MAC Architecturefor BroadbandSatelliteAccess

Systems

April 20,2000

Abstract

MAC Architecturefor BroadbandSatelliteAccessSystems

Tallal O. Elshabrawy

In recentyears,the telecommunicationsindustryhasexpandedtremendously. A ten-

dency of integratingvariousbusinessrevenueswith the conventionalcommunicationsys-

temsis becomingmoreandmorepopularto achieveglobal informationservices.Theinte-

grationis triggeredby increasingdemandsof costumersto accessvarioustypesof broad-

bandmultimediaservices. The accesssystemcan be implementedon many platforms;

from line feedascable,fiber, or coppernetworksto wirelessasradioor satellitenetworks.

Broadbandsatelliteaccessis a leadingcandidateto contribute to suchdevelopmentdueto

satellites’distinctivefeaturesof globalcoverageoversinglehopsanddistanceinsensitivity.

However, assatellitenetworkspossessratherlongerdelaysandboundedresources,aMAC

layerthatcanefficiently shareresourcesover a minimumpossiblebandwidthis mandatory

to thesuccessof satelliteaccess.ExistingMAC protocolsarenot ableto achieve optimum

performance.Hence,designof a new MAC becomesinevitable. The new MAC should

introducea novel structurewith certainbehavioral sequencesandan efficient accesstech-

nique.In this thesis,weproposeaMAC architecturethataimsto addresssuchrequirement.

We utilize a novel accesstechniquebasedon anenhancedCFDAMA protocol.We alsoin-

troduceanew conceptof two level differentialscheduling.Wepresentformalmodelsbased

on SDL to verify the validly of the devisedsystem.Finally, we build an OPNETsimula-

tion modelto demonstratequantitative systemoperationandserve asa nucleusmodelfor

possiblefuture researchinvolving performanceoptimizationin satellitenetworksover the

devisedarchitecture.

iii

Acknowledgments

Firstof all, I would like to thankGodfor grantingmetheability andtheloveof conducting

research.Secondly, I would like to thankmy family; my motherandfatherfor guidingme

andencouragingmetowardsgraduatestudies.I thankyou a lot for your love andsupport.

I want to thankmy sisterfor her love. Your visits every summerminimizedthe loneliness

I felt away from my family andfriends. I would alsolike to thankmy cousinfor helping

memove to Montrealandapply to ConcordiaUniversity. Thankyou for your supportand

friendship.

I want to expressmy gratitudeto my supervisors;Dr. T. Le-NgocandDr. S. Tahar. I

wantto thankDr. Le-Ngocfor initiating theideaof my researchandcontinuouslycontribut-

ing with his technicalinputsthroughmy research.I want to thankhim alsofor conducting

our researchmeetings,andhelpingmeimprovemy researchskills aswell asunderstandthe

valueof teamwork. I wantto thankDr. Taharfor giving metheopportunityto startgraduate

studiesandguidingmethroughmy researchprovidingmewith constructiveinputsandhelp-

ing meimprovemy writing skills. I alsowantto thankhim for helpingmesmoothlymake

thetransitionfrom undergraduateto graduatestudies.I would like to thankDr. F. Khendek

for introducingme, throughhis course,to formal methods,especiallySDL, which helped

mea lot in themodelingpartsof my thesis.I wouldalsolike to thankConcordiaUniversity

for supplyingmewith thenecessaryfacilitieswithin ahealthyresearchenvironmentandthe

CanadianInstitutefor TelecommunicationsResearch(CITR) for supportingmefinancially

throughmy research.Finally, I would like to thankmy colleaguesin my researchteamfor

participatingin thegroupresearchandhelpingmewith our interestingdiscussionsprogress

in my research.I especiallywant to thankmy friend MohamedAshour for participating

with me in partsof my OPNETmodel. Your assistancehasalwaysbeenof greathelp for

iv

methroughmy thesis.

v

Godhasblessedmewith myfamily. I dedicatethis thesisto them,To mymotherWadouda,my

fatherOsama,andmylittle sisterEthar

vi

Contents

List of Figures xi

List of Acronymsand Abbreviations xiii

1 Intr oduction 1

1.1 BroadbandAccess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 BroadbandAccessServices . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 BroadbandSatelliteAccess . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.4 BroadbandSatelliteDeployment . . . . . . . . . . . . . . . . . . . . . . . 5

1.5 SatelliteSystemsImplementation . . . . . . . . . . . . . . . . . . . . . . 7

1.6 ContributionsandOutlineof theThesis . . . . . . . . . . . . . . . . . . . 10

2 BroadbandAccessSystems 12

2.1 DAVIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.1.1 DAVIC Organization . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.1.2 DAVIC Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.1.3 DAVIC GeneralSystemConfiguration. . . . . . . . . . . . . . . . 15

2.1.4 DAVIC DeliverySystem . . . . . . . . . . . . . . . . . . . . . . . 17

2.2 BroadbandAccessNetwork Technologies. . . . . . . . . . . . . . . . . . 18

vii

2.2.1 CableNetworks . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2.2 Digital SubscriberLine . . . . . . . . . . . . . . . . . . . . . . . 21

2.2.3 Fiberto theHome . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.2.4 BroadbandWirelessAccess . . . . . . . . . . . . . . . . . . . . . 22

2.2.5 Multi-channelMulti-point DistributionSystem . . . . . . . . . . . 23

2.2.6 SatelliteAccess . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.3 BroadbandSatelliteSystems . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.3.1 Brief History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.3.2 InteractiveBroadbandSatelliteSystems. . . . . . . . . . . . . . . 25

2.3.2.1 InteractiveDirect BroadcastSatellite . . . . . . . . . . . 25

2.3.2.2 TR34.1SATATM . . . . . . . . . . . . . . . . . . . . . 27

2.3.3 SatelliteChallenges . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.4 MAC AccessTechniques. . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.4.1 FixedAssignment . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.4.2 RandomAccess . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.4.3 DemandAssignment . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.4.4 CombinedTechniques . . . . . . . . . . . . . . . . . . . . . . . . 32

2.4.4.1 CombinedRandom/Reservation . . . . . . . . . . . . . 32

2.4.4.2 CombinedFree/DemandAssignmentMultiple Access . 33

2.5 SchedulingStructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.5.1 CentralizedScheduling . . . . . . . . . . . . . . . . . . . . . . . 34

2.5.2 Centralized/DistributedScheduling . . . . . . . . . . . . . . . . . 34

2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

viii

3 BroadbandSatelliteAccess 36

3.1 BroadbandSatelliteAccessConfiguration . . . . . . . . . . . . . . . . . . 37

3.2 BSA System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.3 BSA ProtocolStacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.4 BSA MAC Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.4.1 BSA MAC Architecture . . . . . . . . . . . . . . . . . . . . . . . 43

3.4.1.1 BSA MediumAccessScheme . . . . . . . . . . . . . . 44

3.4.1.2 DynamicCapacityAllocation . . . . . . . . . . . . . . 45

3.4.1.3 DynamicServices . . . . . . . . . . . . . . . . . . . . . 47

3.4.2 MAC Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.4.2.1 MAC ManagementMessages. . . . . . . . . . . . . . . 48

3.4.2.2 IllustrativeExamplefor ProtocolOperation . . . . . . . 50

3.4.3 BandwidthAllocationandScheduling . . . . . . . . . . . . . . . 51

3.4.3.1 TransmissionOpportunities. . . . . . . . . . . . . . . . 52

3.4.3.2 STSStructurefor DynamicCapacityAllocation . . . . . 52

3.4.3.3 MCSStructurefor DynamicCapacityAllocation . . . . 56

3.4.3.4 Examplefor AdmissionProcedureovertheProposedStruc-

ture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.5 DataTransmissionProtocolundertheEnhancedCFDAMA Scheme . . . . 58

3.5.1 GeneralAssumptions . . . . . . . . . . . . . . . . . . . . . . . . 58

3.5.1.1 Traffic TransmissionModes . . . . . . . . . . . . . . . 60

3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4 BSA Formal Description and Validation 63

4.1 SDL Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

ix

4.1.1 BSA SystemModel . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.1.2 STSBlock Type . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.1.3 MCS Block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

4.1.4 BTS Block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.1.5 SatelliteBentPipe . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.2 SystemValidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.3 DescriptionandVerificationModels . . . . . . . . . . . . . . . . . . . . . 79

4.3.1 DescriptionConstituents. . . . . . . . . . . . . . . . . . . . . . . 80

4.3.2 VerificationConstituents. . . . . . . . . . . . . . . . . . . . . . . 80

4.4 Evaluationof SDL FormalSpecificationfor BSA . . . . . . . . . . . . . . 81

5 Effects of Buffer Sizeon Performanceof Prediction-BasedCFDAMA 85

5.1 CFDAMA Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

5.2 CFDAMA ProtocolOPNETModel . . . . . . . . . . . . . . . . . . . . . 87

5.2.1 ModelAssumptions . . . . . . . . . . . . . . . . . . . . . . . . . 87

5.2.2 ModelOperation . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

5.2.3 PerformanceMeasurements. . . . . . . . . . . . . . . . . . . . . 92

6 Conclusion 94

x

List of Figures

1.1 GeneralConfigurationof aBroadbandAccessSystem. . . . . . . . . . . . 3

1.2 FormalandNon-FormalDevelopmentCycles . . . . . . . . . . . . . . . . 9

2.1 GeneralDAVIC Configuration . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2 GeneralDAVIC System. . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.3 CabledNetwork Architecturein DAVIC . . . . . . . . . . . . . . . . . . . 17

2.4 SatelliteNetwork Architecturein DAVIC . . . . . . . . . . . . . . . . . . 18

2.5 Examplesof AccessNetwork Architectures . . . . . . . . . . . . . . . . . 19

2.6 DOCSISGeneralConfiguration . . . . . . . . . . . . . . . . . . . . . . . 20

2.7 An IDBS SatelliteNetwork Configuration . . . . . . . . . . . . . . . . . . 26

3.1 GeneralConfigurationof aBroadbandSatelliteAccessSystem. . . . . . . 37

3.2 BSA SingleDomainBlock Model . . . . . . . . . . . . . . . . . . . . . . 38

3.3 OverallBSA StackStructure . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.4 3-DimensionalBSA StackModel with BTS andSTSactingasForwarders

at theNetwork Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.5 MF-TDMA Structurein BSA . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.6 An Examplefor STSInitialization . . . . . . . . . . . . . . . . . . . . . . 50

3.7 STSDCA Block Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 53

xi

3.8 MCSDCA Block Structure. . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.9 DataTransferunderNormalModeOperation . . . . . . . . . . . . . . . . 60

3.10 DataTransferunderAccumulativeModeOperation . . . . . . . . . . . . . 61

4.1 SDL ProcessConstructsNotation . . . . . . . . . . . . . . . . . . . . . . 65

4.2 BSA StructureModel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.3 STSBlock TypeStructure . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.4 STSMAC Management. . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.5 MCSBlock Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

4.6 MCSMAC Management. . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.7 BTSBlock Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.8 SatelliteBentPipe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

4.9 An Examplefor MSC Observersof SpecificProperties . . . . . . . . . . . 78

5.1 CFDAMA DataManagerfor Terminalsin theSDL Model . . . . . . . . . 88

5.2 CFDAMA DataManagerfor Terminalsin theOPNETModel . . . . . . . . 89

5.3 CFDAMA SystemConfigurationin OPNETModel . . . . . . . . . . . . . 91

5.4 AverageDelayperSlotandAverageCell LossRatefor OneTerminalunder

Dif ferentLoads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

xii

List of Acronymsand Abbreviations

ADSL AsymmetricDigital SubscriberLine

ATM AsynchronousTransferMode

BSA BroadbandSatelliteAccess

BTS BaseTranceiverStation

BWA BroadbandWirelessAccess

CATV CableTV

CDMA CodeDivisionMultiple Access

CFDAMA Combined/FreeDemandAssignmentMultiple Access

DAMA DemandAssignmentMultiple Access

DAVIC Digital Audio-VisualCounsil

DCA DynamicCapacityAllocation

DHCP DynamicConfigurationHostProtocol

DOCSIS DataOverCableSystemInterfaceSpecifications

DS-DL DownstreamDown-Link

DS-UL DownstreamUp-Link

DSL Digital SubscriberLine

DVB Digital VideoBroadcasting

xiii

FDMA Frequency DivisionMultiple Access

FEC ForwardErrorCorrection

FTTC FiberTo TheCurb

FTTH FiberTo TheHome

GEO Geo-stationaryEarthOrbit

GII GlobalInformationInfrastructure

IDBS InteractiveDistributionBroadcastSatellite

IP InternetProtocol

LAN LocalAreaNetwork

LLC LogicalLink Control

LMDS LocalMulti-point DistributionSystem

MAC MediumAccessControl

MCS MasterControlStation

MCS-DL MasterControlStationDown-Link

MCS-UL MasterControlStationUp-Link

MF-TDMA Multiple Frequency TimeDivisionMultiple Access

MMDS Multi-channelMulti-point DistributionSystem

MPEG-2 Moving PictureExpertGroup-2

xiv

ONU OpticalNetwork Unit

OPNET OptimizedNetwork EngineeringTools

PBX PrivateBranchExchange

PSTN PublicSwitchedTelephoneNetwork

QCM QueueandCapacityManager

QoS Quality of Service

RSVP ReservationProtocol

SAT-MAC SatelliteMediumAccessControl

SDL SpecificationandDescriptionLanguage

SNMP SimpleNetwork ManagementProtocol

STS SubscriberTranceiverStation

TDMA TimeDivisionMultiple Access

TFTP Trivial File TransferProtocol

US-DL UpstreamDown-Link

US-UL UpstreamUp-Link

VDSL Veryhighbit-rateDigital SubscriberLine

VoD VideoonDemand

xv

Chapter 1

Intr oduction

Thetelecommunicationsindustryis oneof themostpromisingandanticipatedglobalbusi-

nessestowardsachieving universalinformationservices.In recentyears,it hasbeentremen-

douslyexpandedandits evolution is orientedtowardsintegrationof conventionaltelecom-

municationsindustrywith theemerging interactive Internetindustryaswell astheevolving

entertainmentindustry (e.g., CableTV (CATV), Video on Demand(VoD), etc.). There-

fore, it is foreseenasanew prominentbusinessarenawith enormouspotentialandbusiness

opportunities. Moreover, with the continuousadvancementsin network and information

technologies,it is reasonableto expectthateverypieceof informationin therealworld will

eventuallybedigitized,stored,andprocessed.Thissetsthestagefor all industriesandbusi-

nessesto becombinedtogetherandintegratedwith thetelecommunicationsindustry. Such

integrationis evident in the caseof electroniccommerce(e-commerce)[1], which on the

long run will grow to becomeoneof themostimportantserviceson thenetworks thatcan

createnew businessopportunities,new businessoperationsandnew industries[2]. Thisem-

phasizesthecorerole expectedfrom thetelecommunicationsindustryto play in achieving

theGlobalInformationInfrastructure(GII) [3]. TheGII is anticipatedto becomeaninfras-

1

tructurethat facilitatesthe development,implementation,and interoperabilityof existing

andfutureinformationservicesandapplicationswithin andacrossthetelecommunications

andinformationtechnologies.

1.1 BroadbandAccess

Oneof the long-termtargetsfor telecommunicationsnetworks in the multimediaerais to

supportany typeof telecommunicationsservices- distributionaswell asinteractiveservices

to residentialsubscribersor users[4]. Thecontinuouslyevolving broadbandaccesssystems

addressall the requirementsin orderto achieve this objective. The termaccessinvolvesa

subscriberaccessto someothernetwork, suchas, the Internet,a privatenetwork, a tele-

phony network or any otherbackbonenetwork. Telecommunicationssystemsin that case

canbe divided into threeparts;accessnetworks, corenetworks andsubscribernetworks.

Theaccessnetwork is definedasa network entity providing accesscapabilitiesfor various

serviceapplicationsto accessvariousserviceproviderslocatedat theedgesof thecorenet-

work. Alone, it doesnot representa completeend-to-endcommunicationsystem.Thecore

network may be the Internet,Public SwitchedTelephoneNetwork (PSTN),etc. The sub-

scribernetwork representstheend-userdoingtheaccess.It doesnotnecessaryconstituteof

asingleuser. Rather, aninterfacefrom somenetwork on thesubscriberside(LAN or PBX)

morelikely representsthestandardconfiguration.

Figure 1.1 depictsthe generalconfigurationof a broadbandaccesssystem. Various

broadbandaccesssystemtechnologiesarecurrentlyunderdevelopmentover cable,optical

fibers,wirelessandsatellite,etc. It is worth mentioning,asindicatedin [5], that in 1996,

theaccessnetwork accountedfor US$6billion of telecommunicationsbusinessworldwide.

It is growing at an averagerateof approximately27%/year. By 2000-2001it will have a

2

Access Network

Backbone Network

Subscriber Network

Subscriber Network

Figure1.1: GeneralConfigurationof aBroadbandAccessSystem

valueof aboutUS$19billion.

1.2 BroadbandAccessServices

Thewholetelecommunicationsresidentialindustryis servicecentered.Successfulservices

are driven in part by advancednetwork and information technologies.The other part is

market dependent,wherethe envisionedservicesshouldaddressconsumers’perceptions,

marketingsensitivity, cost/profitanalysis,andadvancednetwork andinformationtechnolo-

gies[2]. Thedevelopmentof broadbandaccesssystemsis drivenby andcoincideswith in-

creasingusers’demandfor transparenttransmissionsof datatraffic, especiallymultimedia,

betweensubscribersandbackbonecorenetworksin orderto supportenvisionedmultimedia

services.A multimediaserviceis a servicehandlingseveraltypesof presentationmediain

a synchronizedway from theconsumers’point of view. Themediamaybetext, graphics,

sound,image,andvideo,organizedto providevariouswaysof access.Multimediaservices

envisionedto bedeployedover theaccesssystemsmay includeasymmetricservices,such

3

as,Internetaccess,VoD(VideoonDemand)andDigital Audio/VideoMulti-castor symmet-

ric services,suchas,digital telephony andvideoconferencing.Otherareasof application

andinvestigationincludeelectroniccommerce,telemedicine,city informationservices,in-

telligenttransportationsystems,distancelearning,electroniclibrariesandmuseums,etc.

