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PhysicalLayer:CommunicatingSignals

PurposeofthePhysicalLayer

Therole

of

the

OSI

physical

layer

is

to

encode

the

binary

digits

that

represent

data

link

layer

frames

intosignalsandtotransmitandreceivethesesignalsacrossthephysicalmedia(copperwires,optical

fiberandwireless)thatconnectnetworkdevices.Thedatalinkframecontainsastringofbits

representingapplication,presentation,sessionandtransportandnetworkinformation.Thesebits

musttraveloveraphysicalmediumsuchascoppercableoraglassfiberopticcable,orwirelessly

throughtheair.Thephysicalmediumiscapableofconductingasignalintheformofvoltage.

Toprepareadatalinkframeforthejourneyacrossthemedium,thephysicallayerencodesthe

logicalframewithpatternsofdatathatwillmakeitrecognizabletothedevicethatwillpickitupon

theotherendofthemedium.

Physicallayerelements:

Thephysicalmediaandassociatedconnectors

Arepresentationofbitsonthemedia

Encodingofdataandcontrolinformation

Transmitterandreceivercircuitryonthenetworkdevices

Afterthesignalstraversethemedium,theyaredecodedtotheiroriginalbitrepresentationsofdata

andgiventothedatalinklayerasacompleteframe.

Figure81:thefullencapsulationprocessandthetransmittingofencodedbinarybits.

PhysicalLayerOperation

Eachmedium

has

unique

signaling

used

to

represent

the

bits

in

the

data

link

frames.

The

frames

remainunchangedastheycrosstothenextdevice.

Table81:Signaltypesforeachofthemediaatthephysicallayer

Media SignalType

Coppercable patternsofelectricalpulses

Fiberopticcable patternsoflightpulses

Wireless patternsofradiotransmissions

Whenthephysicallayerputsaframeoutontomedia,itgeneratesasetpatternsofbits,orsignal

pattern,thatcanbeunderstoodbythereceivingdevice.

Tomark

the

beginning

and

end

of

frames,

the

transmitting

device

uses

abit

pattern

that

is

unique

andisonlyusedtoidentifythestartorendofframes.

PhysicalLayerStandards

Thephysicallayerdefineshardwarespecifications,includingelectroniccircuitry,mediaand

connectors.(Figure82)Theengineerswhodesignedthespecificationsofthephysicallayerhadto

considerseveraldifferentmediastandardstocompleteatransmission.

Thestandardsofthephysicallayer:

physicalandelectricalpropertiesofthemedia

mechanicalproperties(materials,dimensionsandpinouts)oftheconnectors

bitrepresentationbythesignals(encoding)

definitionofcontrolinformationsignals

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PhysicalLayerFundamentalPrinciples

Communicationatthephysicallayerisaprocessinvolvingphysicalcomponentsthatcarryencoded

datasentoutasasignalappropriatetothemedium.ThefollowingthreecomponentsofLayer1

communicationarekeytounderstandinghowthephysicallayerfunctions:

Physicalcomponentscarrythemessageinareliableandconsistentmannersothatthereceiver

getsthemessageasitwassent

Encodingisanothermajorfunctionofthephysicallayer.Thebitsintheencapsulateddatalink

layerframeneedtobegrouped,orencoded,intopatterns.Aftertransmission,thereceiving

Layer1devicedecodespatternsandhandstheframeuptothedatalinklayer.

Anotherfunctionofencodingiscontrolinformation.Thephysicallayerinsertsacontrolcodeto

indicatethebeginningandendofframes.

Signalinginvolvesdetermininghowtorepresentthebinarybitonaspecificmedium.

Theprocessesofencodingandsignalingcompletethepreparationofdatafortransport(Figure83).

PhysicalSignalingandEncoding:RepresentingBits

SignalingBitsfortheMedia

Eachmethodfindsawaytoconvertapulseofenergyintoadefinedamountoftimeknownasabit

time.BittimeisthetimeittakesforaNICatOSILayer2togenerate1bitofdataandsentitoutto

themediaasasignal.Thetypeofsignalwithinthebittimedependsonthemethodofsignalingused.

TheamountofrealtimeabittimeconsumesdependsonthespeedoftheNIC.

