi range demystified tutorial - university of cyprus and coverage xirrus… · type of antenna...

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