1.3 BroadbandSatelliteAccess

As previously mentioned,in recentyears,numerousbroadbandmultimediaserviceshave

beenintroduced.With MPEG-2[6], Digital VideoBroadcasting(DVB) [7], [8] is already

a reality. Video-conferencingandVoD (Video on Demand)servicesaswell asnumerous

multimediaapplicationsover theInternetwill not take long beforethey becomeefficiently

deployed. Expectedpopularityof theseserviceshasled to rigorouscompetitionbetween

currenttechnologiesin order to serve as the underlyinginfrastructure. DataOver Cable

SystemInterfaceSpecifications(DOCSIS)[9], [10] is alreadyimplementedby thecablein-

dustryin someregions.Digital SubscribersLine (DSL) [11] technologieslately developed

aVeryhighbit-rateDSL (VDSL) [12] to consolidateits competitionchances.IEEE802.16

BroadbandWirelessAccess(BWA) [13] taskgroupwasrecentlyformedto investigatewire-

lessaccess.Othertechnologiesinvolving fiber networksarealsounderinvestigationsuch

as,Fiberto theHome(FTTH) [14] andFiberto theCurb(FTTC) [14].

Advancementsin satellitetechnologieshasencouragedsatelliteproviders to envision

themselvesasstrongcompetitors.However, thesatellitecompetitionfor thebackbonecore

network role seemsquestionablefrom both the costanddelaystandpointsmainly, dueto

its well-known longer delay over shorterdistancesas well as its power and bandwidth

constraints.Satelliteprovidersseemto have a betterchancewith satellitesasaccessnet-

works to the backboneor as interconnectionsbetweenremotenetworks. In that context,

4

the former referredsatellitedrawbacksarebalancedout. Global connectivity over single

hopsallows for wide areacoveragewhile saving lots of switchingandrouting delaysas

well assequencingproblemsthatothertechnologieswill experienceoversimilar distances.

Reachabilityto remoteinaccessibleareasprovescost-effectivein low populationruralareas.

Satellitetransmissionsarebroadcastin nature.Thisallows for cost-effectivedeploymentof

broadcast/multi-pointservices,especially, in regionswith infrastructuredeficiencies.Satel-

lite systemsdiffer from other systemsin that the network provider (satelliteprovider) is

usually independentof the serviceprovider (VoD server for example). This allows satel-

lite networksto becomemoresuitedin developingservicescharacterizedby a competitive

environment.Competitivenessguaranteesbetterpricing for end-users.Suchconfiguration

will alsoencouragenumerousserviceprovidersto join themarket without worrying about

transmissionequipment.

1.4 BroadbandSatelliteDeployment

Broadbandservicesare far more complicatedthan traditional basebandservices. While

basebandservicessimply usually involve only a singledatatype, broadbandservicesare

characterizedby network integration,high bandwidthdemands,multimediaservicescom-

plexity, quality of service(QoS) requirements,and traffic fluctuation. The operationof

futuretelecommunicationsnetworkstherefore,is expectedto bemorecomplicatedthanthe

currentsituation. Taking telephoneserviceasan exampleof basebandservices,the ser-

vice hasa fixed bandwidthrequirementfor its connection.Although theremay be many

servicefeatures,suchas,call forwardingandcall waiting, to beaddedto thebasicservice

mechanism,establishinga call remainsthe major task. Broadbandserviceson the other

hand,demandmuchmorethanthis. A VoD servicefor exampleneedsto handlenot only

5

connectionestablishmentbetweena client andits server in orderto carryaudio,video,and

possiblytext streams,but also the variouscontrol messagessuchas forward, pause,and

rewind [15]. VideosignalsunderMPEG-2compressionarecomparatively bandwidthhun-

gry andproduceburstyvariablebit-ratetraffic. Moreover, real-timeaswell asnon-real-time

multimediaservicesareusuallycharacterizedby stringentQoSrequirements.

Deploying a broadbandservicewith necessaryefficient transportof data, voice and

video in termsof bandwidth,reliability anddelay, thereforeis far morecomplicatedand

requirescomplex engineeringefforts. A key elementin satisfyingsucha requirementwill

be thedevelopmentof anefficient MediumAccesscontrol (MAC) protocol/technique,es-

pecially, over sharedmedia. A MAC protocolspecifiesthe messagesequencesnecessary

to establishandmaintainconnectionsbetweenterminalsacrossthesatellitelinks involved

in broadbandcommunications.The MAC techniqueon the otherhand,defineshow users

will efficiently sharethecapacityallocatedachieving maximumsubscribersunderminimum

bandwidthrequirements.

Thedesignof anadequateefficient MAC layer for satellitenetworks representsa seri-

ouschallengeandis oneof the hottestresearchareas,assatellitesystemspossessseveral

characteristicsthathinderdecentperformanceof regularMAC. Developmentof new access

techniquesthatefficiently utilize thelimited bandwidthis required.Thesetechniquesmust

achieve rapidchannelaccessby userssharingthebandwidthin orderto minimizesatellite

long delayeffects. SatelliteMAC shouldalsosupportflexible reconfigurablechannelsto

facetheharmfuleffectsof non-stabilityandinterferenceof thesatellitemedium.Moreover,

assatellitesystemsareconfigureddifferentlyfrom regulartelecommunicationssystems,the

satelliteMAC mustachieve propercoordinationbetweenthevariousterminalsinvolvedin

datacommunications.

6

1.5 SatelliteSystemsImplementation

Thetrendin implementingcurrenttelecommunicationssystemsis orientedtowardsthede-

velopmentof networks that satisfy the requirementsof an opensystem. An openaccess

systemhasits end-userterminalsindependentof theemployedaccesstechnology. Hence,

accessnetwork providersintroducedtheconceptof interfaceterminalsasintermediategate-

waysthat connectthe customerswith their correspondingaccessnetwork. The advantage

of suchconfigurationis that it promotesmassproductionof end-userequipmentat lower

pricesandrendersflexibility to theirproducers.In satellitesystems,this is evidentin anair-

interfaceterminalbetweenusersandthesatellitemedium.Theair-interfaceterminalshould

convey the responsibilityof addressingandmanagingall the satelliterequirements.As a

result,all theeffortsondevelopmentof anefficientsatelliteMAC layershouldresideat the

air-interface. It is obvious thatcorrectdevelopmentof this air-interfaceterminalis critical

to thesuccessof satellitesystems.

Satellitenetworksasall communicationsystemsarecomplex, concurrent,safety-critical

andrequirestandardization.Descriptionsgivenin non-formalnaturallanguageand/ordia-

gramsarehardto avoid ambiguityandnot easyto analyzeor processautomaticallyasthey

lack explicit technicalpresentations.Generallyover theyears,protocolimplementationfor

telecommunicationssystemswasdonenon-formally. Implementationwasa direct transla-

tion from annon-formalspecification.Thoughfinal productswerereleasedafternumerous

debuggingiterations,still many faultsdueto designandimplementationerrorswereexperi-

encedthroughpracticaluseresultingin hugewastedinvestments.Thiswouldbecompletely

unacceptablewith satellitesystems,whereonly the launchingprocesswould costmillions

of dollars. Over the last two decades,a new trend hasemerged for telecommunications

systemsdevelopmentthroughthe useof formal techniques[16]. Formal techniques(FT)

7

arecapableof definingclear, plain andunambiguoussystemsin orderto achieveconsistent

standards.They havebeensuccessfullyutilized for descriptionandvalidationof communi-

cationprotocols.While systemdescriptioninvolvesbehavior andrequirementspecification,

validationinsuressystemcorrectness.

Theadvantageof describingsystemsusingformal languageslies in their representation.

Usingformal languagesfor systemspecificationcanbeconsideredasanintermediatestage

beforeimplementationthat achievesprecisedescriptionandspecifiestechnicalattributes.

Moreover, somelanguagesastheSpecificationandDescriptionLanguage(SDL) [17] com-

plementthat by visuality throughutilizing a graphicalrepresentation.Anotherinteresting

aspectof formal languagesis their ability to employ abstractionswithout affecting the in-

tegrity of thedescription.In thatscenario,descriptionandconsequentsimulationsareable

to utilize abstracted(input/outputbehavior) systemconstituentsto reducecomplexity while

achievingclearerunderstanding.Thiscanbeutilizedtosimplify thedescriptionof interfaces

to thesystemof interestandpresentmutualinteractions.Again in somelanguageslikeSDL

thatareobjectoriented,abstractionsallow for easiermodeldevelopment.Modelsarebuilt

in stageswith continuousextensionsin orderto gradually, yet simply achieve a complete

model. Moreover, assomeformal languagesrepresenta standardlanguage(e.g.,SDL), an

automaticcodegenerationis alwayspossible,thus,relieving someof the implementation

efforts.

One fundamentalconstituentof systemdevelopmentis the testing. It is usuallycon-

ductedon implementationsto confirmtheir functionality. This is a tediousprocedureespe-

cially within non-formalsystemdevelopmentdueto thecomplexity of currentcommunica-

tion systems.A complex communicationsystemmightrequiremillions/billionsof testcases

to confirmits functionality. Evenwith themosteffective testingtechniques,faultsarestill

8

Non-FormalSpecification of

Needs

Compilation

Non-Formal SoftwareDevelopment Cycle

Testing andDebugging

High-LevelLanguage Code

System Designand Concepts

Executable Code

Compilation

Simulation andValidation

Code Generationand Manual Translation

Non-FormalSpecification of

Needs

FormalSpecificationand Design

Testing andDebugging

Executable Code

Formal SoftwareDevelopment Cycle

High-LevelLanguage Code

Modification inthe design isrequired

SyntaxModificationif required

SyntaxModificationif required

Repeat untilValidation Ok

Automatic TestCase Generation

Manual Generationof Test Cases

Manual Generationof Test Cases

Figure1.2: FormalandNon-FormalDevelopmentCycles

usuallydiscovered. Formal techniquesprovide a way to minimize the requiredtestingby

employing formalvalidationandverificationat earlierstages.With thedevelopmentof for-

mal verificationtechniquesasreachabilityanalysis,systemverificationinsurestheabsence

of deadlocks,unspecifiedreceptions,live-locks,etc. in systemoperation.Validationuses

theverificationprocessto confirmdifferentbehavioral scenariosinsuringthatthedeveloped

systembehavior coincideswith the requirements.Successfulverification and validation

guaranteecorrectnessof thedesignandtheconsequenttestingwouldbesimplified.A com-

parisonbetweenformal andnon-formalsystemdevelopmentcyclesis shown in Figure1.2.

As shown in thefigure, theadvantageof formal techniquesis evident in the independence

betweenthedesignandtheimplementationphases,while non-formaldevelopmentrequires

goingbackandforth betweenthetwo phasesandit is usuallydifficult to distinguishbetween

designandsyntaxerrorsin testing.Hence,oncethevalidationof theformalspecificationis

9

completed,wecanexpectlessiterationsin theimplementationtestphaseandthereforeless

total developmenttime.

Althoughformal methodsarepowerful tools for systemsspecificationandverification,

their building semanticslack theability of providing quantitativeperformanceanalysisfor

thedevelopedsystems[18]. Performancemeasuresarevery importantto evaluateandcom-

parecommunicationsystems.Hence,equivalentsimulationmodelsmustbe built to meet

commonstandards.Oneof themostpopularsimulationtoolsis theOptimizedNetwork En-

gineeringTools(OPNET)[19]. Combiningperformanceandbehavior underformal meth-

odsis averyhot researchissue.

1.6 Contrib utions and Outline of the Thesis

Wecansummarizethecontributionsof our researchwork in thefollowing:

1. Constructinga flexible MAC layer architecturethat canbe employed in residential

two-way interactiveservicesoversatellite.

2. Employing anenhancedCFDAMA accesstechniquebasedonpredictionfor dynamic

capacityallocation.

3. Employing a flexible two-level schedulingstructurebasedon a differentialservices

approachwith minimumcomplexity.

4. Defining behavioral descriptionfor two modesof datatransferoperationunderthe

prediction-basedCFDAMA protocol.

5. Developinga formal model for the devised MAC layer behavioral descriptionand

verification.

10

6. Developinga performancesimulationmodelthatdemonstratesperformanceanalysis

of theutilized accesstechnique.

Therestof thethesisis organizedasfollows:

In Chapter2, we discussissuesrelatedto thestate-of-the-artof broadbandaccesssys-

tems. We describestandardsbodiesandhighlight variousaccessnetworks technologies.

Wethenconductasurvey of interactivebroadbandsatelliteaccesssystemsanddiscussopen

issuesin the field. Then in Chapter3, we describean interactive residentialbroadband

satelliteaccesssystem.We show its configurationstructure,constituentsandproposedpro-

tocol stacks.We emphasizetheMAC layerspecificationaswe describedynamiccapacity

allocation,accesstechniques,schedulingstructures,behavior descriptionanddatatransfer

operation.In Chapter4, wedevelopa formalmodelfor thedevisedMAC layer. Wediscuss

modeldescriptionandpresentverificationandvalidationresults. Then in Chapter5, we

built anOPNETsimulationmodelfor theutilized accesstechniqueandpresentanexample

for performanceanalysis.Finally, in Chapter6, we summarizeaswe concludeour work

andsuggestpossiblefurtherrelatedresearchstudies.

11

Chapter 2

BroadbandAccessSystems

The term broadbandliterally meanslarge bandwidth.With inevitable increasein thecus-

tomers’needsto accessmultimediainformation,popularityof broadbandsystemsis contin-

uouslyontherise.Successfuldeploymentof thesesystemshowever, necessitatesnumerous

efforts in all domainsof the field in orderto achieve efficient cost-effective servicesfrom

theperspective of serviceandnetwork providersaswell ascustomers.In this chapter, we

discussissuesrelatedto thestate-of-the-artof broadbandaccesssystemsaswe addressthe

variouseffortsandtechnologiesthatenvisionbroadbandservicesandaim for systemsstan-

dardization.Wethenshift our interestto focusondeploymentof suchservicesoversatellite

networks. We presentexamplesof currentefforts andhighlight their drawbacks.We then

discusstheopenissuesin satellitesystems.In theend,weaddressoneof themostimportant

challengesof satelliteaccessrelatedto developmentof anefficientMAC layer.

12

2.1 DAVIC

Soon,asweapproachclosertowardsachieving theGII [3], digital audio-visualserviceswill

dominatethe telecommunicationsmarkets. The vision of sharedaudio-visualinformation

transmittedback and forth over telecommunicationsnetworks betweenserviceproviders

andend-usersis steadilybecomingmoreandmoreevidentwith ceaselessefforts of devel-

opmentandimplementationof globally agreedaudio-visualstandards.TheDigital Audio-

Visual Council (DAVIC) is a well-recognizedtechnologystandardsleaderin this domain

and, through its specificationsand cross-industryrole, actively promotesthe successful

worldwidegrowth of interactive digital audio-visualapplicationsandservices[20]. In this

section,we presenta brief discussionof the DAVIC organizationwork, technologyand

specifications.

2.1.1 DAVIC Organization

TheDigital Audio-VisualCouncil(DAVIC) wasfoundedin October1994basedin Geneva.

It is a non-profitmakingassociationwith a membershipin morethan200companiesfrom

over 25 countriesworldwide and hastaken the leadershipof promotingand developing

broadbanddigital services. The DAVIC membershipis representedby all sectorsof the

audio-visualindustryincludingthecomputer, consumerelectronicsandtelecommunications

manufacturingsectors,andthe broadcasting,telecommunicationsandcablecompaniesas

well assomegovernmentandresearchorganizations.DAVIC membersincludemajor in-

dustryplayersincludingMicrosoft,BT, AT&T, Intel, andtheBBC [21]. Moreover, DAVIC

is alsocommittedto operatewith otherstandardsorganizationsandis opento integrationof

thepartialsolutionsprovidedby their standardsandspecificationswheneverpossible.

Since1994,DAVIC expertshave succeededin producingfive separatereleasesof the

13

DAVIC specification. Eachof theseindustry specificationsis backward compatible,and

specifiesneededinteroperabilityandfunctionalcapabilitiesof consumers,contentproviders,

serviceproviders,aswell asdeliverysystemsto achievethetargetservices.Earlierversions

of DAVIC specificationsonly involvedaudio-visualservices,e.g.,TV, VoD astheinitial pur-

posefor DAVIC wasfavoring thesuccessof emergingdigital audio-visualapplicationsand

services.However, with theInternetcurrentlybeingaccessedworldwideby some50million

PCowners,recentversionsof DAVIC specificationsspecifytoolsfor Internetservices,e.g.,

web browsing. DAVIC’s interrelationwith the Internetprovidesan evolutionarypathfor

the Internettowardsexcellentmultimediadistribution with guaranteedconnectionaswell

asdefinedquality of service(QoS)1 (usingbandwidthreservationanddifferentialservices)

in termsof bandwidth,latency, loss,andjitter [22]. Full detailson thelatestDAVIC specifi-

cation1.4couldbefoundin [23]. Thenext DAVIC release1.5is expectedto investigateTV

anywhere on the Intra-netdesignlevel. Examplesof existing implementationsthatutilize

DAVIC specificationscouldbefoundin [20].

