DOCSIS 3.1 Operational Integration and Proactive Network Maintenance Tools

Enhancing Network Performance Through Intelligent Data Mining

and Software Algorithm Execution (aka More with Less!)

A Technical Paper prepared for the Society of Cable Telecommunications Engineers By

Brady Volpe

Founder The Volpe Firm / Nimble This

3000 Old Alabama Rd., Suite 119-434, Johns Creek, GA 30022 USA Ph: +1-404-424-8202 x.201 brady.volpe@volpefirm.com

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Overview This paper will first provide a brief primer on DOCSIS and DOCSIS pre-equalization. It will then discuss the high level changes in DOCSIS 3.1 which must be considered from a migration standpoint from today’s networks. This will lead the reader into a conversation about being reactive versus proactive and venture into the greater discussion of Proactive Network Maintenance. Proactive Network Maintenance will be reviewed and how it can help identify impairments and assist the cable operator in DOCSIS 3.1 migration. Finally the paper will discuss the powerful new PNM features that have been integrated into the DOCSIS 3.1 specification. Examples are provided so that the reader will have a comprehensive understanding of the power PNM will have in a DOCSIS 3.1 network.

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Contents

Section1–OverviewThispaperwillfirstprovideabriefprimeronDOCSISandDOCSISpre-equalization.ItwillthendiscussthehighlevelchangesinDOCSIS3.1whichmustbeconsideredfromamigrationstandpointfromtoday’snetworks.ThiswillleadthereaderintoaconversationaboutbeingreactiveversusproactiveandventureintothegreaterdiscussionofProactiveNetworkMaintenance.ProactiveNetworkMaintenancewillbereviewedandhowitcanhelpidentifyandlocateimpairmentsaswellasassistthecableoperatorinDOCSIS3.1migration.FinallythepaperwilldiscussthepowerfulnewPNMfeaturesthathavebeenintegratedintotheDOCSIS3.1specification.ExamplesareprovidedsothatthereaderwillhaveacomprehensiveunderstandingofthepowerPNMwillhaveinaDOCSIS3.1network.

Section2–DOCSIS1.x,2.0,&3.0Primer(excludes3.1)DataOverCableServiceInterfaceSpecification(DOCSIS)iseffectivelyatransparentEthernetbridgeoverahybridfiber/coax(HFC)network.Therearetwo(2)functionalcomponentsinaDOCSISnetwork,thecablemodem(CM)onthesubscribersideandtheCMTSintheheadendorhubsite.TheCMTStypicallycommunicateswiththeCMsononeormore6MHzwide(8MHzinEuro-DOCSISdeployments),64-or256-QAM(quadratureamplitudemodulation)digitallyencodedRFsignalsonthedownstreampathofanHFCnetworkbetween108MHzand1GHz.TheCMscommunicatewiththeCMTSusingoneormorequadraturephaseshiftkeying(QPSK),8-,16-,32-,or64-QAMdigitallyencodedRFsignals,transmittedonanupstreamHFCfrequencybetween5to85MHz.Thedigitaldata,transportedviadigitallymodulatedcarriers,containsMediaAccessControl(MAC)information,whichenablestheCMstocoexistwithotherCMsbyusingaTimeDivisionMultipleAccess(TDMA)schemeorsynchronouscodedivisionmultipleaccess(S-CDMA).Inessence,theCMTSisthesystemscheduler,whichcoordinatesthepowerlevel,frequency,transmittime,andpre-equalizationofallCMsignalsontheDOCSISnetwork.ByvirtueofthefactthatCMsandtheCMTSareabletocommunicatedigitaldatawitheachotherovertheHFCnetworkforthepurposeof“command-and-control”processes,theyarealsoabletotransmitpacketscontainingothernon-DOCSISMACrelateddata.ThisiswhatfundamentallyfacilitatestheabilitytosendEthernettrafficbi-directionallyoveranHFCnetwork.TheCMTS-CMDOCSISnetworktransportsIPbasedtrafficinthesamemethodthatisusedtocommunicateMACprotocolbetweenthedevices.NowthattheIPtrafficcantraversetheHFCnetwork,endusersarealsoabletoutilizethisnetworkforthepurposeoftransmittingcontentdestinedforthemultitudeofavailabledatanetworkservicessuchasemail,webbrowsing,IPvideo,andvoiceoverIPtelephony(VoIP).Insummary,eachuserisassignedauniquecablemodem,whichconformstotheDOCSISspecification.TheCMTSworksasasystemschedulerenablingmanycablemodemsto