Threepossiblevariationsofasignalthatcanrepresentencodedbitsare:amplitude,frequencyand

phase.Amplitudeisameasureofthevariationofthesignalcycle.(Figure84)

Itisessentialthatalldevicesonthenetworkusethesamemethodsothatthemessagesfrom

sendingdevicescanbereadbythereceivingdevices.

NonreturntoZero

Thesignalingmethodknownasnonreturntozero(NRZ)samplesthevoltagelevelonthemedium

duringabittime.Alowvoltageisrepresentedby0andahighervoltagerepresenting1.

NRZhasnoconstantzerovoltage,soadditionalsignalingissometimesnecessaryforsynchronization

withotherdevices.(Figure85)

ManchesterEncoding

Manchesterencoding

is

asignaling

method

that

looks

for

achange

in

voltage

in

the

middle

of

abit

time.Avoltagechangefromlowtohighwithinthebittimerepresentsa1.(Figure86)

Manchesterencodingisthesignalingstandardfor10BASETEthernet.

Encoding:GroupingBits

Theprocessofaddingsignalpatternstoidentifyimportantsignaltransmissionsisthesolutionused

atthephysicallayer.Asignalpatternletsdevicesworkmoreefficientlybyallowingthemtoignore

unimportantsignalsonthemediaandpayattentiononlytoimportantsignals.

BeforeeachframeistransmittedatLayer1asasignal,itisencodedwithsignalpatternsthat

announcetothereceivingdevicewhentheframestartsandstopsandwhichpartoftheframehas

datato

be

passed

to

OSI

Layer

2.

(Figure

87)

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Codegroups,anothermethodofencoding,canimprovenetworkefficiencyandsignalreliability.A

codegroupisasymbolthatisapredefinedsmallgroupofbitsthatrepresentsalargergroupofdata

bits.Codegroupingispartoftheencodingprocessandhappensbeforethesignalisputonthe

media.(Figure88)Codegroupscanhelppreventseveralreliabilityproblemsthatcommonlyarise

withtheuseofhigherspeednetworks.Theyimproveperformanceonhigherspeednetworksinthe

followingfourareas:

reducingbitlevelerror

limitingtheeffectiveenergytransmittedintothemedia

helpingtodistinguishdatabitsfromcontrolbits

providingbettermediaerrordetection

Codegroupsensureseveraltransitionsbetween1and0.Thebalancingof1sand0sinthesignalcan

alsohelpreduceerrorratesbypreventingcomponentsfromoverheating.Theycanimproveerror

detection.Codegroupscanenhancethedistinctionsinasignalbyusingthreedifferenttypesof

symbols:

datasymbols:thedatasentdownfromthephysicallayer

controlsymbols:theLayer1patternsdenotingthebeginningandendofframes

invalidsymbols:bitpatternsnotallowedonthemediathatcanindicateaframeerror

Data-CarryingCapacity

Eachphysicallayermediumcarriesdataatadifferentspeed.Therearethreedifferentwaysto

analyzethetransferspeedofdataonamedium. Allthreearemeasuredbythesamestandardof

bitspersecond:

theoreticallyasbandwidth

practicallyas

throughput

qualitativelyasgoodput

Bandwidthisthecapacityofamediumtocarrydatainagivenamountoftime.Thestandardisin

bitspersecond(bps)>bandwidthinkilobitsandmegabits.Thebandwidthmeasurementtakesinto

accountthephysicalpropertiesofthemediumandthesignalingmethodappliedtoit.(Table82)

Throughputistheactualtransferrateofdataoverthemediuminaperiodoftime.Bandwidthis

thecapacityformovingdata.Throughputisalsomeasuredinbitspersecond.Factorsthatinfluence

throughput:theamountandtypeoftraffic,thenumberofnetworkdevicesencounteredonthe

networkbeingmeasured,

Throughputcannot

be

faster

than

the

slowest

link

of

the

path

from

source

to

destination.