2.1.2 DAVIC Objectives

The goalsof DAVIC aredeclaredas ‘... to identify, select,augment,developand obtain

endorsementbyformalstandardsbodiesof specificationsof interfaces,protocolsandarchi-

tecturesof digital audio-visualapplicationsand services’[21]. The objective is to define

minimumtoolsandcorefunctionsrequiredby digital audio-visualsystemsfor end-to-end

interoperabilityacrosscountries,applications,andservices.In orderto guaranteeinteroper-

ability acrossdifferentsystemsandapplications,DAVIC’sapproachprecludesthedefinition

of a system;instead,non-system-specificcomponents,or “tools”, aredefined.Thesetools1The term QoSdefinesparametersasdelay, throughput,jitter, etc. that shouldbe sustainedduring the

lifetime of a certainconnection.

14

Figure2.1: GeneralDAVIC Configuration

shouldhave the capability to be employed in a variety of differentsystemsaswell as in

differentpartsof thesamesystem.

2.1.3 DAVIC GeneralSystemConfiguration

ThegeneralDAVIC systemmodelis shown in Figures2.1and2.2 [21]. It consistsof 5 en-

tries: thecontentprovidersystem(CPS),theservicesprovidersystem(SPS),andtheservice

consumersystem(SCS),whichareinterconnectedby theCPS-SPSdeliverysystemandthe

SPS-SCSdeliverysystem.As shown in thefigure,thesystemdefinesfiveinformationflows

[21]:

S1: Principalserviceslayerpeerflow for uni-directionaltransferof encodedaudio,

videoanddatafrom theserver to theset-topunit.

S2: Applicationserviceslayerpeerflow for bi-directionalexchangeof thecontrol

15

Figure2.2: GeneralDAVIC System

informationfrom anapplicationservicelayersourceobjectto apeerdestination

object.

S3: Session/Transportserviceslayerpeerflow for theconnectioncontrolof a ses-

sion.

S4: Network serviceslayerpeerflow for theconnectionof thenetwork androuting

throughthenetwork.

S5: Administrationof network elements,e.g.,network management.

For thesefive flows, DAVIC hasdefineda full protocolstackbaseduponthe appropriate

andcurrentlyavailableprotocols.

16

Figure2.3: CabledNetwork Architecturein DAVIC

2.1.4 DAVIC Delivery System

The term ‘delivery system’involvesany meansof conveying informationfrom oneentity

to anotherin order to supportthe servicesenvisionedby DAVIC. Figure2.1 shows how

thevariousentitiesof thegeneralDAVIC modelareconnectedto thedelivery systemand

definesthereferencepointsthroughwhich connectionspass.Deliverysystemsmaybenet-

worked or non-networked. Non-networked systemsincludephysicalstoragemedia,such

as,CD-ROM, discandtape.Networkeddelivery systemsincludeCablednetworks,Radio

networks(transportsignalsusingelectromagneticwavesandareessentiallybroadcastnet-

works),suchas,terrestrialandsatellite,andHybrid networks,suchas,Multi-channelMulti-

point DistributionSystem(MMDS) andLocalMulti-point DistributionSystem(LMDS). It

is worth mentioningthatthescopeof this thesiswill focuson theaccessnetwork (Satellite

network) delivery systembetweenreferencepointsA1 andA9 (Figure2.2) with empha-

sison theupstreamup-link at A1 referencepoint. Figures2.3 and2.4 [21] show general

structuresfor CabledandSatellite(anexamplefor Radio)networksasdescribedby DAVIC.

17

Figure2.4: SatelliteNetwork Architecturein DAVIC

2.2 BroadbandAccessNetwork Technologies

As indicatedabove, oneof the areasof DAVIC’s study involvesdelivery systems.In the

accesssystemsarchitecture,the key delivery componentresidesin the accessnetwork as

it is consideredthe linking bridgethatshouldefficiently supportcommunicationsbetween

consumersandtheserviceproviders.Moreover, sinceaconceptof multi-serviceintegration

over commonaccessnetworks is becomingmorepopulardue to the economicalbenefits

of sharingthe accessresourcesamongservices,the accessnetwork, especially, the MAC

layer must be carefully designedto accommodatenumerouspossibledivergent services.

Variousbroadbandaccesssystemtechnologiesarecurrentlyunderdevelopment.Figure2.5

shows examplesof accessnetwork architecturesas describedin [24]. It is importantto

notethat mostof thesetechnologiesmay alwayssharesimilar end-systemparticipantsto

ensureanopensystempolicy. Dif ferencesareusuallyratherevident in theaccessmedium

itself. In thefollowing sub-sections,weprovideexamplesof effortsconductedin theaccess

technologyfield. It is alsoworthwhile to mentionthat thesetechnologiesmay participate

18

AccessNode

AccessNode

AccessNode

AccessNode

AccessNode

AccessNode

FTTH

xDSL

MM DS/LMDS

HFC

Satelli teFTTC/VDSL

Metall ic

Metall ic

Optical Fiber

ONU

Optical Fiber

Optical Spli tter

ONU

ONU

ONU

ONU

Optical Fiber

ONU

CoaxialCable

Satelli te

Metall ic (Upstream controlline)

Optical FiberBase

Figure2.5: Examplesof AccessNetwork Architectures

19

Figure2.6: DOCSISGeneralConfiguration

in implementinga (completeor partial)DAVIC compliantsystem,which is not necessarily

developedby theorganizationitself.

2.2.1 CableNetworks

TheDataOverCableSystemInterfaceSpecification(DOCSIS)[9], [10] wasintroducedfor

cableoperatorsin deploying high-speeddatacommunicationssystemson cabletelevision.

It describesterminalstructuresanddatainterfacesthatwill provide betterperformanceon

cablenetworks. Subscribersaccessthe network througha CableModem(CM) andtheir

transmissionsarerealizedat thehead-endby aCableModemTerminationSystem(CMTS).

A generalconfigurationfor the DOCSISsystemis depictedin Figure2.6 [10]. The envi-

sionedservicesrely on efficient bi-directionaltransferof datatraffic over anall-coaxialor

hybrid-fiber/coaxcablenetworkorganizedin a tree-and-branch [9] architecture.Thesystem

usesupstream(CM to CMTS direction) frequenciesbetween5-30 MHz anddownstream

(CMTS to CM) frequencieswith a lower edgebetween50 and54 MHz andanupperedge

rangingfrom 300to 860MHz. Signalsmaybetransmittedatashighas10-15Mb/s. DOC-

SIS 1.1 [10] wasrecentlyreleasedto complementthe former DOCSIS1.0 [9]. DOCSIS

1.0only supportedstaticnetwork connectionsinitiatedduringsubscriberregistration(when

20

usersturn terminalson). DOCSIS1.1ontheotherhand,introducedadynamicdimensionto

thespecificationby definingdynamicservices.Dynamicservicesenablechangingattributes

of existing connectionsaswell asestablishingnew connectionsduringterminaloperation.

DOCSIS1.1 alsoenvisionedtheuseof somedifferentiatedschedulingservices.However,

specificcategoriesarenotyetcompletelystandardized.WehaveconsideredtheDOCSISas

a candidatetowardsdevelopmentof anefficient satelliteMAC. Detailson suchprocedure

andon theDOCSISMAC specificationitself will bepresentedin thenext chapter.

2.2.2 Digital SubscriberLine

Digital SubscriberLine (DSL) [11] offersbroadbandservicesoverspectrumbandsbeyond

the4kHz voicechannelsof twistedpairs.In thepast,bandwidthallocatedby thetelephone

company switch to voice calls limited the bandwidthof the voice modem. DSL technol-

ogy revealedthe capabilityof phonelines of carryingvery high dataratesif the narrow

bandswitchescanbeavoided. AsymmetricDigital SubscriberLine (ADSL) is oneof the

currentdevelopedDSL systems.It definesan asymmetrictransmissionsystemin which

downstreamsignalsusethebandwidthof severalhundredkb/srequiredfor Internetaccess

or several hundredMb/s requiredfor video transmission,while the upstreamusesband-

width sufficient for servicecontrolsignalswith lower initial investmentin facilities.ADSL

is capableof transmissionin a rangeof severalhundredmetersto severalkilometers.The

latestDSL systemis Very high bit-rateDigital SubscriberLine (VDSL) [12]. It is a high

bit-rateversionof ADSL that offers downstreamsignalsup to 30 Mb/s. However, dueto

thefactthatits transmissionis limited to lessthan1.5kilometersthereis a restrictionon its

applications.

21

2.2.3 Fiber to the Home

Fiber to theHome(FTTH) [14] is a fully opticalnetwork from theserviceprovider to the

consumer. Theopticalmultiplexedsignalis broughtto a splitter in thevicinity of a group

of customers.Thereareoptical splittersof differentratios,but themosttypical ratio used

is 1 to 16. This meansthat themultiplexedsignalis split to 16 differenthouseholds.Since

the optical signalhasto be convertedto electricalat the customer’s premises,an Optical

Network Unit (ONU) hasto be installedat the end of the network. Becausethe ONU

areexpensive, it hasbeensuggestedthat the resourcesof a singleONU shouldbe shared

amongseveralcustomers.Figure2.5 suggestswhat theFTTH accessnetwork might look

like. Applicationsof optical fibers result in fewer restrictionson transmissiondistances

and bandwidthsfor future serviceprovisioning. If we can achieve cost-effective optical

components,it maybeconsideredanidealline-feedaccesssystem[24].

2.2.4 BroadbandWir elessAccess

BroadbandWirelessAccess(BWA) is a wirelesssystemdesignedto deliver data,voice

andvideo with low delay. In BWA, multimediaservicesareprovided by basestationsto

businessor homenetworks in a star topology. As larger chunks(on the orderof 1 GHz)

of microwave andmillimeter wave spectrumarebecomingavailable,BWA becomesmore

liable for deployment in several regions. Most of the BWA frequency allocationsare in

the 24, 28, 31 or 40 GHz bandsandhave the capability to deliver dataratesin the tens

of Mb/s range. It shouldbe mentionedthatBWA is differentfrom the traditionalcellular

systemin two significantaspects.Firstly, BWA assumesfixedterminalsandthusdoesnot

supportmobility and the associatedcomplexity due to hand-over and fading. Secondly,

BWA operatesat high frequency, which limits the cell size [13]. The IEEE 802.16task

22

groupwasrecentlyformedto develop802standardsfor BWA.

2.2.5 Multi-channel Multi-point Distrib ution System

A typicalMulti-channelMulti-point DistributionSystem(MMDS) receivesscrambledsatel-

lite or cableTV signalsatacentrallocation.Thecentralstationthende-scrambles,digitizes,

multiplexes,andre-transmitsthereceivedsignalsby specialtransmitters(SHF).TheSuper

High Frequency (SHF, typically in GHz) transmittersthendistributethesignalsthroughout

thecoveragearea.Antennasinstalledon subscribers’roofs thenwill receive thesesignals

anddistribute themwithin the homeor building throughcoaxialcableinto a set-topbox

locatednearthetelevision. MMDS configurationis shown in Figure2.5.

Onemajorlimiting characteristicof wirelesscablesystemsis therequirementof line-of-

sight(LOS) transmission.Suchconstraintinducesa crucialdecisionon thelocationsof the

tranceivers.A preferablelocationwouldbeamountainpeak,with ahighangleof elevation

andfew obstaclesthatblock thesignal[25]. MMDS typically operatesin frequency bands

of lessthan 10 GHz, whereasPoint-to-Multi-point wirelesscommunicationssystemsfor

multimediabroadbandaccessservicescalled Local (L)MDS operatein millimeter-wave

frequency bandsover10GHz.

2.2.6 SatelliteAccess

Satellitecommunicationsystemscanoffer two-way interactive broadbandmultimediaser-

vicesoverwiderregionsthanall previouslydescribedtechnologiesdueto thenaturalbroad-

castcapability. They have theadvantageof possiblequicker deploymentin comparisonto

line-feedsystems,especially, in regionswith minimum infrastructures.They alsoprovide

line-of-sightadvantagesovermobilenetworks,especially, in crowdedregionswheresignal

23

blockingof wirelesstransmissionsis inevitable.Moredetailsaboutsatellitesystemswill be

presentedin thenext section.

2.3 BroadbandSatelliteSystems

In this section,we focuson presentingvarioussystemsdeployed for interactive services

over satellitenetworks andhighlight their drawbacks. We thenrevert to discussthe open

issuesin thefield with emphasison thedevelopmentof anefficientMAC layer.

2.3.1 Brief History

Satellitetechnologydatesbackto asearly as1947whenArthur C Clarke proposedGeo-

stationaryEarthOrbit (GEO)Satellites.Thefirst satellitelaunchedthoughwasa low earth

orbit known asSputnikdevelopedby the Russiansin 1957. The successof Sputniksig-

naledthebeginningof anew erain telecommunicationscharacterizedby globalworldwide

coverage. In 1965,Early Bird (akaIntelsatI) beganthe eraof commercialGEO satellite

communications.Afterwards,GEOsatelliteservicesstartedto spreadrapidly to provideIn-

ternationalaswell asRegionaltelephoneservicesoverwideareas.Currently, GEOsatellite

applicationsarevery popularin theTV broadcastingindustry, e.g.,Directv, PrimeStar, etc.

Low EarthOrbit (LEO) satellitesystemsarealsobecomingmorepopularwith Orbcomm

(1998),GlobalStar(2000),andICO-Global(2002).

Onthecourseof satellitedevelopment,complexity of equipmenthaschangeddrastically

from 34 kg, 240 telephonecircuits in the 1965Early Bird to 3000kg, 8 - 15 kW power,

1,200kg payloadin a year2000large GEOindicatingthehugeinvestmentsdedicatedfor

thesatelliteindustry. It is reportedthatexpectedrevenuesfrom all satellitecommunications

24

servicesshouldreach$75billion by 2005[26].

2.3.2 Interacti veBroadbandSatelliteSystems

Satellitesarecurrentlymainly usedfor TV andvoicecommunicationsworldwide. As the

most attractingfeatureof satellite is its naturalbroadcasting,video servicesby satellite

earned$17Billion in 1998.Hence,Wall StreetJournalcalledDirectBroadcastSatelliteTV

theGreatestTechnologyDevelopmentof theCentury[27]. Howeverasusers’demandsfor

multimediaInternet-basedservicesincrease,satellitetechnologyis challengedto enhance

itscapabilitiesby introducingauserinteractiondimensionto its structure.Satellitenetworks

operationwill no longerbeconfinedto a videoprovider broadcastingits datato customers.

Userswill have to be able to transmitdatathemselvesfor satelliteoperatorsto have any

chanceto survive in a highly competitive telecommunicationsmarket. As a matterof fact,

communicationssatelliteshave beenusedin theInternetfrom thebeginning,whereoneof

the first global networksusingthe Internetprotocolswasthe Atlantic SAT-NET intercon-

nectingtheARPANET with researchnetworksin Europein theyears1979-1985.However,

thescopeof futuresatellitenetworksin orderto sustainits establishedsuccessmustsurpass

just interconnectingpurposes.In thefollowing sub-sections,wepresentexamplesof differ-

entefforts to constructan interactive satellitesystem.We alsopoint out thedrawbacksof

eachsystemandhow they areavoidedunderthescopeof ourwork.

2.3.2.1 Interacti veDir ect BroadcastSatellite

A network configurationdescribedin [27] proposesa forward channelsuppliedby Direct

BroadcastSatellite(DBS). The returnlink to a server is provided by someothernetwork

(telephone,separatesatellitechannel,etc.) in order to achieve interactive datadevices.

25

Client

Client

STB

Return LinksISP

STB

Direct BroadcastSatelli te

Server

Server

MasterStation

ForwardLinks

Figure2.7: An IDBS SatelliteNetwork Configuration

ThesesystemsaretermedInteractive (I) DBS. Figure2.7 [27] shows an IDBS configura-

tion. TheIDBS asdescribedin [27] mayusedifferentdatalink protocolsdependingon the

satellitechannelused.IDBS-A usestheWegenersystemfor digital sub-carrierswheredata

packetsfor IDBS aremultiplexedover up to eight digital audiochannelsonto oneanalog

TV channelfor a netdatarateof 192kb/s(still limited in transmissioncapacity).IDBS-V

usesVery Small ApertureTerminals(VSAT) modemswith integratedFEC codesfor data

ratesof 384kb/sto 2 Mb/s. IDBS-D reliesonthepopularMPEG-2/DVB standardsandrep-

resentsthemostadvancedsolution,asmultimediaapplicationsrequirethehigherdatarates.

TheMPEG-2standardsdefinehow compressedvideoandaudioareencodedasPacketized

ElementaryStreams(PES)and transported.This asynchronousTime-Division Multiplex

(TDM) systemutilizesa packet transmissionsystemwith fixedlengthcellsof 188bytesto

attaindataratesof 4 or 8 Mb/s beingcommonfor television quality. The MPEG-2/DVB

26

protocolarchitecturecanbebrokendown into threelevels;Physicallayerdefinedby EBU-

DVB documents[28]. Thedatalink providestransportover 188byteslong packets. Level

threeprovidesadaptationlayers.More detailson MPEG-2andDVB maybefound in [6],

and[7], [8] respectively.

It is notedthat IDBS assumesanasymmetricalnetwork accessconfigurationthatenvi-

sionsa separatenetwork for thereturnlink; usuallyis a terrestrialnetwork. Ratesattained

on this returnlink aremuchlower thantheforwardlink. Suchaconfigurationmightappear

relevant for web browsing like applications;especiallythat measurementsof typical web

sessionsanduserbehavior show a ratio of about10:1 to 20:1 betweenincomingandout-

goingdatastreams.However, underthepressureof users’demandsfor serviceintegration,

theallocatedbandwidthwill neverbesufficient to deploy somekey servicessuchasIP tele-

phony or videoconferencing,etc. Moreover, usingterrestrialnetworkswith its equipment

andfacilities for returnlinks, yet addsa new playerto a satelliteaccesssystem.This will

resultin highercostsif comparedto systemsthatintegratetheforwardandreturnpathsover

a sharedmediumunderthesameprovider. It is alsoworth mentioningthat recentlyIDBS

hastried to useseparatesatellitechannelsfor thereturnlink. However, suchefforts arenot

yet standardized.

