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Chapter 4. Multiservice Provisioning Platform Architectures

Chapter 4. Multiservice Provisioning Platform Architectures

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TraditionalService-ProviderNetwork

Architectures

WithinMultiserviceProvisioningPlatform(MSPP)architectures

therearetraditionalservice-providernetworkarchitectures:the

PSTN,FrameRelay/ATM,SONETIP/MPLS,andtransport

networktypessuchasIOF,access,andprivatering

deployments.ThischapteralsocoversheritageOSSbecauseit

playssuchalargeroleintoday'sMSPPequipmentproviders'

plans.



PublicSwitchedTelephoneNetworks

Thosewhoarefamiliarwithpacket-switchedrouting,whichis

thebackboneoftheInternetandusesInternetProtocol(IP),

knowthattheInternetistheamalgamationoftoday'sdata

networks(seeFigure4-1).Thepublicswitchedtelephone

network(PSTN),showninFigure4-2,isanalogoustothe

Internet,inthatitistheamalgamationoftheworld'scircuitswitchedtelephonenetworks.AlthoughthePSTNwasoriginally

afixed-lineanalogtelephonesystemnetwork,ithasevolved

intoanalmostentirelydigitalnetworkthatnowincludesboth

mobileandfixedtelephones.



Figure4-1.TheInternetAmalgamatingNumerous

DataNetworks



Figure4-2.PSTNAmalgamatingNumerousPSTNs



JustastherearemanystandardssurroundingtheInternet,the

PSTNislargelygovernedbytechnicalstandardscreatedbythe

ITU-T.ItusesE.163/E.164addresses(knownmorecommonly

astelephonenumbers)foraddressing.

ThePSTNistheearliestexampleoftrafficengineeringusedto

deliver"voiceservice"quality.

Inthe1970s,thetelecommunicationsindustryunderstoodthat

digitalserviceswouldfollowmuchthesamepatternasvoice

services,andconceivedavisionofend-to-endcircuit-switched

services,knownastheBroadbandIntegratedServicesDigital

Network(B-ISDN).Obviously,theB-ISDNvisionhasbeen

overtakenbythedisruptivetechnologyoftheInternet.

TheprimarysectionofthePSTNthatstillusesanalog

technologyisthelast-milelooptothecustomer;however,only

theveryoldestpartsoftherestofthetelephonenetworkstill

useanalogtechnologyforanything.Inrecentyears,digital

serviceshavebeenincreasinglyrolledouttoendusersusing



servicessuchasdigitalsubscriberline(DSL)andISDN.

Manypunditsbelievethatoverthelongterm,thePSTNwillbe

justoneapplicationoftheInternet;however,theInternethas

somewaytogobeforethistransitioncanbemade.Thequality

ofservice(QoS)guaranteeisoneaspectthatmustimprovein

VoiceoverIP(VoIP)technology.Insomecases,private

networksrunbylargecompaniesareconnectedtothePSTN

onlythroughlimitedgateways,suchasalargeprivate

automaticbranchexchange/telephone(PABX)system.A

numberoflargeprivatetelephonenetworksarenotevenlinked

tothePSTNandareusedformilitarypurposes.

ThebasicdigitalcircuitinthePSTNisa64-kbpschannelthat

wasoriginallydesignedbyBellLabs,calledaDS0,orDigital

Signal0.Tocarryatypicalphonecallfromacallingpartytoa

calledparty,theaudiosoundisdigitizedatan8kHzsample

rateusing8-bitpulse-codemodulation.Thecallisthen

transmittedfromtheoneendtotheotherthroughtheuseofa

routingstrategy.

TheDS0sarethemostbasiclevelofgranularityatwhich

switchingtakesplaceinatelephoneexchange.DS0sarealso

knownastimeslotsbecausetheyaremultiplexedtogetherina

time-divisionfashion.MultipleDS0saremultiplexedtogetheron

higher-capacitycircuitssothat24DS0smakeaDS1signal.

Whencarriedoncopper,thissignalisthewell-knownT-Carrier

system,T1(theEuropeanequivalentisanE1,containing32

64-kbpschannels).Inmodernnetworks,thismultiplexingis

movedasclosetotheenduseraspossible,usuallyinto

roadsidecabinetsinresidentialareas,orintolargebusiness

premises.Figure4-3showsthecustomerDS0asitpasses

throughthelocalloopandintothecentraloffice(CO1),whereit

is"trunked"fromoneCOswitchtoanother.Fromtheswitching

network,thecustomer-premisesDS0scanbe"switched"to

theirdestination.