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resideonthesameRFnetwork.TDMAand/orS-CDMAareemployedincablemodemcommunicationssothateachmodemisallocatedacertainfinitetimeoverwhichitmaytransmitandreceiveIPdata.IPdatadestinedforaparticularuserissenttothatuser’smodembytheCMTSononeormoredownstreamRFchannels.ThisisthewayanEthernetnetworkisabletobetransparentlybridgedfromadatabackbonetoasubscriber’shomeorbusinesslocation.

Section3–WhatisDOCSISPre-EqualizationDOCSISpre-equalizationisafeaturethatwasfirstaddedintheDOCSIS1.1specification.Theobjectiveofpre-equalizationistoimproveupstreamperformanceinthepresenceofcertainRFimpairments.Theseimpairmentsinclude,butarenotlimitedto,frequencyresponse,micro-reflections,andgroupdelay.ThemethodinwhichDOCSISpre-equalizationimprovesupstreamperformanceinthepresenceoftheseRFimpairmentsissimple.TheCMTSlooksatmessagescomingfromthecablemodemandevaluatesthesignalqualityofthemessages.IftheCMTSdeterminesthatthemessagescanbeimprovedbypre-equalization,theCMTSsendsequalizeradjustmentvaluestothecablemodem.Thecablemodemappliestheseequalizeradjustmentvalues,calledcoefficients,toitspre-equalizer.Theresultisthatthecablemodemtransmitsapre-distortedsignaltocompensateforimpairments,causingthedistortion,betweenthecablemodemandtheCMTS.Asthepre-distortedsignaltraversestheHFCnetworkitwillexperiencetheeffectsofRFimpairments.Bythetimethepre-distortedsignalfromthecablemodemarrivesattheCMTSitwillnolongerhaveanyoftheoriginalpre-distortion,astheRFimpairmentswillhavetransformeditbackintoanear-idealsignalthattheCMTSintendedtosee.Iffurtheradjustmentsarerequired,theCMTSwillsendmorepre-equalizercoefficientvaluestothecablemodemandthecyclerepeats.ThiscyclerepeatsatleastonceeverythirtysecondsforeverycablemodemintheDOCSISnetwork,providedpre-equalizationisenabledintheCMTS.Anillustrationofacablemodemsignalisperhapsthebestwaytodemonstratepre-equalizationinaction.Figure1belowshowsanupstreamcablemodemsignalasseenattheCMTS.ThisRFsignalshowssignificantroll-offduetoplantimpairments.ThiswouldcausetheCMTStohavedifficulty-demodulatingsignal,resultingincodeworderrors,lostsubscriberdataandpoorsubscriberqualityofexperience(QoE).

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Figure1:CableModemSignalwithoutPre-Equalization

OnceDOCSISpre-equalizationisenabledontheCMTSforthisparticularupstream,theCMTSwillinstructthecablemodemtopre-distortthesignalsitistransmittingviaitsinternalequalizer.Thepre-distortionwouldresultinasignalthathashigheroutputatthehighfrequenciesandlessoutputatthelowerfrequencies.Thiswouldbeamirrorimageofthesignalseeninfigure1.Theresultistheresponseshowninfigure2below,wherethesignalattheCMTSisflataftergoingthroughtheRFimpairments.

Figure2:CableModemSignalwithPre-Equalization

NowthevalueofDOCSISpre-equalizationshouldbeclear.Whatwasonceaverypoor,lookingsignalattheCMTS(figure1),isnowanear-perfectsignalattheCMTS(figure2)thankstopre-equalizationinthecablemodem.