2.3.2.2 TR34.1SATATM

Basedondemandfrom theindustry, theCommunicationsandInteroperabilitySection(CIS)

of the TelecommunicationsIndustryAssociation(TIA) satellitecommunicationsdivision

hasstartedthis standardizationprocess.TR34.1,thestandardscommitteefor CIS, hasde-

finedasetof satellitebasedATM [29] network architecturesfor futurephysicallayerspec-

ification. The architecturesdefinedby TR34.1are presentedin [30] andcanbe broadly

27

groupedinto two categories;satelliteATM (SATATM) architecturesfor transparentsatel-

lites, andSATATM architecturesfor satelliteswith on-boardswitches.In orderto provide

ATM andInternetservicesat requiredQuality of Service(QoS)levelsin a very bandwidth

efficientmanner, theTR34.1considersaDemandAssignedMultiple Access(DAMA) satel-

lite network asanidealplatformto respondto expectedvariablebandwidthdemands.One

suchnetwork is theCOMSAT Linkway 2000wide areanetworking andswitchingsystem

thatprovidesaccessandtransportfor packet switchedservices(ATM, IP/LAN, andFrame

relay),andcircuit switchedservices(ISDN andSignalingsystemNo. 7) [30]. It provides

a full-meshmulti-servicesatellitenetwork with a multi-frequency Time Division Multiple

Access(TDMA) [31] satelliteair-interfaceandunifiedframe,cell, andcircuit modetrans-

port services.

ATM packet sizeof 48 byteswasprimarily chosento specificallycarry voice packets

rangingbetween32 and64 kb/swith requiredquality of service.However, with the inva-

sion of multimediaservices,especially, video with variablelengthpackets,ATM packets

will not necessarilyremainthemostefficient transportsystemfrom theutilization point of

view. Sincehigherswitchingspeedsarefeasibledueto fasterprocessingcapabilities,sys-

temswith ratherlongeryetstill fixedsizepacketswill achievetherequiredqualityof service

with minimumoverhead.Moreover, ATM is currentlyevenbeingchallengedby non-fixed

packetssystemssuchasintegratedanddifferentialservicesof theInternetEngineeringTask

Force(IETF). As a result,wefind thatnewly deployedsystemsshouldbereluctantto com-

mit to aspecifictechnology. To succeedin achieving anopensystem,thedesignof aflexible

systemwith anindependentprotocol(SAT-MAC) thatallows convergencefor ATM or any

otherupperlayerprotocolwheneverneededis preferred.

28

2.3.3 SatelliteChallenges

While satellitenetworksindeedpromiseto becomewidely deployedasaccessnetworksfor

interactive residentialbroadbandservices,yet a lot of openissuesmuststill be addressed

beforesatelliteachievestheanticipatedsuccess.Oneof themostcommonresearchissues

focusesondevelopingextensionsto TCPin orderto overcometheslow startproblemunder

the long delayencounteredby satellitesystems.Antennatechnologyis alsoexperiencing

continuousdevelopmentsin orderto producerobustcost-effective terminals.Moreover, in

orderto respondto theeffectsof interferenceandfadingof thesatellitemedium,powerful

FECcodingalgorithmsaredesirable.Otheropenissuesincludeon-boardprocessing,beam

switching,protocolsuites,etc.

One of the most importantstudiesinvolves developmentof an efficient air-interface

MAC layerin termsof utilizationaswell ascomplexity. It shouldbenotedthatmostsatellite

studiesconductedin this domainup to this point usually involved the downstream(from

providersto customers)andhow to encodedataover the broadcastchannel.This issueis

nearlysettledwith advancementsin MPEG-2technologies.However, with the increasing

users’demandsfor interactive services,the challengeswitchestowardsthe capabilityof

multiplexing the maximumnumberof usersefficiently over smallerbandwidthswithout

affectingtheQualityof Service(QoS).Thetraffic patternoverdownstreamsatellitechannels

canalwaysbeassumedto achievearatherhighutilizationasaresultof consistentbroadcast

transmissionof datafrom a centralbasestationto variouscustomers.On the otherhand,

upstreamtransmissionsrepresenta distributed system. With customers’behavior being

lessconsistentas well, the burdenon the return link capabilitiesincreasesand requires

standardization.This triggeredthe necessityof developingan efficient MAC layer at the

air-interfaceterminalin theupstreamdirectionin orderto respondto thechallenge.

29

In this thesis,we proposea MAC architecturethat aimsto answerthe above require-

ments.Thescopeof thedevelopedMAC emphasizesonintroducinganew accesstechnique

with minimumdelayandhigh utilizationaswell asa novel distributedschedulingarchitec-

ture to minimize systemcomplexity. The integral functionality of both constituentswill

employ a DynamicCapacityallocation(DCA) approachto dynamicallyassignresources.

Completesystemdescriptionwill be presentedin the next chapter. In the following sub-

sections,we will presenta brief survey on both the MAC accesstechniquesanddifferent

schedulingstructuresin satellitesystems.Weshouldalsopoint out thatotherissuesrelated

to the MAC layer suchasschedulingalgorithms,admissioncontrol anduserparameters

controlareoutof thescopeof this thesis.

2.4 MAC AccessTechniques

MAC protocolsaredesignedto enablecommunicatingstationsat diverselocationsto reg-

ulatetransmissionsof their packetsandmanagenetwork bandwidthin orderto utilize the

network resourcesasefficiently aspossible.If theindividualtraffic loadperconnectionwas

high and the interconnectionpatternswerestatic,a fixed capacityallocationwould have

beenapplicable.Unfortunately, traffic in residentialserviceapplicationssupportingmulti-

mediais ratherburstyandusers’behavior is non-consistent.Hence,capacityallocationhas

to bemoredynamicto copewith thereal-timetraffic demandssothatsatelliteresourcesmay

efficiently besharedby a largepopulationof earth-terminals.Many accesstechniqueshave

contendedto leadthatrole [32], [33]. In this section,wedescribesomeof thesetechniques

anddiscusstheir suitability in satelliteaccessnetworks.

30

2.4.1 Fixed Assignment

In the fixed-assignmentmultiple accesstechnique,the allocationof channelbandwidthto

a station is a static assignmentand independentof other stations’activities (i.e., circuit

switching).Examplesof fixedassignmenttechniquesareFrequency, Timeor CodeDivision

Multiple Access(FDMA, TDMA or CDMA) [31]. Fixed assignmenthasthe advantage

of beingsimpleandwith minimum delay. However, with bursty traffic, fixed assignment

resultsin a hugewastein bandwidthresourcesrenderingvery low utilization. Moreover,

with channelsbeingcompletelydevotedto singleusersover extendedtime periods,fixed

assignmentallowslimited numberof usersthatcansimultaneouslyaccessthemediumwith

therequiredqualityof service.

2.4.2 RandomAccess

Randomaccessallows terminalsto instantlytransmittheir datapacketsassoonasthey ar-

rive, independentof otherstations. As it representsa distributedsystem,datapacketsof

variousterminalsaredestroyed from time to time dueto possiblecollisions. Examplesof

randomaccesstechniquesincludepureALOHA andslottedALOHA [29] proposedfor ear-

lier stagesof satellitecommunications.Randomaccesshastheadvantageof beingsimple.

It alsorequiresnosetupphase,whichotherschemes,suchas,reservationmightuse.At low

load,retransmission(dueto possiblecollisions)is negligible, whichoffersterminalsinstant

channelaccess.However, with increasingloads,collisionratesgrow exponentiallyandcon-

sequentpacketretransmissionbecomesnon-acceptable,especially, in real-timeapplications

andqualityof servicecannotbeguaranteed.It is worthwhileto mentionthatthemostinno-

vative randomaccessschemescanonly provide anupperboundof achievableutility in the

regionof 0.4-0.5.

31

2.4.3 DemandAssignment

Demandassignmentallows dynamicallocationof capacityon demandin responseto sta-

tion requests.Thealgorithmreservesbandwidthbasedon terminaldemandsandtherefore

rendershigh utility. Thereservationschemeitself might befixedor random.Examplesof

demandassignmenttechniquesincludePriority OrientedDemandAssignment(PODA) and

FIFO OrientedDemandAssignment(FODA) [32]. Demandassignmentis bestsuitedfor

jitter-tolerantqueueabletraffic. In spiteof its high utility, demandassignmentin satellite

environmentexperiencesinevitable setup (reservation) delaysof two roundtrips (around

500ms)addedto anotherroundtrip delayof datatransmissionfor a totalof threeroundtrip

delays.This is ratherunacceptablefor interactiveasymmetricreal-timeapplications,where

userscontinuouslyproduceshortcontrol commands,especially, given that randomaccess

canavoid suchdelaysunderlow loads.

2.4.4 CombinedTechniques

Multimedia traffic containsboth real-time(e.g.,voice andvideo) and jitter-tolerant(e.g.,

dataandgraphics)andrequiresa combinedtechniqueto be efficiently transported.Com-

bined techniquesaim to provide fast accessfor low load conditionsandshortmessages,

suchas, inquiry messagesin interactive datasessions,and to attain high utility of satel-

lite resourcesat high traffic load. In the following sub-sections,we provide examplesfor

combinedtechniques.

2.4.4.1 CombinedRandom/Reservation

A random/reservationschemeachieveslow delayatlow loadswith highutility of reservation

schemes.Bandwidthmaybecategorizedinto reservedandunreservedslots. Jitter-tolerant

32

traffic maybeconveyedover both typesof slots. Real-timetraffic on theotherhand,only

usesreserved slots. Sincesatelliteprovidersusespotbeamsto achieve channelreuse,the

scheduleratthemastercontrolis requiredtomonitorunreservedchannelsandreliablydetect

collisionsasterminalswill not beableto detectcollisionswith simultaneoustransmissions

on otherbeams.This will leadto very longcollision resolutionperiods;i.e.,a terminalwill

not be able to detecta collision beforetwo roundtripdelaysand is forcedto remainidle

within that period. In caseof a singlebeam,terminalscould be able to detectcollisions

instantaneouslyandthereforemightattemptretransmissionin thenext unreservedslot.

2.4.4.2 CombinedFree/DemandAssignmentMultiple Access

Contentionfreeprotocolscancontrol theworst delayin caseof real time applications.In

combineddemandassignmentwith fixed assignmentor with free assignment,unreserved

slotsareeitherfixedor freely assigned.TheCombinedFreeDemandAssignmentMultiple

Access(CFDAMA) providesbetterperformancedueto its dynamicfeature.By combining

freeassignmentwith demandassignment,CFDAMA protocoloffersmuchshorterdelaysat

low andmediumtraffic loads,while maintainingthe high channelutility of DAMA tech-

niques. Reservation in theseschemesmay be pre-assignedrequestslots,piggybackingor

any otherspecialalgorithm.A studyin [33], [34] showssuperiorityof CFDAMA overother

accesstechniquesin satelliteenvironments.CFDAMA mightbeevenenhancedby allowing

terminalsto reserveaheadof timebasedonaprediction.If thepredictionalgorithmhasac-

ceptableaccuracy, thenew enhancedCFDAMA will evenprovide betterperformancewith

lowerdelays.Detailson thatschemeandthedevisedprotocolbehavior arepresentedin the

next chapter.

33

2.5 SchedulingStructur es

Schedulingis mandatoryto complementanefficient accesstechniqueto achieve high per-

formance.It determineshow themastercontrolstationdistributesavailablecapacityondif-

ferentterminalsbasedon a specialalgorithm. Thesimplestschedulingalgorithmsarefirst

comefirst servedandroundrobin. However, schedulingalgorithmsareout of thescopeof

this thesis.In DynamicCapacityAllocation (DCA), theschedulermustdynamicallyassign

slotsto variousterminalseachtime frame(24 ms) . Hence,it requiresa lot of computing

anddataprocessing.Accordingly, schedulingstructureswith lesscomplexity arealways

desirable.Thissectiondescribestwo possibleschedulingstructures.

2.5.1 Centralized Scheduling

In centralizedscheduling,the schedulersuperviseseachandevery connectionwithin the

network. As a resultof serviceintegration,eachterminalis expectedto continuouslypro-

ducenumeroussimultaneousconnections.Thiswill overloadtheschedulerwith long look-

up tablesandwill leadto longprocessingdelays.

2.5.2 Centralized/Distributed Scheduling

As mentionedabove, the scheduleris obliged to control a large numberof connections

over a hugepopulation. In order to reducethe complexity, schedulersmay be structured

into two levelsof scheduling;amacrolevel responsiblefor handlingtheaggregaterequests

of eachterminalanda micro level, whereeachterminaldistributesgrantedcapacityover

local connections.While the mastercontrol stationperformsmacrolevel scheduling,it

is the terminalsthemselves that handlemicro scheduling. As the mastercontrol station

34

then dealsonly with total terminal demandsand not single connections,the scheduling

processingduty becomesdistributedrenderinglower complexity andminimumprocessing

delays.However, theoverall bandwidthcontrolstill residesat themastercontrolstationin

a centralizedmanner. Full detailson thatschedulingstructurewill bediscussedin thenext

chapter.

2.6 Summary

In thischapter, wehave indicatedtheimportanceof broadbandaccessin futuretelecommu-

nicationssystems.Accordingly, we highlightedthestructure,objectivesandachievements

of DAVIC asoneof themostactive standardsbodiesin thefield. We have alsodiscussed

variouscompetingtechnologiescontendingfor theaccessnetwork role andhencepointed

out the essenceof developmentof efficient satellitenetworks to achieve simpleandcost-

effectiveglobalcoverage.Consequently, wepresentedtheboundingfactorsthatlimit direct

successfuldeploymentof broadbandinteractivesatelliteaccessandfocusedour intereston

developmentof an efficient MAC layer at the air-interfaceterminalsto achieve maximum

utilizationandminimumdelay.

In thenext chapter, we will discussthroughlya proposedMAC architectureto address

theaboverequirements.Weshow systemstructures,behavior sequences,accesstechniques

andschedulingarchitectures.

35

Chapter 3

BroadbandSatelliteAccess

Broadbandsatelliteaccessis anaccesssystemwith a satellitesegmentrepresentingits ac-

cessnetwork. It addressesthesamemarketsandservicesasotheraccesstechnologiessuch

as, wireless,copper, cable,etc. As indicatedin Chapter2, most servicesdeployed over

currentconventionalsatellitesystemsareonly one-way andusually rely on a phoneline

for the returnpath (upstream)to provide interactive features. Any failuresin the public

network will directly terminateany communicationpossibility. In broadbandsatelliteac-

cess,two-way interactivemultimediaservicesaresupportedoversharedbandwidth.In this

chapter, we describea proposedbroadbandsatelliteaccesssystem.We show its configura-

tion structure,constituentsandproposedprotocolstacks.We emphasizeon theMAC layer

specificationdueto its critical role in systemimplementation.Themainscopeof thedevel-

opedMAC includesdynamiccapacityallocationundera novel accesstechniquebasedon

an enhancedCFDAMA andsimpleschedulingstructuresaswell asthe protocolbehavior

specification.

36

BackboneInternet

Subscriber

VoD Service Provider

Subscriber

Satellite AccessNetwork

MCS

Figure3.1: GeneralConfigurationof aBroadbandSatelliteAccessSystem

3.1 BroadbandSatelliteAccessConfiguration

Figure 3.1 shows the generalconfigurationof a broadbandsatelliteaccesssystem. The

figure indicatesthat via the satellite,subscribersmay accessa VoD serviceor an Internet

backboneforming two accessdomains.Within eachaccessdomain,aserviceprovider ren-

dersits applicationservicesto a groupof subscribers.A MasterControl Station(MCS)

supervisedby thesatelliteprovider is necessaryin orderto regulatemediumaccessamong

subscriberswithin thesamedomainaswell asacrossdifferentdomains.This is oneof the

maindifferencesthatdistinguishsatelliteaccessfrom othertechnologies.In thesetechnolo-

gies, the serviceandnetwork providersarecommonlysupervisedandusuallyphysically

locatedat thesamestation.On theotherhandin satellitesystems,theserviceprovider and

the network provider (satellite)are separate.Propercoordinationandsignalingbetween

bothprovidersis thereforeessentialto guaranteesuccessfulaccess.Thiscanbeachievedby

implementinga carefullydesignedMAC layer. TheMAC layerarchitecturemustbecon-

figuredto dynamicallymultiplex themaximumnumberof terminalswith highestpossible

utilizationandoptimumuseof satelliteresourcesover thesatellitenetwork.

In general,satellitecommunicationsis onetypeof BWA (BroadbandWirelessAccess).

37

Downstream Direction

Upstream Direction

STS

STS

MCS

MCS-ULUS-DL

DS-UL

US-DL

MCS-DL

DS-DL

US-UL

MCS-DL

BTS

Figure3.2: BSA SingleDomainBlock Model

However, the IEEE BWA groupexcludessatellitefrom their specifications.Our proposed

systemis part of a generalBroadbandSatelliteAccess(BSA). However, from this point

andthroughouttherestof this thesis,we will usethetermBSA to specificallyrefer to our

proposedbroadbandsatelliteaccesssystem.

3.2 BSA System

Figure3.2 depictsthe proposedblock modelfor the BSA system.The systemconsistsof

threemainstations:

� BaseTransceiver Station(BTS): It representsthegateway to thebackbonenetwork.

Subscribers’informationandregistrationfiles to bedownloadedduringSTSinitial-

izationarelocatedthere.

� SubscriberTransceiverStation(STS):It is connectedto theuserpremisesequipment.

38

It representsthe air-interfaceterminal. The interfaceto the userequipmentis stan-

dardizedto satisfyrequirementsof anopensystem.

� MasterControlStation(MCS):Regulatestheaccessto thesatellitemedium.

It is worthwhile to mentionthat we adoptsimilar terminology, asBWA, IEEE 802.16for

BTSandSTS,howeverwemustagainhighlight thattheMCSis uniqueto satellitesystems.