Goodputisthetransferrateofactualusabledatabits.Goodputisthedatathroughputlessthe

protocoloverheadbits,errorcorrections,andretransmissionrequests.Goodputaccountsforbitsare

devotedtoprotocoloverhead.Goodputisthroughputminustrafficoverheadforestablishing

sessions,acknowledgmentsandencapsulation.(Figure810)

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PhysicalMedia:ConnectingCommunication

TypesofPhysicalMedia

Thephysicallayerdefinestheperformancestandardsforthephysicalcomponentsofanetworksuch

ascopperandfibercablesandtheconnectorsusedonthem.Thephysicallayeralsodefineshowbits

arepresentedintheformofvoltage,lightpulsesandradiosignals.(Table83)

CopperMedia

Themostpervasivemediainusefordatatransferinlocalnetworksiscopper.Coppercableisthe

mostcommonmediumforconnectingnetworkdevices.Copperconnectshoststodevicessuchas

routers,switches,andhubswithinaLAN.Coppermediahasstandardsdefinedforthefollowing:

typeofcoppercablingused

bandwidthofthecommunication

typeofconnectorsused

pinout

and

color

codes

of

connections

to

the

media

maximumdistanceofthemedia

Copperisaneffectivemediumbecauseitconductselectricalsignalsverywell,butithasits

limitations.Datatravelsoncoppercablesassmallpulsesofelectricalvoltage.Thevoltageisquite

lowandeasilydistortedbyoutsideinterferenceandsignalattenuation.Attenuationisthelossof

energyinasignalasittravelslongerdistances.Thetimingandvoltagevaluesofthesesignalsare

susceptibletointerferenceornoisefromoutsidethecommunicationssystem.Theseunwanted

signalscandistortandcorruptthedatasignalsbeingcarriedbycoppermedia.

Theadvancesincoppercabledesignhaveimproveddatatransferratesbyreducingtheeffectsof

noiseand

signal

attenuation

on

the

wire.

Therearedifferenttypesofcoppercabledesignedtomeetthespecificneedsofdifferentnetworks.

Themostcommonisunshieldedtwistedpair(UTP)cabling.Othersarecoaxialcableandshielded

twistedpaircables.

UnshieldedTwisted-Pair(UTP)Cable

ThemostcommoncoppernetworkmediaisUTP.UTPinEthernetconsistsofeightwirestwistedinto

fourcolorcodedpairsandthenwoundinsideacablejacket.Thecoloredpairsidentifythewiresfor

properconnectionattheterminals.(Figure811)

ThetwistingofpairsinUTPcableispartofthecableengineeringdesign.Whenwirescarryelectrical

current,they

can

create

an

electromagnetic

field

that

can

cause

interference

on

other

wires

in

the

cable.Becauseeachwireinthepaircarriescurrentintheoppositedirection,keepingthemclose

togetherwithtwistingwillcausethemagneticfieldsonthewirepairtocanceleachother:crosstalk.

StandardsforUTPcableinstallations:

cabletypes

cablelengths

connectors

cabletermination

methodsoftestingcable

Thereare

several

categories

of

UTP

cable.

The

most

common

UTP

cable

connector

in

LAN

devices

is

anRJ45connector.

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Therequiredorderofthewiresintheconnector,calledthepinout,variesaccordingtowherethe

cablefitsinthenetwork.TheorderofthewiresinthepinoutsisdefinedbyTIA/EIAstandards.

Figure813showsthecolorpatternsforTIA/EIA568Aand568Bpinouts.

Table84:thesecabletypesarenotinterchangeable,anditisacommonmistaketousethewrong

wirefortheintendedpurpose.

OtherCopperCableTypes

Coaxialcable,alsoknownascoax,hasasingle,coatedcopperwirecenterandanoutermetalmesh

thatactsasbothagroundingcircuitandanelectromagneticshieldtoreduceinterference.Theouter

layeristheplasticcablejacket.(Figure814)Coaxcarrieshighfrequencyradioandtelevisionsignals

overwire.

Hybridfiber-coax(HFC)combinestheelectricalpropertiesofcoaxandthebandwidthanddistance

benefitsoffiberopticcable.

CoaxcableconnectstoahostsNICandotherdeviceswithabarrelconnector.(Figure815)

Shieldedtwisted-pair(STP)cableisaLANtechnologythathasbecomelesscommonly.STPcable

combinestwomethodsofnoisereductionbytwistingthepairsofwireinsidethecabletoreduce

interferenceandthenshieldingthecableinawiremesh.(Figure816)

CopperMediaSafety

Becausecoppercarrieselectricalcurrent,thereisaninherentriskinusingit.Apotentialdangeris

overextendingcablerunsbetweenbuildingsandfloorswithinbuildings.Theyaresusceptibletothe

effectsoflightning.