Section4–DOCSIS3.1ChallengesDOCSIS3.1isthelatestDOCSISspecification releasedinlate2013.ItsprimarygoalistoextendthelifeofDOCSISbyprovidingsubstantiallyhigherdownstreamandupstreamdata

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ratesthroughhighordermodulations.ThehigherordermodulationsinanHFCnetworkareachievedbyusingOrthogonalFrequencyDivisionMultiplexing(OFDM)andanimprovederrorcorrectiontechniccalledLowDensityParityCheck(LDPC).ThedetailsofOFDMandLDPCareoutsidethescopeofthispaper,howeverwewillfocusonthemodulationandbandwidthincreasesinDOCSIS3.1andhowthesewillimpacttheplantoperations.ThefirstkeychallengeinDOCSIS3.1istheaforementionedhigherordermodulationformats.InpreviousversionsofDOCSISthehighestmodeofquadratureamplitudemodulation(QAM)usedwas256-QAM.Thismeans28or8-bitspersymbolarerepresentedintheconstellation.InDOCSIS3.1,modulationsofupto4096-QAMwillbesupported.At4069-QAMthereare12-bitspersymbolor212.ThismakesforahigherrequirementonthedownstreamMER(roughly35dBMER)versusthecurrent31dBMERneededfor256QAM.Itiseasiesttovisualizethephysicaldifferencebetween256-QAMand4096-QAMbylookingattheirrespectiveconstellationdiagramsshowninfigure3below:

Figure3.256-QAMvs.4096-QAMConstellationDiagram

Hereweseethat256-QAMhasagreatdealofspacingbetweenconstellationpointswhile4096-QAMlooksmorelikeablurrymess.Thisnecessitatesaverycleandownstream.InotherdocumentsonecanreadhowLDPCwillhelpduringtherecoveryprocessincompensatingforerrorsthatmayoccur,howeveritmustbeunderstoodthatthereisnofixinganoisydownstream.Similarlyintheupstreamthecurrenthighestordermodulationis64-QAM.InDOCSIS3.1upto1024-QAMwillbesupported.JustasinthedownstreamtheupstreammustimprovefromanMERstandpointinordertosupport1024-QAM.Figure4providesavisualindicationofthedifferencebetween64-and1024-QAM.

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Figure4.64-QAMvs.1024-QAMConstellationDiagram

For64-QAMweseeevenlargerspacingbetweenconstellationpointsthanin256-QAM.Thisisexpectedintheupstreamduetothelargeringressandimpairmentstypicaloftheupstream.The1024-QAMconstellationisnotnearlyascongestedasthe4096-QAMconstellationinfigure3asindividualsymbolscanstillberecognized.However1024-QAMisstillaverycomplexmodulationtobeusedinupstreamtransmissiononanHFCnetwork.Further,upstreamDOCSIS3.0andlesserchannelshaveamaximumbandwidthof6.4MHz.InDOCSIS3.1theminimumbandwidthofaDOCSISchannelwillbe24MHz.Thisisroughlytheequivalentoffour(4)legacyDOCSISchannelsstackedside-by-side.ItwillrepresentasubstantialRFenergyincreasetoanalogreturnpathopticallasersresultinginpotentiallaserclipping.Figure5showsacomparisonbetweenanexisting6.4MHzDOCSISchannelanda24MHzDOCSIS3.1channel.

Figure5.6.4MHzUpstreamvs.24MHzUpstreamChannels

Tobesuccessfulitisclearthatupstreamimprovementsandpreparationswillberequired.ThisistheopportunityforProactiveNetworkMaintenance(PNM)now,longbeforethefirstDOCSIS3.1networkisturnedup.

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Section5–ProactiveNetworkMaintenance(PNM)ProactivenetworkmaintenanceorPNMisthepracticeofmaintaininganetworkinanon-reactivemethod.Thisstatementisnotmeanttobesarcastic,asonlyuntilthepastseveralyears’cableoperatorshavereliedonreactivetools.SuchtoolsprovidemetricssuchasFECstatistics,MER,SNR,orevenofflinemodemcounts.Thechallengewiththesemetricsarethattheyoftenprovidetoolittleinformationtoolate.Inmanycaseswhenthemetricsarrivetheyarecustomerimpacting.ThroughtheadventofthePNMinitiativeandtheInGeNeOsworkinggroupsatCableLabs,proactivenetworkmaintenancehasbecomeaviablealternative.Thisisprimarilyaccomplishedthroughthedataacquisitionofcablemodempre-equalizationdata.Uponobtainingthedata,itisprocessedthroughalgorithmsandupstreamimpairmenttaps,distancetoimpairmentsandcorrelationofmodemswithcommonimpairmentscanbedetermined.TheresultingoutputcanbesummarizedinatypicalPNMapplicationasshowninfigure6below.