Theconfigurationshown is basedonaPoint-to-Multi-Pointstructureto form astartopology

for only oneaccessdomain.Multiple domainshowever, maycoexist in theaccessnetwork.

Upstreamdirectioninvolvesdatatransmissionsfrom STSto theBTS.Downstreamdirection

involvesdatatransmissionsfrom theBTS to STS.Thesatellitecanhave non-regenerative

transpondersandactsasa bentpipe whereup-link transmittedsignalsareamplified, re-

transmittedandswitchedat RF onto thecorrespondingdown-link beams.As indicatedin

thefigure, traffic andcontrol informationmaybe transmittedover threedifferentpossible

paths:

1. STScontrolandtraffic informationtransmittedover theUS-UL (UpstreamUp-link)

areamplifiedandregeneratedby thesatelliteover theUS-DL (UpstreamDown-link)

wherethey canbereceivedby theBTSandMCS.

2. BTStraffic informationtransmittedovertheDS-UL (DownstreamUp-Link) is ampli-

fied,regeneratedandbroadcastby thesatelliteoverDS-DL (DownstreamDown-link)

to bereceivedby differentSTS

3. MCS control informationtransmittedover theMCS-UL (MCS Up-link) is amplified

and regeneratedby the satelliteover MCS-DL (MCS Down-link), whereit canbe

receivedby theBTS andSTS.

39

SAT MAC

IP

PHY

LLC

SNMP

TCP/UDP

PHY

DLC

Application

TCP/UDP

RSVPIP

PHY

DLC

Application

TCP/UDP

RSVP IP

Convergence

PHY

DLC

RSVP IP

PMD

LLC

TFTP

UDP

RSVPIP

SAT MAC

DHCPSNMP

Convergence

PHY

DLC

RSVPIP

PMD

LLC

SNMP

UDP

RSVP IP

SAT MAC

DHCPTFTP

CPESTSBTSProvider

SNMP: Simple Network Management Protocol.DHCP: Dynamic Host Configuration Protocol.TFTP: Trivial File Transfer Protocol.RSVP: The Reservation Protocol.DLC: Data Link Protocol.LLC: Logical Link Control.CPE: Customer Premises Equipment.

MCS

Figure3.3: OverallBSA StackStructure

3.3 BSA ProtocolStacks

Figure3.3depictstheoverallstackstructurefor aBSA system.Themainfocusof ourstudy

involvesthe interfacesto the satellitemediumat the air-interfaceterminal. Designof in-

terfacestowardstheserviceprovideranduserequipmentshouldusestandardizedprotocols

andareassumedto satisfytherequirementsof anopensystem.An opensystemfacilitates

massproductionof customerequipmentindependentof theaccesstechnologyused(cable,

wireless,satellite,etc.).As shown in thefigure,theBTSandSTSmayoperateasforwarding

agents(bridging or at the network level asin Figure3.3) andalsoasend-systems(hosts).

As hosts,the applicationlayer supportsa numberof protocols. SNMP (SimpleNetwork

andManagementProtocol)is responsiblefor network management.DHCP(DynamicHost

ConfigurationProtocol)andTFTP(Trivial File TransferProtocol)areusedduringSTSini-

tialization procedures.Figure3.4 shows a threedimensionalmodelfor the BTS andSTS

actingasforwardersat thenetwork layer. Thestackis dividedinto two planes.In thedata

plane,thefollowing layersmaybedefined:

� Upperlayers:TheproposedMAC is flexible andmayindependentlysupportInternet

40

Data Plane

Control Plane

RSVP

LLC

MAC TCDAV TC

DigitalAudio/Video(BTSonly)

Upper Layer

Physical Layer

DataInterface

ServiceInterface

Interface

MAC Mang.

Convergence

BSA MAC

Figure3.4: 3-DimensionalBSA StackModel with BTS andSTSactingasForwardersattheNetwork Layer

protocolor ATM services.We assumethatreal-timetraffic over theselayersmayby-

passtheLLC anddirectlyaccesstheMAC assupportingservicesmayacceptslightly

erroneouspacketsratherthanafford delaysresultingfrom retransmissions.

� ConvergenceLayer: It encapsulatesProtocolData Units (PDU) framing of upper

layersinto thenativeBSAMAC/PHYPDUandtranslatesupperlayerQoSparameters

into BSA MAC constructs.It alsomapsupperlayer’s addressesinto corresponding

BSA addresses.

� BSA MAC Layer: This is the main focusof our study. It guaranteesefficient data

transmissionoverthesatellitemedium.DetailsontheMAC layerarepresentedin the

next sections.

� SegmentationandRe-assembly:Thisprocessat theMAC TransmissionConvergence

(TC) sub-layeris responsiblefor segmentingvariablelengthMAC framesinto equal

lengthpacketsto be reassembledonceagainat theotherside. It is essentialin MF-

TDMA ascapacityallocationsby theschedulerat theMCSmaybedistributedovera

41

rangeof carrierfrequencies.Transmissionof MAC framesover fixedsizepacketsis

essentialto satisfyQoS[35].

Notethat in theBTS only, broadcastMPEG-2encodeddigital audio/videomaybypassthe

MAC protocollayeranddirectly accessthephysicallayer in thedownstreamasit usually

conveys distribution traffic anddoesnot needtheMAC accessfunctionalityusedwith up-

streamtransmissions1. In the control plane,the generalstructurecanbe summarizedinto

thefollowing layers:

� A Control Protocol: As an illustrative example,we assumethe IETF Reservation

ProtocolRSVP[36]. It is thecontrolprotocolusedfor resourcesreservation in IPv6

(IP thenext generation)[37]. In RSVP, datais forwardedto thedestinationacrossthe

pathdeterminedduringtheresourcesreservationphase.

� InterfaceLayer: It translatesRSVPcommandsinto local messagesthatwill beused

for the resourcereservation over the satellitelink. It containsthe dynamicservices

processaswell asadmissionandpolicy functionalities.

� MAC ManagementLayer: It is responsiblefor overallMAC layermanagement.Typ-

ical functionsincludesystemmanagementandSTSregistration.

� LLC Packaging:We encapsulateMAC managementmessagesin LLC packagingin

consistency with theDOCSISspecifications[9], [10].

1MPEG-2transmissionsover theupstreammustutilize theMAC accessfunctionality. All upstreamcon-nectionsdefinepoint-to-pointlinks betweenanSTSandthecorrespondingBTS.

42

3.4 BSA MAC Layer

As indicatedearlier, akey elementto thesuccessfuldeploymentof aBSA systemis thede-

velopmentof a suitableMAC protocol/techniqueto efficiently sharethesatelliteresources.

TheMAC Layershouldaddressthefollowing challenges:

1. Satellitemediumlongdelayandits resourceconstraint.

2. ExpectedburstyInternetandmultimediatraffic of envisionedservices.

3. QoSrequirementsfor real-timeandnon-real-timedataexpectedto be conveyed by

theBSA system.

Capacityof thedownstreamlinks maybeassumedto befixed,astheaggregatebroadcast

transmissionis consideredto have high loadandis rathersmooth(i.e., thepeak-to-average

ratio is closeto unity). Transmissionover theupstreamandMCS links is burstywith high

peak-to-averageratio and necessitatesan efficient MAC. The MAC must satisfy certain

delayboundsandattainmaximumutilizationatthesametime. As anillustrativeexample,in

thefollowing discussionswefocusononeaccessdomain.Weassumethatin aBSA system,

theSTScanonly communicatewith eachothervia theBTS. We shouldalsomentionthat

weconsiderInternet-basedupperlayersspecifically, IPv6andRSVPfor integratedservices.

However, wemustagainemphasizethatourdevelopedarchitectureis completelyconsistent

with otherupperlayersdependentondifferentialservices.

3.4.1 BSA MAC Ar chitecture

We have chosentheDOCSIS1.1RF-interface[10] specificationasour building block due

its popularity in addressingthe samerangeof servicesanticipatedover satellitenetworks

43

andwe alsoadopttheir messageterminology. However, ascableandsatellitemediarender

diversecharacteristicsaswell asdifferentconfigurations(with satellitesystemsemploying a

MCS),largemodificationsto theDOCSIS1.1infrastructureshouldbeintroducedto achieve

optimumperformanceover satellite.In thefollowing sub-sections,we presentvariousfea-

turesof our BSA MAC layer. We alsohighlight aswell asjustify thedifferenceswith the

DOCSIS1.1specification.

3.4.1.1 BSA Medium AccessScheme

Terminalsare assumedto deploy a Multiple Frequency Time Division Multiple Access

(MF-TDMA) [33] accessschemewidely usedin satellitecommunications.It offers sim-

ple connectivity, greatflexibility andcompatibility with digital transmission.Superiority

of MF-TDMA over FDMA/TDMA employedby DOCSISis attributedto a bettertrunking

efficiency dueto a largerpoolof availablephysicalchannels.In MF-TDMA, thebandwidth

is first dividedinto a numberof frequency bands.A streamof time slotsis recognizedover

eachfrequency band. A channelin MF-TDMA is definedby a time-frequency slot in a

two-dimensionalframeasshown in Figure3.5. All STSmayaccessthesetime-frequency

slotsin a sharedmanner. Capacityallocationin MF-TDMA canberepresentedby a time-

frequency map indicating the time-frequency slots assignedto eachterminal. Note also

asshown in the figure, that mini-slots in the time-framemapmight vary in the time-slot

sizesor the transmissionbandwidth. Sinceeachtime-slotmusthave a segmentationand

re-assembly(SAR) header, variationin slot sizessupportsthecapabilityof decreasingthe

resultingoverheadwhensmallerpoolsof physicalchannelsareallowed underlow loads.

Variationin transmissionbandwidthon theotherhand,givestheBSA systemtheflexibility

of supportingterminalswith variousmodemtransmissionratecapabilities.

44

time (t)

STS-9STS-7

STS-7STS-4Carrier f3

Carrier f4

Carrier f2STS-2 STS-1

Carrier f1STS-1 STS-3 STS-2

Carrier f6f6

STS-8

Carrier f5 STS-6 STS-10

Band-1

Band-2

Band-3

Figure3.5: MF-TDMA Structurein BSA

3.4.1.2 Dynamic Capacity Allocation

Traffic in residentialapplicationssupportingmultimediaserviceshasdifferent typeswith

diversecharacteristicsandis ratherburstyin nature.Hence,thecapacityallocationscheme

hasto becarefullychosento attainbetterperformance.Terminalswith real-timetraffic re-

questsatelliteresourcesinfrequentlybut with relatively highcapacityandastringenttiming

requirement.Accordingly, in BSA, we employ DynamicCapacityAllocation (DCA) for

assigningdatatraffic. In DCA, satelliteresourcesaredynamicallyallocatedto registered

terminalsdependingon therequest,scheduler’s status,priority, etc. DynamicCapacityAl-

location(DCA) will thereforegiveenoughflexibility to guaranteethatsatelliteresourcescan

beefficiently sharedby thelargestpossiblepopulationunderminimumpossiblebandwidth

requirements.SuccessfulDCA necessitatesthefollowing two complementaryfunctions:

45

AccessTechnique Randomaccesstechniquesbasedon ALOHA [29] wereinitially pro-

posedfor satellitesystems.ALOHA is contentionbasedwith low utility andenvisionedser-

viceswill not toleratedelaysaccompaniedwith retransmissionsdueto collisions. In BSA,

we proposeto usea CFDAMA-basedMAC accesstechniquefor dynamiccapacityalloca-

tion. CFDAMA [33] is a hybrid accesstechniquewhereun-requestedbandwidthis freely

allocatedto usersaccordingto a pre-determinedalgorithm. DOCSIS1.1 employs a hy-

brid reservation/contentionbasedaccesstechnique.As discussedin Section2.4,contention

degradesutilization anddelayperformancein satellitesystems.We have also discussed

CFDAMA superiorityin satellitenetworksover traditionaltechniquesin thatsection.

Furthermore,in BSA we proposean enhancedCFDAMA accesstechnique.By com-

bining predictionwith CFDAMA, betteraccessdelayperformancemay be achieved. In

thatscheme,STSutilize a predictionfunction in their demands.STSrequeststo theMCS

will not reflecttheir instantaneousneeds.Rather, anticipatedbandwidthaftertwo roundtrip

delayswill bedemanded.Moreover, freeassignmentby theMCScanbebasedonapredic-

tion of terminals’demandsandactivity. While thefirst approachpromisesfasteraccessfor

demandassignmenttraffic, thesecondapproachwould promotebetterutilization of freely

assignedtraffic. It is worthwhileto mentionthatwe only allow contentionin thebeginning

initialization proceduresasuserssignalto theMCSwhenturningon their terminals.

MCS Scheduling MCS areobligedto controla largenumberof connectionsovera huge

population. In order to reducethe MCS complexity, we have structuredtwo levels of

schedulingin our system;a macrolevel responsiblefor handlingthe aggregaterequests

of eachSTSterminalanda micro level whereeachterminaldistributesgrantedrequests

over local connections.While macrolevel schedulingis doneat the MCS, the terminals

themselves(STS)handlemicroscheduling.As theMCSdealsnow only with total terminal

46

demandsandnot singleconnections,the schedulingprocessingduty becomesdistributed.

Distributedschedulingachievesan MCS with lower complexity andminimum processing

delays.However, overallbandwidthcontrolstill residesat theMCSin acentralizedmanner.

To furtherdecreasevariablesprocessedby theMCS, connectionsbetweenanSTSandthe

MCS on theupstreamup-link maysimply bedefinedby only a limited numberof standard

servicecategories(real-timeandnon-real-timefor example)with minimumparametersper

category (requiredaveragebandwidthfor example).This meansthatthemaximumnumber

of logical connectionswith an STSthat the MCS hasto handleis alwaysboundedby the

numberof categoriesableto supportall upperlayerconnectionswithin thatSTS.This re-

lievestheMCSfrom theburdenof managingnumerousconnectionsperSTSwith minimum

informationto beprocessed.

3.4.1.3 Dynamic Services

In BSA, we adopttheDOCSIS1.1 conceptof dynamicservicesto supportQoS.QoSde-

finesparametersasdelay, throughput,jitter, etc. thatshouldbesustainedduringthelifetime

of a connection.Dynamicservicesallow terminalsto dynamicallyadd,modify or delete

connectionsbasedon upperlayerrequests.Eachconnectioncorrespondsto a singlediffer-

entiatedservicecategory. Basedon their QoSparameters,upperlayerconnectionswill be

categorizedinto localservicecategorieswith specificequivalentparametersvia a translator.

Requestswith theresultingparametersandconsequentnegotiationswill thenbeconveyed

throughdynamicserviceprocedures.Servicecategoriesarealsoproposedby DOCSIS1.1,

however, they arenot yet well-standardized.ThecurrentDOCSIS1.1 specificationrather

reliesonsingleconnectionsbetweenthesubscribersandtheequivalentMCS.

47

3.4.2 MAC Protocol

TheMAC protocolspecifiestheMAC layerbehavior anddefinesmessagesequencesacross

thesystem.A MAC Framedefinestheunit of informationexchangedbetweenMACentities.

MAC framesmaybeoneof thefollowing types:

� DataFrames:DataframesmaycarryanIP packetor avariablenumberof ATM cells.

� MAC SpecificFrames:SpecificframesmaybeRequestframesusedfor reservation

requestsor MAC managementframes,whichcarrytheMAC managementmessages.

MAC managementplaysa key role in theMAC protocoloperationasit controlsandman-

agesall theprotocolmainfunctions.Managementmessagesareencapsulatedin LLC infor-

mationframes,which in turnareencapsulatedwithin MAC framingin consistency with the

DOCSISspecifications.

3.4.2.1 MAC ManagementMessages

Thedifferentmanagementmessageschosencoincidewith DOCSIS1.1terminologyandare

describedasfollows:

� SYNC (Synchronization):A SYNCmessageis transmittedperiodicallyby theMCS.

STSuseit to establishMAC sub-layertiming. TheMCS broadcastsinsidethis mes-

sagethecurrentstateat transmissionof a clock insidetheMCS to offer timing refer-

ence.

� UCD (Up-link ChannelDescriptor):A UCD messageis transmittedperiodicallyby

the MCS. It definesthe characteristicsof possibleSTSup-link channelbandswith

similar characteristics.Typical descriptorswould includemini-slot size,symbolrate,

modulationtypeandotherphysicalparameters.

48

� MAP (Allocation Map): A MAP messageis regularly sentby the MCS in orderto

organizeandregulatebandwidthallocationsonupstreamup-links.Basedonschedul-

ing decisions,it describestime asa variablenumberof variable-lengthtransmitop-

portunitiesover a bandof frequencies.Opportunitiesinclude request,data, initial

maintenance(ranging),stationmaintenance(ranging),andregistration.

� RNG_REQ(RangingRequest):RNG_REQmessagesaretransmittedby an STSat

initialization andperiodicallyon requestfrom MCS to determinenetwork delayand

requestfor time,powerandfrequency adjustmentsoveracertainbandof frequencies

describedby acertainUCD.

� RNG_RSP(RangingResponse):A RNG_RSPmessagemustbe transmittedby the

MCS in responseto a received RNG_REQ.Typical carriedparameterswill include

timing andpoweradjustmentaswell asfrequency fine tuning.

� REG_REQ(RegistrationRequest):A REG_REQmessageis transmittedby anSTS

duringtheinitializationphase.It carriesSTSconfigurationparametersasdownloaded

from theBTS.Theseparametersareusedby theMCS in registeringtheSTSwith the

satellitenetwork.

� REG_RSP(RegistrationResponse):A REG_RSPmustbetransmittedby theMCSin

responseto REG_REQ.TheMCSmaymodify someof theREG_REQparametersin

theresponse.

� REG_ACK (RegistrationAcknowledgment):A REG_ACK messageis transmitted

by theSTSin responseto a REG_RSPfrom theMCS. It confirmsthereceptionand

acceptanceof theSTSto theQoSandotherregistrationparametersasreportedby the

MCS in theREG_RSP.