Figure4-3.TrunkingandLocalLoopArchitecture

Relationship



[Viewfullsizeimage]



Thetimeslotsarecarriedfromtheinitialmultiplexertothe

exchangeoverasetofequipmentthatiscollectivelyknownas

theaccessnetwork.Theaccessnetworkandinterexchange

transportofthePSTNusesynchronousopticaltransmission

(SynchronousOpticalNetwork[SONET]andSynchronous

DigitalHierarchy[SDH])technology,althoughsomepartsstill

usetheolderPlesiochronousDigitalHierarchy(PDH)

technology.PDH(plesiochronousmeans"nearlysynchronous")

wasdevelopedtocarrydigitizedvoiceovertwisted-paircabling

moreefficiently.Thelocaltelephonecompanies,alsoknownas

localexchangecarriers(LECs),serviceagivenareabasedon

geographicboundariesknownasalocalaccessandtransport

area(LATA).LATAisageographicareathatdefinesanLEC's

territory.CallsthatcrossaLATAboundarymustbecarriedby

aninterexchangecarrier(IXC),asshowninFigure4-4.

Withintheaccessnetwork,anumberofreferencepointsare



defined.MostoftheseareofinterestmainlytoISDN,but

onetheVreferencepointisofmoregeneralinterest.Thisisthe

referencepointbetweenaprimarymultiplexerandan

exchange.



Figure4-4.LocalTelephoneCompanyCOs

ConnectedbyLong-Distance,orIXE,Carriers



FrameRelay/ATMNetworks



FrameRelayisatraditionalpacket-basedtelecommunications

servicethattakesadvantageofcharacteristicsoftoday's

networksbyminimizingtheamountoferrordetectionand

recoveryperformedinsidethenetwork.Streamliningthe

communicationsprocessresultsinlowerdelayandhigher

throughput.

FrameRelayoffersfeaturesthatmakeitidealtointerconnect

local-areanetworks(LANs)usingawide-areanetwork(WAN),

asshowninFigure4-5.Traditionally,LANswereinterconnected

bydeployingprivatelinesorbycircuit-switchingoveraleased

line.However,thisapproachhasseveraldrawbacks.The

primaryweaknessofthislegacyapproachisthatitbecomes

prohibitivelyexpensiveasthesizeofthenetworkincreases,in

boththenumberoffacilitymilesandthenumberofLANs.The

reasonforthehighcostisthathigh-speedcircuitsandports

mustbesetuponapoint-to-pointbasisamonganincreasing

numberofbridges.Inaddition,circuit-modeconnectivityresults

inalotofwastedbandwidthfortheburstytrafficthatistypical

ofLANs.



Figure4-5.ServiceProviderFrameRelay

NetworksConnectedThroughanATMNetwork



[Viewfullsizeimage]



Ontheotherhand,traditionalX.25packet-switchednetworks

requiredsignificantprotocoloverheadsandhavehistorically

beentooslowatprimarilysupportinglow-speedterminalsat

19.2kbpsandlower.FrameRelayprovidesthestatistical

multiplexinginterfaceofX.25withoutitsoverhead.Inaddition,

itcanhandlemultipledatasessionsonasingleaccessline,

whichreduceshardwareandcircuitrequirements.FrameRelay

isalsoscalable,meaningthatimplementationsareavailable

fromlowbandwidths(suchas56kbps)allthewayuptoT1

(1.544Mbps)orevenT3(44.736Mbps)speeds.



ConnectiontotheLAN

Inthepastdecade,significantadvancementsincomputingand

communicationstechnologyhavereshapedthebusinessmilieu.

Withthecostofprocessingpowerfalling,PCsandhigh-powered

workstationshaveproliferatedexponentiallyandarenowan

integralpartoftheenduser'sworld.Thishasresultedinan

explosioninthedemandanduseofpersonalcomputers,

workstations,andLANs,andhasalteredthecorporate

informationsystem.Themajorchangesincludethefollowing:

CorporateorganizationTraditionally,informationsystems

werearrangedinahierarchicalstructurewithacentralized

mainframesupportingalargenumberofusers.Withthe

emergenceoftoday'stechnology,distributedcomputing

environmentsbasedonLANsaresupplementingtraditional

hierarchicalmainframearchitectures.Nowinformationflows

onalaterallevel(peertopeer)bothwithinorganizations

andtooutsidegroups.