Figure6.CommercialPNMMonitoringSolution

Ascanbeseen,thecablemodemsonagivenCMTSarequantifiedasred,yellowandgreenbasedonthecriticalityofupstreamimpairmentsanalyzedbythepre-equalizationdatastoredinthecablemodems.Aredmodemshownonthisdashboardisverydifferentthanaredmodem,whichwouldbeshownonastandardSNMPorNMSdashboard.TheSNMPdashboardwilltellyouwhenamodemisreportingproblems,likebadSNR,MERor

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uncorrectablecodeworderrors.ThisPNM-baseddashboardisproactivebecauseitshowswhenamodemmayhaveproblemsinthefuture.Thisisbecausepre-equalizationinthecablemodemiscompensatingforupstreamimpairments.IdeallythemodemisabletofullycompensatefortheimpairmentsandthesubscriberandtheSNMPmonitoringsystemisnotawareoftheexistenceoftheimpairment.Onethingiscertain,however;cableplantsdonotgetbetterovertime.Corrosion,water,wind,andtimewilltaketheirtoll.Thetapvaluesandinchannelfrequencyresponsewilldeteriorateovertime.SoitissafetoassumethatmostoftheredmodemswilleventuallystartshowingupontheSNMPmonitoringsystemsand/orresultinsubscribercalls.Similarlytheyellowmodemswillbecomeredmodems.Soitisaconsciousdecisiontodecidetobeproactiveinsteadofreactive.SmallandlargecableoperatorshaverealizedthepowerofPNManditsadoptionrateisrapidlygrowing.ThegoalinallofthisisusingPNMtechnologyistobeproactiveandidentifyandfixproblemsbeforethesubscriberknowstheyexist.Evenfurther,itisdesirabletotakethisopportunitytofocusonimprovingtheHFCplantinpreparationforDOCSIS3.1.Thiscanbewiththeinsighttoimpairmentsthatwerenotpreviouslyvisible.

Section6–PNMSpectrumAnalysisWhilenotquiteasproactive,someoftheoutcomesfromthePNMworkinggroupswereadvancedfunctionalitiesthattookadvantageofexistingDOCSIShardwaretoprovidemoreinsighttotheHFCplant.Mostnotablyareupstreamanddownstreamspectrumanalyses.Typicallyupstreamanddownstreamspectrumanalysisrequiredexpensivehardwarefromtestequipmentvendors.Inadditionreturnpathmonitoringintheheadendinvolvedextensiveheadendsplittersandcabling.Theadditionofthesesplittersandcablesaddloss,pointsoffailureandingressandaddOPEXforthemanpowertoinstallandmaintaintheequipment.ThePNMsolutionforupstreamspectrumanalysisistousethebuilt-inspareFFTineveryCMTS.ThisaddsvirtuallynoextraprocessingtotheCMTS.FurthersinceitusestheintelligenceoftheCMTSoneisabletofilteronanycablemodemMACaddresswiththeadditionalpowerofseeingingressundertheDOCSISchannel.SeeingingressundertheDOCSISchannelisaccomplishedbecausetheCMTSknowsbothwhenmodemsaretransmittingdataaswellwhenmodemsarenottransmittingdata.AstheCMTSknowswhennomodemsaretransmittingitcanprovideRFdataontheseno-transmitperiods.ThisprovidesRFspectrumofingressnoiseundertheDOCSISchannels,whichisvirtuallyimpossibletoobtainusingconventionalheadendmonitoringequipment.Figure7showsanexampleofaPNM-basedupstreamspectrumanalyzerusingCMTShardwareforthecaptureoftheupstream.