49

Power_On

STS

SYNC

RNG_RSP

INI_RNG_REQ

MAP

UCD

SYNC

SYNC

MAP

SYNC

DHCP_REQ (data)

DHCP_RSP (data)

TFTP_REQ (data)

TFTP_REQ (data)

REG_RSP

REG_REQ

REG_ACK

BTS MCS

Figure3.6: An Examplefor STSInitialization

3.4.2.2 Illustrati veExample for Protocol Operation

To illustratethefunctionalityof theMAC management,we presentthe following example

shown in Figure3.6to describeanSTSinitialization procedure:

As soonasan STSpowerson, it scansfor an appropriatedown-link channelandwill

acquiresynchronizationwith theMCS by receiving two consecutive SYNC messages.On

thesamedown-link channel,theSTSmustthenreceiveanappropriateUCD transmittedby

theMCS. An appropriateUCD definesa bandof channelswith similar physicalattributes

thatcoincidewith theSTScapabilities.Following that,a rangingprocessis thennecessary

for theSTSto performtiming, power andfrequency adjustments.TheSTSshouldusethe

first initial rangingopportunityover the first received MAP message.A randomback-off

algorithmis usedto resolvecollisions2. OncetheINI_RNG_REQ(Initial RangingRequest)2Note that contention-basedallocationsareallowed only whenterminalslog on to the satellitenetwork.

TheMCS cannotrecognizeinactive terminalsandallows themto join thenetwork over regularly transmittedinitial maintenanceopportunities.Probabilityof collisions is low becauseusersare lesslikely to log on atexactly thesamemoment.

50

transmissionis successfultheSTSis assignedatemporaryID in theRNG_RSPandany fur-

therranginginstructionswill berecognizedasopportunitiesassignedto thetemporaryID or

thestationID acquiredafterregistration.To obtainits IP addressaswell astheIP addressof

theconfigurationfile from theBTS(serviceprovider), theSTSmustestablishIP connectiv-

ity via DHCP. Usingthis informationtheSTScandownloadtherequiredparameterfile via

TFTP. DHCPandTFTPcommandsarecarriedasdataframesusingMCSallocationsto the

temporaryID. In caseany of thesetwo processesis delayedor non-successfulboththeMCS

andSTStimeoutandthetemporaryID will belost. Re-initializationof theSTSis required

in thatcase.SuccessfulDHCPandTFTPrendertheSTSimplicit registrationto theserver

network. STSmustalsoperforma registrationprocedurewith the MCS to gainaccessto

thesatellitenetwork. Theregistrationshouldbebasedontheparametersof thedownloaded

file, which definesSTSmodemcapabilities,allowedbandwidth,thecorrespondingaccess

domain,policy data,etc. Uponregistrationsuccess,STSis givena stationID identifiedby

theBTSandis givenauthorityto exchangedatatraffic with theserviceprovider.

It is worthmentioningthattheMCSalwaysmonitorsregisteredSTSdatatransmissions

andtakesnecessaryproceedings(stationmaintenance)to maintainacceptableSTSranging

parameters.

3.4.3 Bandwidth Allocation and Scheduling

Allocationsandtransmissionopportunitiesoverfuturetimeframesrelyondynamiccapacity

allocation(DCA) assumingtheenhancedCFDAMA techniqueandaredefinedby theMAP

managementmessagesregularlytransmittedby theMCS.In thissub-section,wepresentthe

differentallocationopportunitiesin BSA andshow theSTSandMCS block structuresfor

DCA.

51

3.4.3.1 TransmissionOpportunities

A MAP messageconveying allocationdescriptionis constitutedof a groupof consecutive

MAP elements.A MAP elementis composedof a MAP opportunity, addressandallocated

slots.MAP opportunitiesarecategorizedasfollows:

� Initial Maintenance:This definesa contentioninterval wherenew stationsmay join

thesatellitenetwork throughinitial ranging.

� StationMaintenance:Thisdefinesintervalsassignedby theMCSfor STSto perform

routinestationrangingto maintaintiming, powerandfrequency settings.

� Request:In this interval the MCS providesopportunitiesfor STSto reporttheir in-

stantaneousbandwidthdemands.

� Datagrants:TheMCS in this interval allocatesgrantsto STSfor datatransmission.

� Registration:ThisdefinesintervalswhereSTSmaytransmittheirregistrationrequests

andacknowledgments.

� NULL: This is usedto indicatetheendof a time frame’s allocations.

3.4.3.2 STSStructure for Dynamic Capacity Allocation

Figure3.7depictsablockstructurethathighlightstherequiredcomponentsfor theproposed

dynamiccapacityallocationandschedulingin theSTS.It alsospecifieswherethesecompo-

nentsfit within theoverallMAC layerandhow will they interactwith theMAC management

aswell aswith upperlayers.Thefiguredefinesthefollowing blocks:

� UpperLayerControl: We assumeRSVP. Requestsfor new connectionsareinitiated

thereandforwardedtowardsthedynamicserviceprocess.

52

Data and requests

Translator

Category Requests

Upper Layer Control

Local Policy and Admission

Dynamic Service Process

Traffic Selector

TrafficPredictor

CapacityAllocator

MAC Management

To Local IDs

Upper Layer Traffic

AdmissionInformation

Queue and Capacity Manager

RequestResponse

Convergence Layer

MAC Layer

Convergence Layer

Upper Layers

Category Compliant Traffic

MAP information,Data and RequestTriggers

RegistrationParameters

FeedbackInformation

AdmissionInformation

Inst. Primary Queue Size

Controls Category DataTransmission

QCM Update

PrimaryCompliantTraffic

PrimaryData andRequestQueues

RegistrationParameters

Controls Primary DataTransmission

Inst. Category Queue SizeCategoryData andRequestQueues

Primary Requests

ManagementMessages

DynamicService Messages

Figure3.7: STSDCA Block Structure

53

� Translator:It hastheresponsibilityof translatingrequestedupperlayerQoSparame-

tersinto localparametersrecognizedby localadmission.For example,RSVPdefines

QoSin termsof a tokenbucket model.Thetranslatorwill convert thecorresponding

bucketattributesinto local parametersasdelay, jitter, cell loss,etc.

� LocalPolicy andAdmission:It definestheSTSpolicy. It alsohandlesadmissionof all

connectionrequestsfrom upperlayer control. Translatedupperlayer requestsfrom

singleconnectionsarecategorizedaccordingto internalcriteria into certainservice

categories(real-timeandnon-real-timefor example).Admissionmight immediately

refusea connectionrequestlocally; in caselocal resourcesarecompletelyexhausted

or if theconnectionviolatestheterminal’scapabilities.Admissionmechanismsmight

alsodecideto multiplex requestsoveranexisting contractin caseit is under-utilized.

Otherwise,theadmissionmustconsulttheMCS andconnection-relatedrequestsfor

thecorrespondingcategory areissued.Theserequestsaccompaniedby traffic param-

etersrelatedto that specificcategory will be transmittedover the MAC asdynamic

servicecommands.RegistrationinformationandperiodicQueueandCapacityMan-

ager(QCM) updateshelpin admissiondecisions.Theadmissionmustalsoupdatethe

QCM with the currentadmittedresourcestatusto be utilized in the predictionpro-

cess.It will alsofeedinformationaboutthevariousconnectionsto thetraffic selector

to helpit monitorcomplyingtraffic.

� DynamicServiceProcess:The dynamicserviceprocessis the meansby which the

STSconveys its requestto theMCSanddefinesthebehavior of mutualnegotiations.

� Traffic Selector:The traffic selectorhasthe responsibilityof forwardingcompliant

usertraffic into dataqueuesof the correspondingcategory. It also interleavesover

54

eachtime frame,traffic from differentlocal connectionsbeforeforwardingtheminto

thequeuesto avoid monopolizingof thequeuesby asingleconnection.

� DataQueues:Datafrom all connectionsof thesamecategoryenterthecorresponding

dataqueue.Instantaneousqueuestatusupdatesarereportedto help in theprediction

process.Theprimaryqueueis utilized by controlmessagesandis usedasanescape

mechanismfor non-categorizeddatapackets.

� QueueandCapacityManager:It hastheresponsibilityof managinganddistributing

grantedresourcesfrom the MCS. Basedon queuestatusandadmissioninformation

theQCM will predictandrequesttherequiredbandwidthandallocategrantedalloca-

tions.Thequeueandcapacitymanageris composedof:

1. Traffic Predictor:Thetraffic predictorhastheresponsibilityof requestingthe

necessarycapacityfrom theMCSto servethepacketsin thedataqueues.The

traffic predictorperiodicallyreceivesupdatesfrom the dataqueueson their

currentstatusand recordsinstantaneousdatarequests.The predictoralso

maintainsa recordandreceivesregularupdatesfrom admissionon the traf-

fic parametersof currentlyacceptedconnections.All provided information

will be usedin predictingthe anticipatednumberof datapacketsafter two

roundtrips (requestsfrom STSto MCS andback). Fine-tuningis achieved

by comparingpredictedrequestswith actualqueuesizeat receptionof allo-

catedgrants.

2. CapacityAllocator: It receivesdescriptionof thegrantsfrom MAC manage-

mentandupdatesthepredictorby theallocatedgrantsto modify its prediction

if necessary. It alsomanagesthequeuesandtheir timing. In caseof multiple

55

OutputQueue

Convergence Layer

MAC Layer

SchedulingInstructions

Dynamic ServiceProcess Poli cy and Admission

SchedulerMAC management

Reg. Request,Registration Parameters

Request Frame

Connection Request

Dynamic serviceMessages

Connection Response

Dynamic serviceResponse

SchedulingStatus

PollingInformation

AdmissionInformation

Management messagesMCS-Bands

Reg. Response

Figure3.8: MCS DCA Block Structure

queues,the allocatormay decideto move allocationsfrom onecategory to

another(real-timeovernon-real-timefor example).Excessbandwidthif any

will alsobeshuffledbasedon local algorithmsaswell asinstantaneoussizes

of thedifferentqueues.

3.4.3.3 MCS Structure for Dynamic Capacity Allocation

Figure3.8depictsablockstructurethathighlightstherequiredcomponentsfor theproposed

dynamiccapacityallocationandschedulingat theMCS. It alsospecifieswherewill these

componentsfit within theoverall MAC structureandhow they will interactwith theMAC

managementaswell aswith upperlayers.Thefiguredefinesthefollowing blocks:

� Policy andAdmission:It carriespolicy informationacquiredduringregistration.Ad-

missiondecisionsdependon thecurrentnetwork statusandregularupdatesreceived

for the currentschedulerstatus. The policy and admissionblock will also always

56

regularlyupdatetheschedulerwith latestadmissioninformation.

� Scheduler:It hastheresponsibilityof allocatingcapacitygrantsto all requestingSTS.

STSrequestsrepresenttheir overall requirementsper category. Theschedulerregu-

larly receivesadmissioninformationandusesit to monitorcomplyingstations.It is

alsofrequentlypolledfrom MAC managementto allocatemanagementopportunities

(RangingandRegistration)to specificstations.Schedulinginstructionsareforwarded

to theMAC managementto betransmittedasMAP messages.

3.4.3.4 Example for AdmissionProcedure over the ProposedStructure

Thefollowing examplewill give a scenarioto illustrateadmissionprocedureandhelpun-

derstandinteractionsbetweenthe systemcomponents:Assumingan STS registeredto a

contractthat specifiesan averageof 3 slotsper time framewith a maximumof 5 for the

real-timeservicecategory. If auseris trying to runamultimediaapplicationwhich requires

morecapacitythanis currentlyavailable(anaverageof 8 slotsanda maximumof 10 slots

for example),localadmissionwill needconsultationfrom theMCSbeforeit canacceptthe

connection.Theadmissionwill requestthedynamicserviceprocessto handlethemodifi-

cation. In that particularscenario,the dynamicserviceprocesswill senda requestto the

MCS for an increaseof 5 slotsin theaveragegrantsandanincreaseof 5 slotsin themax-

imum allowablegrantsperonetime framein thereal-timeconnection.Uponreceptionby

theMCS dynamicserviceprocess,the requestis forwardedto thepolicing andadmission

process.Thepolicing is performedaccordingto registrationparametersreceivedduringini-

tialization. Admissiondecisionis basedon theavailability of therequestedresourcesafter

consultingschedulerupdates.If themodificationis acceptedtheresponsewill beforwarded

backto theSTSandmodificationswill takeplacein futureframesreportedby broadcasting

57

MAP messages3.

3.5 Data TransmissionProtocol under the EnhancedCF-

DAMA Scheme

Subscribersin residentialapplicationsservicesareexpectedto producereal-timeaswell as

non-real-timetraffic. Non-real-timetraffic involvesbulk servicesasftp, http, etc. It has

the leastpriority andcan toleratecertainperiodsof delay. Real-timetraffic may involve

two categoriesof service;Short Impulseservicesare asymmetricin their natureandare

characterizedby muchmoretraffic on thedownstreamthantheupstream.Exampleof short

impulseservicesis VoD. ContinuousBurst servicesontheotherhand,arerathersymmetric

with muchlongerpacketsbut theapplicationmay toleratesinglepacket lossesevery once

and a while. Examplesof theseservicesare IP telephony and video conferencing. An

intermediatecategory might alsobedefined,whererathershortbut continuouscommands

mustbetransmittedon theupstream.Thesecommandsarevery sensitive to lossanderror.

Examplesof sucha category includeInternetgamingandvirtual shopping.In this section,

we focuson real-timetraffic andproposetwo modesof datatransmissionoperationunder

theenhancedpredictionbasedCFDAMA protocol.

3.5.1 GeneralAssumptions

1. Satellitepropagationroundtrip delayis 480ms,while thesatelliteMAP time frame

describesa periodof 24 ms . This meansthat thepropagationdelayis 20 timesthe3Note that MAP messagesareusuallymonitoredby the BTS on a regular basis. The BTS may usethe

messageinformationto anticipatepossibletransmissionsfrom its varioussubscribers.

58

MAP time frameperiod.

2. Usually per time frame,a singleapplicationwill generatea maximumof onedata

packet4.

3. In residentialservices,more thanoneapplicationmay be in useat the sametime.

However, thenumberof simultaneousapplicationsis bounded.Hence,it is legitimate

to assumethat theaggregatenumberof packets(variablein sizeeach)generatedper

time frameis alsoconstrainedby anupperbound. This providesanestimateon the

upperboundfor maximumnumberof slotsthatmayberequestedby asingleterminal

pertime frame.

4. The predictionalgorithmpredictsthe numberof slotsneededto convey the packets

generatedby activeservicesaftera roundtrip delay.

5. Requestsissuedby theterminaldescribetheir aggregaterequirementwithin acertain

category.

6. In casetheallocationis notenoughto emptythequeue,fragmentationwill beallowed.

Theuserterminalwill continueto attemptto sendthedeferredpacketsover N time

framesandwill thendropthemfrom its queue.

7. A requestmight be transmittedeitherexplicitly over a requestopportunityor piggy-

backedif theuserterminalreceivesa grantat thetimeof theprediction.

4This a is legitimateassumption,aswe assumea MAP frameevery 24 ms. Theexpectedservicesarenotexpectedto generatetraffic at ahigherrate,asall of themarecontrolledby humainbehavior.

59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Prediction for 23 at 3 is transmitted over a request (explicit or piggyback)

Prediction for 22 at 2 is discarded because there is no request opportunity

Prediction for 21 at 1 is transmitted over a request (explicit or piggyback)

Figure3.9: DataTransferunderNormalModeOperation

3.5.1.1 Traffic TransmissionModes

Normal mode In this mode,the STSpredictsmini-slotsdemandsneededafter an exact

roundtrip (20 framesinto the future,neglectingoffsetsdueto processingdelays,packets

transmissiontime, etc). However if the terminal is not able to convey its predictionto

theMCS (no requestopportunityexplicit or piggybacked), it will discardthis requestand

processthe predictionof the next frame. Normal modeoperationis shown in Figure3.9.

Thenormalmodewill bemoreefficient for servicesthatcontinuouslygeneratepacketsover

bursts.In this case,all requestscanbeconveyedoverdatapacketsaspiggybacking.

Accumulative Mode In this mode,the STSpredictsmini-slotsdemandsneededafter a

roundtripdelayplussometolerance(25 framesinto thefuturefor example).SincetheSTS

may not necessarilyget a requestopportunityeachframeandthe predictionis performed

earlier by 5 frames,the STS might convey its un-transmittedrequestwithin the 5-frame

tolerancezone. However, in this casewhenthe STSis ableto senda request,the request

shouldaddressthe aggregatepredicteddemandsfrom the first un-transmittedrequesttill

therequesttransmissiontime. Sincethesedemandsrepresentconsecutive frames,they can

beassumedhighly correlatedwith closedemands.Basedon local criteria in theMCS and

60

1 2 3 4 5 6 7 8 9 10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

Prediction for 28 at 3 is transmitted (offset field in request is 0)

Prediction at 1+Prediction at 2 for 26 and 27 are transmitted (offset field in request is 1)

Prediction for 26 at 1 cannot be transmitted (no explicit request or piggyback)

Figure3.10:DataTransferunderAccumulativeModeOperation

its schedulingstatus,it decidesto distribute the aggregatedemandson the corresponding

frames(equally for example). Aggregatemodeoperationis shown in Figure 3.10. The

aggregatemodewill be more efficient for applicationsthat produceshort randomtraffic

bursts.

3.6 Summary

In this chapter, we have presenteda proposedMAC layer architectureat the air-interface

terminalsto deploy interactive residentialtwo-waybroadbandservicesoversatelliteaccess

networks. We have describedthesystemconfigurationandcorrespondingprotocolstacks.