FiberMedia

Fiberopticcableisverydifferentfromcopper,yetbotheffectivelycarrydataovernetworks.Fiber

opticcable

uses

light

pulses

conducted

through

special

glass

conductors

to

carry

data.

The

cable

is

engineeredtobeaspureaspossibleandtoallowreliablelightsignalstotraversethemedium.

Fiberhasgreaterbandwidthandcanrunmuchfartherthancablewithoutneedingasignalenhanced,

butthehighercostoffiberopticcableandconnectors.

Fiberisananswertothesafetyissuesoflongcopperruns,becausefiberdoesnotcarryvoltageand

currentandbecauseitissaferandcancarrydatamuchfartherthancopper,fiberopticcableis

usuallyconsideredthebestchoiceforbackboneconnections.(Figure817)

Fiberopticcablecancarrylightinonlyonedirection,sofibercablesusuallyincludeapairoffiber

cores.Thisallowsfullduplextransmission.

Thelightcarriedonfibercablesisgeneratedbyeitheralaseroralightemittingdiode(LED)that

convertsthe

data

to

light

pulses.

Atthereceivingend,devicescalledphotodiodesinterpretthelightsignal,decodethebitpattern,and

sendituptothedatalinklayer.(Figure818)

Table85describesthedifferencesbetweensinglemodeandmultimodefiberopticcable.

Dispersionofthelightsignalmeansthatitseparatesasittravels.Fiberopticsareeconomicalon

longer,highspeed,pointtopointbackboneruns,buttheyarenotcurrentlywellsuitedforlocal

connectionsbetweenhostsandothernetworkdevices.

WirelessMedia

Wirelessmediacarryelectromagneticradiosignalsthatrepresentthebinarydataofthedatalink

frame.Wireless

technologies

transmit

and

receive

signals

through

the

medium

of

the

open

atmosphere,whichfreesusersfromhavingtoconnecttoacopperorfibercableconnection.

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Openareasarebestforwirelessconnections.Awirelessconnectionisusuallyslowerthanacable

connection,andbecausethemediumisopentoanyonewithawirelessreceiver,itismore

susceptibletosecuritybreachesthanothermedia.

Commondatacommunicationsstandardsthatapplytowirelessmedia:

StandardIEEE802.11

StandardIEEE802.15:WirelessPersonalAreaNetwork(WPAN)

StandardIEEE802.16

GlobalSystemforMobileCommunication(GSM)

WirelessLAN

AwirelessLANrequiresthefollowingnetworkdevices:

Wirelessaccesspoint(AP):concentratesthewirelesssignalsfromusersandconnects,

usuallythroughacoppercable,totheexistingcopperbasednetworkinfrastructuressuchas

Ethernet

WirelessNICadapter:provideswirelesscommunicationcapabilitytoeachnetworkhost

ThecostsavingsandeaseofaccessarethemajorbenefitsofwirelessLANs,withnetworksecurity

beingthemajorcaveat.

MediaConnectors

Eachmediatypehasitsowntypeofconnectors,andalltheconnectorshavedefinedstandardsto

describethemanufacturingminimumsandtheinstallationrequirements.

Connectionthatmightlookthesamecanhavedifferingpinouts.(Figure820)

Fiberopticcablingismuchmorespecializedthancoppercable.

Threecommonfiberrepairproblemsareasfollows:

Misalignment

Endgapswherefibersdonotcompletelytouch

Poorlyfinishedendscausingpoorclarity

Whenterminatingfiberopticcable,itisimportanttohavetheendsproperlyaligned,fusedand

polishedsothatsignalingremainsstronganddispersionisattheminimum.

Fiberopticlinkscanbegivenabasictestwithaflashlightshininginoneendandlookingforlightat

theother.ItisbesttouseaspecialfiberoptictestdevicecalledanOpticalTimeDomain

Reflectometer(OTDR).

Fiberoptic

connectors

come

in

avariety

of

types.

Straight

Tip

(ST)

for

multimode

and

the

Subscriber

Connector(SC)forsinglemodearetwoofthemostcommontypesinuse.TheLucentConnectoris

gainingpopularityandcanadapttobothsinglemodeandmultimodecables.