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Figure7.PNM-basedUpstreamSpectrumAnalyzer

Fordownstreamspectrumanalysisthecablemodemisused.OnlynewergenerationDOCSIS3.0cablemodemssupportthisfunctionality,whichiscalledfull-bandcapture.Thisadvancedcapabilityinmodemsprovidetheabilitytocapturethefrequencyspectrumfrom54MHzto1GHzandsometimeshigher.Thismeansthateverysubscriberhavingoneofthesemodemsbecomesanend-of-lineprobewithspectrumanalysiscapability.ConsiderasubscribercallingaboutnotbeingabletoviewESPN.OnecouldquicklypulluptheRFspectrumgoingtothesubscriber’shomeandseethereisabigsuck-outwheretheESPNnetworkis.Itwouldthenbepossibletolookaroundneighboringhomesandfindiftheimpairmentisonlyatthesubscriber’shomeorifitisaplantissue.Thisenablesthecableoperatortoknowwhethertosendatechtothesubscriber’shomeortoanRFamplifierfeedingthegroupofhomeseffected.Thispowerfulfunctionalityenablesonetofindtheproblemandnarrowitslocationdowntoaspecificlocationinamatterofminutes.Figure8showsanexampleofalivecapturefromaComcastplantcourtesyofLarryWolcottofComcast.

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Figure8.PNM-basedCableModemDownstreamSpectrumAnalyzer

Intheabovecaptureitisevidentthatasuck-outispresentdirectlywherethesportschannelsarebeingbroadcast.Otherminorimpairmentscanbevisualizedinthespectrumaswell,indicatingthepowerofthisfeature.

Section7–PNMinDOCSIS3.1SofarthediscussionsofPNMhavestrictlybeenassociatedwithDOCSIS1.xthroughDOCSIS3.0.WithoutquestionPNMisextremelypowerandmanycableoperatorsarefindingCAPEX&OPEXsavingsinadditiontotremendoussubscribersatisfaction.DuringthedevelopmentoftheDOCSIS3.1specificationtheCableLabsInGeNeOsworkinggroupcollaboratedwiththeDOCSIS3.1workinggrouptoensurethatPNMhookswereintegratedintothe3.1standard.ThishasresultedintosomeverypowerfulPNMfeaturesintheDOCSIS3.1standardthatwillenablePNMfunctionalityfarbeyondwhatiscurrentlyavailabletoday.TheentireSection9oftheDOCSIS3.1PHYspecificationisdedicatedto“ProactiveNetworkMaintenance”.Figure9belowshowsthecorrespondingfigure9-1fromtheDOCSIS3.1PHY,whichrepresentsanoverviewoftestpointsintheCMandCMTSprovidingnetworkanalyzer,spectrumanalyzerandvectorsignalanalyzerfunctionalityfromaPNMperspective.

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Figure9.DOCSIS3.1PHYFigure9-1 TestpointsinCMandCMTSSupportingProactiveNetworkMaintenance

TheabovediagramandPHYspecificationincludesPNMmetricssuchasdownstreamtransmitandreceivesymbolcapture,widebandspectrumanalysis,noisepowerratio,inchannelestimation,constellationdisplay,MER,histogramsandatonofstatisticsSimilarlyfortheupstreamthereisafeaturecalledaquietprobe,givingtheabilitytodoliveplantNPRtesting,morespectralanalysis,thesameupstreamtapsjustliketoday,lotsofstatistics,histogramsandMER.Thereisunprecedentedinsighttoupstreamsignalqualityandmoreimportantlyimpairments.Somedetailedexamplesofhowthesetypesoftoolswillbeusedinpracticearemodeledinthefollowingparagraphs.Figure10showsthedownstreamsymbolcaptureinpractice.SincethethetransmittedandreceivedsymbolsattheCMTSandcablemodemareknown,theexactqualityofthetransmittedandreceivedDOCSISsignalcanbedetermined.Ifthereisanimpairmentitwillbevisibleanddeterminant.Theimpairmenttype,degreeofimpairmentandimpactonthenetworkwillalsobeevident.Impairmentssuchasingressunderthecarrierwillnotrequiretestequipmenttoestimatetheingress.Cablemodemswillprovidethisinformationtotheuserorapplicationwithextremedetail.Inthecaseoffigure10,theinchannelfrequencyresponseisbeingderivedbythesymbolrecovery,howevermuchmoredetailcanandwillbederived.