We also have shown behavioral sequencesand introduceda new techniquebasedon an

enhancedCFDAMA protocol. Furthermore,we have proposeda novel schedulingstruc-

ture that canreducesystemcomplexity. We have alsoshown examplesof dataexchange

operationover two modesof datatransfer.

In thenext chapter, we developanSDL modelto formally describethedevisedsystem.

Wewill thenverify andvalidatetheresultingmodel,to ensuresystemcorrectness,usingthe

61

ObjectGEODEtools. Finally, we will reflecton our experienceby providing anevaluation

on theuseof formal techniques,especiallySDL, to describe/validatereal telecommunica-

tionssystems.

62

Chapter 4

BSA Formal Description and Validation

In the last chapter, we have describedtheproposedMAC structureat theair-interfaceter-

minalsfor BSA systems.As presented,theimplementationof suchsystemwill necessitate

complex engineeringefforts due to the ambiguityof a plain Englishdescription. Dif fer-

entprogrammersmight understandthespecificationdifferently, which setsa boundaryon

achieving thenecessaryrealstandardization.Thenon-formalspecificationis very far from

implementationtechnicalitiesandsystemvalidity cannotbeguaranteed.Duringthelasttwo

decades,formal techniqueshave beenusedsuccessfullyfor thedescriptionandvalidation

of communicationsystems.A systemis correctif it satisfiesgeneralaswell asspecificpro-

tocol properties.Generalpropertiesof a protocol includethe non-existenceof deadlocks,

unspecifiedreceptions,live-locksandunreachablestates.Specificpropertieson the other

handarederivedfrom thenon-formalspecificationto guaranteea propersystembehavior.

WehavedevelopedaformalmodelusingtheSpecificationandDescriptionLanguage(SDL)

[17] for theproposedBSA MAC with necessaryabstractionsto testits validity. In thischap-

ter, we presenttheconstructedSDL modelandshow validationproceduresandresults.In

theend,we commenton theutilization of formal methods,especiallySDL for description

63

andvalidationof realcomplex systems.

4.1 SDL Model

WehavechosenSDL astheformal languageto modeltheMAC layerandsystemstructure.

SDL is anITU standardformal descriptionlanguagethat is usedto describesystemsusing

graphicalrepresentationsaswell astextual representations.SDL describesasystemasaset

of processes.Transitionsin SDL from onestateto anotheraretriggeredby thereceptionof

signalingmessages.For eachprocess,SDL describestheactionsit is allowed to take and

which eventsareexpectedto happen.In SDL, a systemis dividedinto building blocksthat

communicateusingchannels.Blocksarecomposedof processes.Processes(within ablock)

areconnectedusingsignalroutes.Eachprocessis anextendedfinite statemachine,which

hasits own infinite queueandis assumedto operateindependentlyfrom all otherprocesses.

Figure4.1depictsthedifferentconstructsof anSDL process.

4.1.1 BSA SystemModel

Figure4.2 shows the devisedBSA systemmodel. It is composedof a BTS, a MCS and

two STS of an accessdomaincommunicatingthrougha satellitebent pipe. The MCS

is given MAC address2, the BTS is MAC address3, while the STS are representedby

MAC addresses4 and 5. The two STS are instancesof a commonblock type; SUB-

SCRIBER_TRANCEIVER_STATION_STSandareinitializedby feedingtheircorrespond-

ing MAC addressesasanattributeof thePOWER_ONsignals.We have modeledanMF-

TDMA systemoverasinglebandof frequencieswith similarcharacteristicsto reducemodel

complexity. The modelthereforedepictsonly threechannelbands;CH_1 carriesall STS

64

Start

Decision

Task

ProcedureCall

ProcessCreate

End

Input

State

Output

Figure4.1: SDL ProcessConstructsNotation

65

system BSA

DS_UL

CH_3A

TO_UPPER_LAYER_STS_5

BTS_UPPER_LAYER_DATA

STS_UPPER_LAYER_DATA

TO_UPPER_LAYER_BTS

STS_UPPER_LAYER_DATA

BTS_UPPER_LAYER_DATA

US_DL_2

CH_1B

US_DL_3

CH_1B

MCS_DL_5

CH_2B

MCS_UL

CH_2A

MCS_DL_3

CH_2BUS_UL_4

CH_1A

DS_DL_5

CH_3B

US_UL_5

CH_1A

DS_DL_4

CH_3B

TO_UPPER_LAYER_STS_4

BTS_UPPER_LAYER_DATA

STS_UPPER_LAYER_DATA

ON_OFF_SWITCH_4

POWER_ON,POWER_OFF

ON_OFF_SWITCH_5

POWER_ON,POWER_OFF

MCS_DL_4

CH_2B

SUBSCRIBER_TRANCEIVER__STATION_STS

BASE_TRANCEIVER_STATION_BTS_

MAC_ADDRESS_3

SATELLITE_BENT_PIPE

MASTER_CONTROL__STATION_MCS_

MAC_ADDRESS_2

CH_2 CH_3 CH_1

UPPER_LAYERS SWITCH

MAC_ADDRESS_5:SUBSCRIBER_TRANCEIVER_

_STATION_STS

CH_2 CH_3CH_1

UPPER_LAYERS SWITCH

MAC_ADDRESS_4:SUBSCRIBER_TRANCEIVER_

_STATION_STS

Figure4.2: BSA StructureModel

66

block type SUBSCRIBER_TRANCEIVER_STATION_STS

CH_2 CH_2B

CH_1 CH_1A

CH_3 CH_3BSWITCH POWER_ON,POWER_OFF

UPPER_LAYERS STS_UPPER_LAYER_DATA

BTS_UPPER_LAYER_DATA

CH_1_TRANS

CH_1A CH_3_REC

CH_3B

TO_LLC

MAC_SPECIFIC_FRAME,INI_ADD

MAC_SPECIFIC_FRAME

DATA_TO_MAC

STS_DATA,REQ

DATA_FROM_MAC

REC_DATA

CH_INITIATE_TERMINATE

USE_CH,STOP_USE_CH,INI_ADD

TO_PHY

RE_INIT

TO_UPPER_LAYER_STS

BTS_UPPER_LAYER_DATAFROM_UPPER_LAYER_STS

STS_UPPER_LAYER_DATA TO_DATA_BUF

REQ_TRIG,DATA_TRIG

ON_OFF_SWITCH

POWER_ON,POWER_OFF

TO_MAC_MANGINI_RNG_REQ,UNI_RNG_REQ,REG_REQ,REG_ACK

SYNC,UCD,MAP_MES,RNG_RSP,REG_RSP,INI_ADD

CH_2_REC

CH_2B

TO_MAC

UPSTREAM_MES

DOWNSTREAM_MES,INI_ADD

DATA_RECEIVER DATA_BUFFER__REQUESTER

STS_MAC_MANAGEMENT

LLC_ENCAPSULATOR__DECAPSULATOR

MAC_ENCAPSULATOR_DECAPSULATOR

STS_PHY_LAYER_ENCAPSULATOR__DECAPSULATOR

PHY_LAYER__SCANNER

Figure4.3: STSBlock TypeStructure

(stations4 and5) transmissions,CH_2conveysMCScontrolcommands,while CH_3bears

BTS datapackets. It is worth mentioningthatMCS andBTS arealwaysloggedto thenet-

work, while STSareturnedon andoff by feedingPOWER_ONandPOWER_OFFsignals

via theON_OFF_SWITCH.

4.1.2 STSBlock Type

Figure4.3 depictstheSUBSCRIBER_TRANCEIVER_STATION_STSblock type. As in-

dicatedby the figure, its structurecoincideswith theproposedprotocollayers. Theblock

typeis furtherdecomposedinto thefollowing building blocks:

� PHY_LAYER_SCANNER:Weusetheblockto simulateachannelscanningprocess.

67

On receptionof a POWER_ONtrigger from the environment,the scannerwill acti-

vatethe STS_PHY_LAYER_ ENCAPSULATOR_ DECAPSULATOR functionality

by sendinga USE_CHsignal. A POWER_OFFsignalwill result in deactivationof

that encapsulator/decapsulatorblock througha STOP_USE_CHinstruction. If the

MAC Managementproceduresdictatea re-initializationprocess,theRE_INIT signal

will triggeraSTOP_USE_CHfollowedby a USE_CHto simulatesystemrebooting.

� STS_PHY_LAYER_ENCAPSULATOR_DECAPSULATOR:Theblocksimulatesphys-

ical layerframingcompositionandextraction.WhentheSTSis poweredoff theblock

is in adormantstate.Activity of thisencapsulator/decapsulatorblock is controlledby

thescannervia theUSE_CHandSTOP_USE_CHsignals.

� STS_MAC_ENCAPSULATOR_DECAPSULATOR: It decapsulatesreceivedsignals

andcategorizesthemasdataor MAC SpecificFramemessages.Signalsflowing in

the otherdirection(data,requestandmanagement)areencapsulatedandforwarded

to thephysicallayer. Modelingof erroneouspacketsis doneby droppingoneout of

every100receivedsignalsfor anerrorrateof 0.011.

� LLC_ENCAPSULATOR_DECAPSULATOR: It converts MAC managementmes-

sagesto MAC_SPECIFIC_FRAMEsignalsandviceversa.

� DATA RECEIVER:This block simply receivesdataframesfrom the BTS andfor-

wardsthemto upperlayers.

� DATA_BUFFER_REQUESTER:ThisblocksimulatestheQueueandCapacityMan-

ager(QCM) processaswell astraffic queues.It is madesimpleassuminga single1This valuedoesnot representa realisticsatellitemediumperformance.We only introduceit to beableto

describesystembehavior in caseof corruptedpackets.

68

queueto reducemodelcomplexity. Weassumedatatransmissionoperationunderthe

normalmode. Existenceof a predictionis randomlydeterminedevery time frame.

If a predictionis detected,theSTSmayconvey its requestto theMCS only whena

requestopportunityis sensedvia an explicit REQ_TRIGor implicitly by receiving

a DATA_TRIG (Piggybacking).The queuestatus(emptyor non-empty)eachtime

frameis alsorandomlydeterminedunlessfragmentedpacketsareoutstandingfrom

thepreviousframe.Notethatweareonly interestedin describingall thepossiblesce-

nariospertimeframe,i.e., in acertaintimeframeareceivedgrantmightsatisfyoneof

two possibilities;eitherit is sufficient to emptythedataqueueor outstandingpackets

will remainto beservedin future frames.Thequeuebehavior in betweeneachtime

frameis outof thescopeof thespecification(i.e,weassumethatprioritization,queue

dropping,queuemanagement,etc.,aredoneexternally).

� STS_MAC_MANAGEMENT: Thisblock is responsiblefor STSMAC management.

Figure4.4describesits components.It is consistsof:

1. STS_MANAGEMENT_MESSAGES_DISTRIBUTER: It receivesall man-

agementmessagesandcorrectlydistributesthemto variousprocessesof the

Managementblock.

2. STS_MAC_MANAGEMENT_HANDLER:It managestheoverallSTS_MAC_MANAGEMENT

block. An STSis eitherin an initialization phaseor a datatransfermodeas

determinedby theHandler. It receivesSYNC,UCD, RangingandRegistra-

tion statusupdatesandenforcesre-initializationin caseof any failures.It will

terminateUCD, rangingor registrationprocesseswheneverre-initializationis

in effect.

69

block STS_MAC_MANAGEMENT

TO_DATA__BUF

TO_PHY TO_MAC_MANG

TO_MAC_MANG

TO_MAC_MANG

TO_UCDUCD

TO_RNG

RNG_RSPTO_REG

REG_RSP

TO_SYNCSYNC

TO_MAP

MAP_MES

SYNC_MANG

SYNC_STATUS

UCD_MANG

UCD_STATUS TERMINATE

RNG_MANGRNG_STATUS

TERMINATE

REG_MANG

REG_STATUS

TERMINATE

MAP_TO_REG

REG_TRIG

MAP_TO_RNG

INI_RNG_TRIG,UNI_RNG_TRIG

TO_DATA_BUF

REQ_TRIG,DATA_TRIG

TO_PHYRE_INIT

TO_DIST

SYNC,UCD,MAP_MES,RNG_RSP,REG_RSP,INI_ADD

FROM_REG

REG_REQ,REG_ACK

FROM_RNG

INI_RNG_REQ,UNI_RNG_REQ

STS_REGISTRATION(0,)

STS_RANGING(0,)

STS_UCD(0,)

STS_SYNC(0,)

STS_MAC__MANAGEMENT_

_HANDLER(0,)

STS_MAP__HANDLER

(0,)

STS_MANAGEMENT_MESSAGES__DISTRIBUTER

Figure4.4: STSMAC Management

3. STS_SYNC:It is responsiblefor handlingsynchronizationprocedures.Fail-

ure to receive a SYNC messagewithin a certaintimeout period forcesre-

initialization.

4. STS_UCD:It receivesUCD messages.Thesuitability of theband’sparame-

tersis randomlydeterminedusinganANY construct.A re-initializationpro-

cedureis triggeredif theprocessfails to receiveaUCD messagewithin acer-

tain timeoutperiodor if a receivedUCD messageindicatesanon-compatible

changein theband’sparameters.

5. STS_RANGING:It handlesinitial andperiodicrangingprocedures.It forces

re-initializationin caseof failuresin rangingprocedures.

6. STS_REGISTRATION: It is responsiblefor exchangingregistrationinforma-

tion with theMCS.Registrationfailureforcesa terminalre-initia- lization.

70

block MASTER_CONTROL_STATION_MCSMAC_ADDRESS_2

US_DL_2 MCS_UL

MAC_MANG_AND_SCH

POLL_REG,POLL_RNG_REQ,REGIS_STATIONS

MAP_INS

MAC_TO_SCH

REQ_FRAME

TO_MAC

MCS_MES

UPSTREAM_MES

CH_1_REC

CH_1B

TO_LLC

MAC_SPECIFIC_FRAME

MAC_SPECIFIC_FRAME

CH_2_TRANS

CH_2A

TO_MAC_MANG

SYNC,UCD,MAP_MES,RNG_RSP,REG_RSP

INI_RNG_REQ,UNI_RNG_REQ,REG_REQ,REG_ACK

SCHEDULING

MCS_MAC_ENCAPSULATOR_DECAPSULATOR

MCS_PHY_LAYER_ENCAPSULATOR_DECAPSULATOR

MCS_LLC_ENCAPSULATOR__DECAPSULATOR

MCS_MAC_MANAGEEMNT

Figure4.5: MCSBlock Structure

7. STS_MAP_HANDLER:It receivesandtranslatestheMAP_MESsignal. It

then issuestriggersto the variouscorrespondingprocessesin the samese-

quenceasthatindicatedin theMAP.

4.1.3 MCS Block

Figure4.5depictstheMASTER_CONTROL_STATION_MCSblock. Its structurealsoco-

incideswith theproposedprotocollayers.Theblock is furtherdecomposedinto thefollow-

ing building blocks:

� MCS_PHY_LAYER_ENCAPSULATOR_DECAPSULATOR:As in theSTS,theblock

simulatesphysicallayerframing.

71

� MCS_MAC_ENCAPSULATOR_DECAPSULATOR:It decapsulatesreceivedsignals

andcategorizesthemasrequestframesor MAC SpecificFramemessages.Specific

Framesignalsflowing in theotherdirectionareencapsulatedby this block to besent

over theMCS_UL.

� MCS_LLC_ENCAPSULATOR_DECAPSULATOR: It convertsMAC management

messagesto MAC_SPECIFIC_FRAMEsignalsandviceversa.

� SCHEDULING: The schedulermay receive requestframesfrom STSor is locally

polled for managementopportunities(e.g.,maintenanceor registration).Thesched-

uler randomly(with ANY construct)respondsto requestsor polled messageseach

time frame(to simulatepossiblebehavior scenariosdependenton capacityavailabil-

ity or systempolicy). After eachtimeframetheschedulersendsaMAP_INSto MAC

managementdescribingcontentsof futureMAP messages.A Requestopportunityfor

eachregisteredterminal is randomlydeterminedeachtime frame. Freedatais also

randomlygrantedper registeredterminaleachtime frame. The algorithmby which

requestsandfreedataallocationaredeterminedis out of thescopeof themodel.

� MCS_MAC_MANAGEMENT: This block is responsiblefor MCS MAC manage-

ment. Figure 4.6 describescomponentsof the STS_MAC_MANAGEMENT. It is

constitutedof:

1. MCS_MANAGEMENT_MESSAGES_DISTRIBUTER: It receivesall man-

agementmessagesandcorrectlydistributesthemto variousprocessesof the

Managementblock. It alsoinstantiatesa new Handlerprocessfor every new

STSloggingin to thenetwork.

72

block MCS_MAC_MANAGEEMNT

TO_MAC_MANGTO_MAC_MANG

TO_MAC_MANG

MAC_MANG_AND_SCH

MAC_MANG_AND_SCH

FROM_SYNC

SYNC

REG_MANG

REG_STATUSREG_REQ,REG_ACK

RNG_MANG

RNG_STATUSINI_RNG_REQ,UNI_RNG_REQRNG_TO_SCH

POLL_RNG_REQ

REG_TO_SCH

POLL_REG

SCH_TO_MAP

MAP_INS

FROM_UCDUCD TO_DIST

INI_RNG_REQ,UNI_RNG_REQ,REG_REQ,REG_ACK

FROM_REG

REG_RSP

FROM_RNG

RNG_RSP

FROM_MAP

MAP_MES

MANG_DIST

INI_RNG_REQ,UNI_RNG_REQ,REG_REQ,REG_ACK

REGIS_UPDATE,STATION_UPDATE

TO_SCH

REGIS_STATIONS

MCS_MAC__MANAGEMENT_

_HANDLER(0,)

MCS_UCD__TRANSMITTER

MCS_SYNC__TRANSMITTER

MCS_MAP__TRANSMITTER

MCS_RANGING(0,)

MCS_REGISTRATION(0,)

MCS__MANAGEMENT_

_MESSAGES__DISTRIBUTER

Figure4.6: MCSMAC Management

2. MCS_MAC_MANAGEMENT_HANDLER: Each instanceof this process

managesasingleSTS.It receivesRangingandRegistrationstatusupdatesof

thecorrespondingSTS.Successfulregistrationwould addthecorresponding

terminalto theregisteredterminalslist, while arangingfailureof aregistered

terminalwould removeit from thatlist.