Network-managementspecificationsThemanagement

requirementsoftoday'snetworksaremorecomplexthan



ever.Eachnetworkisadistinctivecombinationof

multivendorequipment.Growthandchangewithina

companyresultinconstantnetworkalterations.The

networkmanagerispressuredtofindacost-effectiveway

tomanagethiscomplexity.

RiseinbandwidthdemandLANshavegrownupfromthe

morphingofPCsandintelligentworkstations,pullingalong

withthemtheworkstationapplicationsduetotheuser's

expectationofobtainingquickresponsetimeandthe

capabilityofhandlinglargequantitiesofdata.TheLAN

pipeline,whichtypicallyrunsat10Mbps,100Mbps,or1

Gbps,mustbecapableofsupportingtheseapplications,so

theapplicationstypicallytransferordersofmagnitudemore

datapertransactionthanatypicalterminal-to-mainframe

transaction.Nevertheless,liketheirterminal-to-mainframe

counterparts,theLANapplicationsarebursty,withlongidle

periods.



BenefitsofFrameRelay

FrameRelayoptimizesbandwidthbecauseofitsstatistical

multiplexingandlow-protocoloverhead,resultinginthe

followingbenefitsofFrameRelay:

ReducedinternetworkingcostsAcarrier'sFrameRelay

networkmultiplexestrafficfrommanysourcesoverits

backbone.Thisreducesthenumberofcircuitsand

correspondingcostofbandwidthintheWAN.Reducingthe

numberofportconnectionsrequiredtoaccessthenetwork

lowerstheequipmentcosts.

Increasedinteroperabilitythroughinternational

standardsFrameRelay'ssimplifiedlink-layerprotocolcan

beimplementedoverexistingtechnology.Accessdevices



oftenrequireonlysoftwarechangesorsimplehardware

modificationstosupporttheinterfacestandard.Existing

packet-switchingequipmentandT1/E1multiplexersoften

canbeupgradedtosupportFrameRelayoverexisting

backbonenetworks.



AsynchronousTransferMode

FrameRelayandAsynchronousTransferMode(ATM)offer

differentservicesandaredesignedfordifferentapplications.

ATMisbettersuitedforapplicationssuchasimaging,real-time

video,andcollaborativecomputer-aideddesign(CAD)thatare

toobandwidthintensiveforFrameRelay.Ontheotherhand,at

T1speedsandlower,FrameRelayusesbandwidthmuchmore

efficientlythanATM.

ATMisadedicatedconnection-switchingtechnologythat

arrangesdigitaldatainto53-bytecellunitsandtransmitsthem

overaphysicalmediumusingdigitalsignaltechnology.

Individually,acellisprocessedasynchronouslyrelativetoother

relatedcellsandisqueuedbeforeitismultiplexedoverthe

transmissionpath.Theprespecifiedbitratesare155.520Mbps

or622.080Mbps,althoughspeedsonATMnetworkscanreach

10Gbps.

KeyATMfeaturesincludethefollowing:

Setupofend-to-enddatapathsusingstandardized

signalingandload-andQoS-sensitiveATMrouting

Segmentationofpacketsintocellsandreassemblyatthe

destination

Statisticalmultiplexingandswitchingofcells



Network-widecongestioncontrol

Advancedtraffic-managementfunctions,includingthe

following:

Negotiationoftrafficpoliciestodeterminethestructure

thataffectstheend-to-enddeliveryofpackets

Trafficshapingtomaintainthetrafficpolicies

Trafficpolicingtoenforcethetrafficpolicies

Connectionadmissioncontroltoensurethattraffic

policiesofnewcustomersdonotadverselyaffect

existingcustomers

CarriershavetraditionallydeployedATMforthefollowing

reasons:

Supportsvoice,video,anddata,allowingmultimediaand

mixedservicesoverasinglenetwork.Thisisattractiveto

potentialcustomers.

Offershighevolutionpotentialandcompatibilitywith

existinglegacytechnologies.

Supportsawiderangeofburstytraffic,delaytolerance,and

lossperformancebyimplementingmultipleQoSclasses.

Providesthecapabilitytosupportbothconnection-oriented

andconnectionlesstraffic.

Offersflexiblefacilityoptions.Forexample,cablecanbe

twistedpair,coaxial,orfiberoptic.



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