DUTCablePlant

SpectrumAnalyzerFunctions

VSA(VectorSignal

Analyzer)Functions

NetworkAnalyzerFunctions

DeviceUnderTest=CablePlant

OtherTestPoints

SpectrumAnalyzerFunctionsInCM InCMTS

• Full-bandspectrum

• NPRnotch

• Triggeredspectrum

VSAFunctionsInCM InCMTS

• USpre-equalizercoeffs

• DSchannelestimate

• Constellationdisplay

• RxMERvssubcarrier

• USequalizercoefficients

OtherTestPointsInCM InCMTS

• FECstatistics• Histogram

• Impulsenoisestatistics

• FECstatistics• Histogram

NetworkAnalyzerFunctionsInCM InCMTS

• DSsymbolcapture

• DSsymbolcapture

• USactive/quietprobecapture

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Figure10.PNMDSSymbolCapture

Ontheupstream,itwillbepossibletotellexactlyhowmuchdynamicrangeisleftinthereturnpathlaser.ThiswillbeaccomplishedwithNoisePowerRatioorNPRtesting.Usingexclusionbands,non-linearity’swillfilluptheopennotchesasshowninfigure11andthenitwillbepossibletopreciselydeterminetheamountofavailabledynamicrange.PreviouslyNPRwasonlysomethingthatcouldbeaccomplishedinthelaboratory,butwithPNMinDOCSIS3.1itwillbepracticaltodoinliveDOCSISnetworks.

Figure11.NPRTestinginLiveDOCSISNetworks

Thefinalexampleistheabilitytodefinitivelyknowanytimethelaserclippedonasymbol-by-symbolbasis.Furtheritwillbepossibletoplotthisinahistogramandcreatesimplethresholdstoset-offtrapswhenlaserclippingoccurs.ThiswillbeeasilytoimplementwithPNMinaDOCSIS3.1networkwiththenewhooksinplace.Figure12providesanexampleofhowthisispossible.

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Figure12.DOCSIS3.1PNMLaserClippingDetection

Thetoprowoffigure12showsanormalOFDMsignalwithnolaserclipping.Thetimedomainchart(upperleft)hasnoclippingeventsandthehistogram(upperright)isperfectlyGaussianshaped.ThebottomrowshowswhathappenswithPNMmetricswhenlaserclippingoccurs.Thetimedomainchartonthelowerleftprovidesvisibleindicationofsignalsbeingclippedastheydonotriseabovethey-axiswherey=2.Thesearehighlightedbyreddots.Whenthisisplottedonahistogram(lowerright)onecanseethesignificantchangefromtheGaussiandistributionandthesharppeakaty=2.Thissharppeakaty=2onthehistogrammakeslaserclippingeventsextremelyeasytodetect.Thisisadramaticchangefromtoday’smethodsoflaserclippingdetectionwhereonemustlookattheRFspectrumabove42MHz(65MHzforEuropeannetworks)andtrytoidentifynon-linearRFenergy.ThelatermethodisfraughtwitherrorandrequiresexpensiveRFspectrumanalyzers.ThePNMmethodinDOCSIS3.1simplyrequirestheCMTSandcablemodemsalongwithasoftwarealgorithm.

Section8–SummaryThispaperexaminedDOCSISpre-equalization,ProactiveNetworkMaintenance(PNM)andthenewfeaturesthatwillcomeinPNMwithDOCSIS3.1.FurtherthepaperdiscussedsomeofthekeychallengesinDOCSIS3.1andhowproactivelyaddressingyournetworktodaycanhelpimproveyournetworkinordertoprepareforDOCSIS3.1.ThereadershouldbeabletotakeawayasenseofthevalueofPNMaswellashowPNMwillcontinuetoincreaseinvalueasDOCSIS3.1arrives.

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CombinedwithupstreamspectrumanalysisintheCMTS,full-bandcapturemodemsfordownstreamspectrumanalysis,theutilizationofPNMtoproactivelytroubleshootnetworksischangingplantmaintenanceinaverypositiveway.Akeystepisfirsttakingtheinitiativeto“beproactive”,thefirsthabitinStephenCovey’s“7HabitsofHighlyEffectivePeople”.ThenextstepistoimplementPNMifitisnotalreadybeingdoneinone’sDOCSISnetwork.TherearemanysuccessstoriesbylargeandsmallcableoperatorsusingPNM.Thetimeisnowtogetproactive.

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Bibliography CableLabs, “Data Over Cable Service Interface Specifications DOCSIS 3.0-Physical Layer Specification,” CM-SP-PHYv3.0-I08-090121, 2009 B. Currivan, “Cable modem physical layer specification and design,” in Cable Modems: Current Technologies and Applications , IEC Compendium and IEEE Press, 1999. CableLabs, “DOCSIS® Best Practices and Guidelines, Proactive Network Maintenance Using Pre-equalization”, CM-GL-PNMP-V02-110623, 2011 T. Wolf and C. L. A. Gatherer, “Low complexity equalization for cable modems,” in IEEE Proc. ISCAS, May 1998, vol. 5, pp. 522–524 CableLabs, “Data Over Cable Service Interface Specifications DOCSIS 3.1-Physical Layer Specification”, CM-SP-PHYv3.1-I01-131029, 2013 References of Honorable Mention:

• Larry Wolcott, Comcast • Alberto Campos, CableLabs • Tom Williams, CableLabs

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Abbreviations and Acronyms Adaptive pre-equalizer A circuit in a DOCSIS 1.1 or newer cable modem that pre-equalizes or pre-distorts the transmitted upstream signal to compensate for channel response impairments. In effect, the circuit creates a digital filter that has approximately the opposite complex frequency response of the channel through which the desired signal is to be transmitted.

CableModem(CM)A modulator-demodulator at subscriber locations intended for use in conveying data communications on a cable television system.

CableModemTerminationSystem(CMTS)Cable modem termination system, located at the cable television system head-end or distribution hub, which provides complementary functionality to the cable modems to enable data connectivity to a wide-area network. Channel A portion of the electromagnetic spectrum used to convey one or more RF signals between a transmitter and receiver.

Coefficient Complex number that establishes the gain of each tap in an adaptive equalizer.

CustomerPremisesEquipmentEquipment at the end user’s premises; may be provided by the end user or the service provider. dBcDecibelbelowcarrier DownstreamIn cable television, the direction of transmission from the head-end to the subscriber. Fast Fourier transform (FFT) An algorithm to compute the discrete Fourier transform (DFT), typically far more efficiently than methods such as correlation or solving simultaneous linear equations.

FiberNodeIn HFC, a point of interface between a fiber trunk and the coaxial distribution. Frequency response A complex quantity describing the flatness of a channel or specified frequency range, and that has two components: amplitude (magnitude)-versus-frequency, and phase-versus-frequency.

GroupDelayThe difference in transmission time between the highest and lowest of several frequencies through a device, circuit or system. Micro-reflection A short time delay echo or reflection caused by an impedance mismatch. A micro- reflection’s time delay is typically in the range from less than a symbol period to several symbol periods. MRLevelMicro-reflectionlevel Modulation error ratio (MER) The ratio of average symbol power to average error power. The higher the MER, the cleaner the received signal.

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postMTTPostmaintaptap

postMTTRPost-MainTaptoTotalEnergyRatio

Pre-equalizer See adaptive pre-equalizer.

SNR signal-to-noise ratio

Tap (1) In the feeder portion of a coaxial cable distribution network, a passive device that comprises a combination of a directional coupler and splitter to “tap” off some of the feeder cable RF signal for connection to the subscriber drop. So-called self- terminating taps used at feeder ends-of-line are splitters only and do not usually contain a directional coupler. Also called a multitap. (2) The part of an adaptive equalizer where some of the main signal is “tapped” off, and which includes a delay element and multiplier. The gain of the multipliers are set by the equalizer’s coefficients. (3) One term of the difference equation in a finite impulse response or a infinite impulse response filter. The difference equation of a FIR follows: y(n) = b0x(n) + b1x(n-1) + b2x(n-2) + ... + bNx(n-N)

TDRTimedomainreflectometer

UpstreamThe direction from the subscriber location toward the head-end.