3. MCS_SYNC_TRANSMITTER:It periodicallytransmitsSYNC messages.

4. MCS_UCD:It periodicallytransmitsUCD messages.Weassumeonly asin-

gle bandof frequencieswith similar characteristics.However, the band’s

attributesmight changeasreportedby the UCD every now andthendueto

variationof themediumconditionsfor example.

5. MCS_RANGING:Eachinstanceof this processhandlesinitial andperiodic

rangingproceduresof asingleSTS.

73

block BASE_TRANCEIVERSTATION_BTSMAC_ADDRESS_3

DS_UL

TO_UPPER_LAYER_BTS

MCS_DL_3US_DL_3

TO_MAC

DOWNSTREAM_MES

UPSTREAM_MES

CH_3_TRANS

CH_3A

MAC_DATADATA_FRAME

DATA_FRAME

TO_LLCMAC_SPECIFIC_FRAME

TO_MAC_MANG

MAP_MES,SYNC

TO_UPPER_LAYER_BTS

BTS_UPPER_LAYER_DATA

STS_UPPER_LAYER_DATA

CH_1_REC

CH_1B

CH_2_REC

CH_2B

BTS_MAC_ENCAPSULATOR_DECAPSULATOR

BTS_PHY_LAYER_ENCAPSULATOR_DECAPSULATOR

DATA__TRANCEIVER

BTS_LLC_ENCAPSULATOR__DECAPSULATOR

BTS_MAC_MANAGEEMNT

Figure4.7: BTS Block Structure

6. MCS_REGISTRATION: Eachinstanceof this processhandlesregistration

proceduresof a singleSTS.

7. MCS_MAPTRANSMITTER:It receivesMAP_INSmessagesfromthesched-

ulerandformatstheminto correspondingMAP messages.

4.1.4 BTS Block

Figure4.7depictstheBASE_TRANCEIVER_STATION_BTSblock. Its structurealsoco-

incideswith theproposedprotocollayers.Theblock is furtherdecomposedinto thefollow-

ing building blocks:

� BTS_PHY_LAYER_ENCAPSULATOR_DECAPSULATOR: As in STS, the block

74

simulatesphysicallayerframing.

� BTS_MAC_ENCAPSULATOR_DECAPSULATOR: It decapsulatesreceivedsignals

andcategorizesthemasdataframesor MAC SpecificFramemessages.Datapackets

flowing in theotherdirectionareencapsulatedby this block to betransmittedon the

downstream.

� BTS_LLC_ENCAPSULATOR_DECAPSULATOR:It convertsreceivedMAC-_SPE-

CIFIC_FRAME signalsinto SYNC andMAP_MES managementmessages.Other

managementmessagesaredroppedby theBTS asthey areirrelevant to its function-

ality.

� DATA_TRANCEIVER: It sendsandreceivesdataframesover thesatellitesystem.

� BTS_MAC_MANAGEMENT: Aspreviouslymentioned,theBTSmightelecttomon-

itor theMAP messagesto anticipateits subscribers’transmissions.Moreover, in order

to adjustits timing, it periodicallyreceivesthebroadcastSYNC messages.

4.1.5 SatelliteBent Pipe

The bentpipe structureis shown in Figure4.8. It consistsof threeprocessesresponsible

for introducingsatellitedelayandbroadcastingframeson thecorrespondingsatellitebands

involvedin thecommunicationprocess.

4.2 SystemValidation

For the verificationprocess,we have utilized a subsetof the modelshown in Figure4.2

by removing oneof the STSterminalsasverificationis usually interestedin independent

75

block SATELLITE_BEND_PIPE

DS_UL

DS_DL_5

US_UL_5

US_DL_3

US_DL_2

MCS_DL_3 MCS_DL_4MCS_UL

DS_DL_4

US_UL_4

MCS_DL_5

DS_ULCH_3A

DS_DL_5

CH_3B

US_UL_5

CH_1A

US_DL_3

CH_1B

US_DL_2

CH_1B

MCS_DL_3

CH_2B

MCS_DL_4

CH_2B

MCS_UL

CH_2A

DS_DL_4

CH_3BUS_UL_4

CH_1A

MCS_DL_5

CH_2B

DOWNSTREAM_SAT_PIPE

UPSTREAM_SAT_PIPE

MCS_SAT_PIPE

Figure4.8: SatelliteBentPipe

peerto peeroperation. The relevanceof suchprocedurewill be thoroughlydiscussedin

the next section. Using the aforementionedpostulate,we have verified the correctnessof

theprotocol’sgeneralpropertiesby runningtheverificationprocessin ObjectGEODE[38].

Resultsprovedtheprotocolfreefrom deadlocks,unspecifiedreceptionsandlive-locks.We

have also derived the most critical specificpropertiesthat we considershouldcover the

validationof thelargestportionof thesystembehavior. Illustrativepropertiesare:

� Failuresin MAC managementprocedures(SYNC, UCD, RangingandRegistration)

at theSTSwill alwaysstimulatere-initializationandinstancesof all theseprocesses

(exceptfor SYNCprocess)areterminated.

� Failuresin MAC managementprocedures(RangingandRegistration)at theMCSwill

alwaysterminatetheseprocessesaswell astheir Handlers.

76

� An STSissuingINI_RNG_REQduring initialization eventuallyexpectsa response

from theMCS or elsetheprocessmustbe terminatedif retriesareexceeded(dueto

collisionsfor example).

� A changein UCD descriptionparametersby theMCSis eitheracceptedor refusedby

theSTS.

� A stationmaintenanceopportunitysensedby anSTSwill triggera rangingprocedure

andtheterminalremainsregisteredonly with successfulranging.

� Registrationprocedureat theSTSis only completeafterreceiving thefirst dataallo-

cationfrom theMCS.

� Registrationprocedureat theMCSis completeby receiving thelastregistrationMAC

managementmessagefrom theSTS(REG_ACK).

� Thesequencefor STSregistrationis alwayssynchronization,acceptableUCD param-

eters,successfulrangingandregistrationprocedures.

� STSarenot allowedto transmitdatapacketson thesatellitenetwork beforethey are

successfullyregisteredwith theMCS.

� Piggybacked requestsareutilized to implicitly convey terminals’predictionsto the

MCS.

� Non sufficient grantswill triggera fragmentationprocedureif remainingcapacityis

enoughto convey at leastonefragmentof theoutstandingpacket.

� TheschedulerMAP allocationsarealwaysregularlybroadcastto thenetwork in man-

agementmessages.

77

STS_Ranging STS_Ranging

UNI_RNG_TRIG UNI_RNG_TRIG

RNG_RSP (Good)

Figure4.9: An Examplefor MSCObserversof SpecificProperties

� Unallocatedcapacityby theschedulereverytimeframeperiodwill befreelyassigned

to variousterminalsasdescribedby CFDAMA.

To validatemost of the above propertiesin ObjectGEODE,they must be translatedinto

equivalentMessageSequenceChart(MSC) [39] observers.An MSCobserver is composed

of oneormany MSCleaveslinkedbyhigh-levelMSCoperator(s),e.g.,AND, OR,REPEAT,

etc. Formal validationis achieved by runningthe verificationprocessagainsttheseMSC

observers. For illustration purposes,an examplefor the translationprocessis presented

by deriving the observer for the following property; "A stationmaintenanceopportunity

sensedby anSTSwill triggera rangingprocedureandtheterminalremainsregisteredonly

with successfulranging". This may be representedby two MSC scenariosthat arelinked

togetherby an OR parameterasshown in Figure4.9. As indicatedin the figure,a station

maintenanceopportunityat a registeredSTSis representedby a UNI_RNG_TRIGsentby

the MAP Handler. The two possiblescenariosthat the propertystatesinvolve either the

78

receptionof a successfulRNG_RSPfrom the MCS or terminatingthe rangingprocessas

a result of rangingfailure (i.e., terminal will no longer be registered). The verification

procedureis doneby simulatingtheBSA systemto thepoint that theschedulerincludesa

UNI_RNGopportunityin its MAP frame.Thefollowing stepis to runaverificationprocess

againstthespecifiedobserver.

Similar translationsweredonefor otherderivedpropertiesandvalidationwasachieved

by runningtheverificationprocessagainsteachof theequivalentobservers.Wehave found

thatnoneof thepropertieswereviolated.

It is worthwhile to mentionthat we werenot ableto formally validatepropertiesthat

involve variabledecisionsanddependon non-message(event)criteria,asnon-sufficient or

unallocatedgrantsfor example. It is even harderwith the randombehavior assumption

(usingANY) dueto the lack of quantitative attributes.However, dueto the importanceof

theseproperties,wehavechosento try to validatethemvia inspectionby runningnumerous

guidedsimulationsandstudyingall possiblescenarios2. Nevertheless,noneof theindicated

propertieswereeverviolatedover thechosenscenarios.

4.3 Description and Verification Models

Whenwe developedthe modelshown in Figure4.2, we have set in mind two goals;sys-

temdescriptionandverification.Theserepresenttwo diverseobjectives.While description

becomesmorerobustby includingmoredetailsto contributeto easierimplementation,the

verificationprocessis moreinterestedin abstracted(i.e., lesscomplex) peerto peeropera-

tion independentof othersystemcomponents.To respondto suchdiversity, we havedevel-2This approachdoesnot offer a true100%validation.Simulationis a partial techniqueandpossiblysome

scenariosarenot studied.

79

opedour SDL with flexibility by merging descriptionandverification-orientedconstituents

underonemodel. Theonly significantdifferencebetweenthedescriptionandverification

models,asindicatedearlieris evidentby removing oneof thetwo STSterminalsto simplify

theverificationprocessandsatisfythepeerto peerrequirement.

4.3.1 Description Constituents

Most of thedescriptionconstituentsresideon theblock (i.e.,configuration)level. It is very

evident that even thoughour larger interestis in the MAC operation,we have chosento

specifylower layerblocks,suchas,thescannerandframingblocksat theSTS.Thereason

is thatwewill beableto observeaclearerdescriptionof MAC operationandits interactions

at the interfacewith the physicallayer by runningsimulationsundersuchcircumstances.

Moreover, in a BSA implementation,numerousscenarioswould always include various

STSjoining andleaving thenetwork. Hence,in our descriptionmodel,we have introduced

two STSterminalsto view theimplicationsof thatontheMCSmanagementoperation.This

helpedusintroducecrucialfunctionalprocessesthatareoutof thescopeof theMAC opera-

tion, but areessentialfor properfunctionality, suchas,theDistributorandtheManagement

Handlerprocesses.Theseprocessesallow thedevisedMCS systemto simultaneouslycon-

trol variousSTS.In thatcase,aclearermulti-terminalbehavior couldberecognizedaswell

throughsimulation.

4.3.2 Verification Constituents

Most of theverificationconstituentsresideon theprocess(i.e., functional)level. Suchan

approachgiveslesscomplex processes,which is very importantastheverificationprocess

consumesalot of memoryresources;i.e.,asimplermodeloffersaneasierverificationproce-

80

dure.Moreover, descriptionof simultaneouseventsthatrequireprecisetiming (ascollision)

is not partof thestandardSDL constructs.It is evenharderto describetheir behavior in a

distributedsystemsuchastheBSA. Eventhoughdescribingsuchbehaviors is still possible,

it will expandmodelcomplexity drastically. To clarify that issuewe presentour approach

in designingthescheduler. In ourschedulerprocessweassumeit randomlyreactsto capac-

ity demandsusingan ANY construct.If the grantbranchis taken, this signifiesavailable

capacity. If thesimulationchoosestheotherbranch,we assumenon-availablecapacityor

a policy violation. For verificationpurposes,we areonly interestedin describingall the

possiblebehavior scenariosandif wedeploy aschedulerthatcounts,addsandsubtracts,we

canexpecta large andcomplex process.Moreover, we will be forcedto utilize a specific

schedulingalgorithm,which is out of thescopeof thedevisedmodelanddeprivesit from

its flexibility .

4.4 Evaluation of SDL Formal Specificationfor BSA

Basedon our efforts in developinganSDL modelfor thedescribedBSA system,we were

ableto evaluatethe utilization of formal methods(advantagesandlimitations), especially

SDL, in thedevelopmentof telecommunicationssystemsin thefollowing:

� EasierSystemImplementation:Systemdescriptionin SDL shouldprovidesignificant

decreasein implementationeffort asit shouldreducethe ratherlarge jump from an

Englishlanguagespecificationto aC program(usuallyusedin softwareimplementa-

tion) implementationthroughanintermediate(semi-technical)model.Moreover, the

possibilityof usingsimulationwill allow implementersto attainaclearerunderstand-

ing of therequiredsystem.

81

� GraphicalHierarchalRepresentation:Oneof themostattractive featuresof SDL lies

in its graphicalrepresentation.Thegraphicalrepresentationpermitsdescriptionsthat

areuserfriendly, easyto understandandmodify. Anotherinterestingfeatureof SDL

is its hierarchalstructureof definingsystems,blocksandprocessesfor building mod-

els. This allows for a clear, representative modeldescription.It alsooffers an easy

procedurefor modeldevelopment.Our BSA model,initially, only involveda single

STSanda MCS at theMAC Managementlevel. Furtherdevelopmentsincludeddata

transferoperation,lowerlayersspecification,BTSdescriptionandmultipleSTSintro-

duction. Moreover, SDL is object-oriented.A station(STS)descriptionwasdefined

onceasa block type. Multiple instancesof thatblock typewill allow for description

of multiple stations.Hence,expandingthescalabilityof themodelis simpleasmore

STSmight besimply introducedby addingnew instancesof theSTSblock type

� SystemAbstraction:As mentionedearlier, abstractionof variousoperationsachieves

simpler, moregeneralandflexible models. We only careaboutpossiblescenarios

ratherthan specificalgorithms. However, due to attainedmodel flexibility , when-

ever specificalgorithmsareneededthey canbeeasilyintegratedwith thedeveloped

models.

� Useof MSCobserversfor validationof specificproperties:MSCobservershavesev-

eralattractive features.MSC is agraphicalinterfaceusedto traceanumberof events

within a process.This allows for a cleardescriptionof thespecificpropertieswhen

definedasMSCobserverscomparedto algebraicformulas.

� SDL Timing: SDL doesnot have standardtiming constructsin its semantics[18].

Thishinderedthepossibilityof attainingacompletesimplesystemspecification.For

82

example, the MAC protocol necessitatespreservation of transmissionsequenceas

definedin MAP frames.Dueto thedifficulty of specifyingthecompleteSYNCoper-

ation,satisfyingsucha requirementover thedistributedsystemis neglectedpertime

framein our model.We canonly guaranteethata terminalwill transmitwithin a cer-

tain timeperiod.However, wearenotableto identify theexacttransmissioninstant.

� Broadcasting:The SDL version(SDL 96) utilized in this modeldoesnot definea

broadcastingor multi-castingfunction.Therefore,to simulateabroadcastingfunction

a datapacket is transmittedon all channelsin thereceiving domain.However, under

suchan assumption,packetsaresequentiallytransmittedratherthansimultaneously

asnormalbroadcastingregularlydoes.

� Delay: In orderto specifythesatelliteroundtrip delay, wehadto introducethesatel-

lite bentpipe block, asSDL canonly specify instantaneoustransmissionor a com-

pletelyrandomdelay. Thebentpipereceivespackets,delaysthemfor acertainperiod

andthenbroadcaststhemon thedown-links. This addsto thecomplexity of thede-

visedmodelwithoutsignificantbenefitsin therequiredMAC specification.

� Validationof PropertiesthatinvolveVariables:Wehavealreadydiscussedthedifficul-

tieswe experiencedin verifying propertiesthatrely on non-messagecriteria(mainly

variables).The involvedpropertiesarecritical to prove theprotocolcorrectnessand

wehadto try to validatethemthroughinspectionaspreviouslydiscussed.

� Behavior vs. Performance:SDL is only concernedwith systembehavior. However,

any telecommunicationssystemmustdefinebothbehavior andperformancerequire-

ments. Systembehavior involves the variousscenariosconductedthroughsystem

operation. Systemperformancedefinesquantitative attributesthat evaluatesystem

83

efficiency. It is a fundamentalcriteriain orderto beableto comparebetweentheper-

formanceof variousprotocolsandidentify the onewith the bestperformancemea-

suresresultssuitablefor optimumoperation.Typical performancemeasuresfor any

telecommunicationssystemincludebandwidthutilization, minimum delay, cell loss

rateandtime jitter, etc. As SDL lacks timing andprobability constructs,it cannot

beusedto verify or evenevaluatesystemperformance.In Chapter5, we introducea

simplifiedperformancemodelfor theprediction-basedCFDAMA protocolto demon-

stratequantitative operationof theutilized accesstechnique.Numerousresearchef-

forts arecurrentlybeingconductedin order to integrateperformanceandbehavior

within SDL [18].

Basedon theabove,asanoverall evaluation,we candeclarethatSDL is a simpleandvery

efficient methodto describeandvalidatetelecommunicationssystemsbehavior despitethe

existenceof somelimitations. In the next chapter, we complementthoseresultsby intro-

ducinganOPNETsimulationmodelfor a simplifiedprediction-basedCFDAMA protocol.

We will usethedevelopedmodelto studytheeffectsof buffer sizeon theperformancein

termsof total delayandcell lossrate.

84

Chapter 5

Effectsof Buffer Sizeon Performanceof

Prediction-BasedCFDAMA

In Chapter3, we have describedthestructureandsystembehavior for theproposedMAC

layerattheair-interfaceterminalsin ourBSA system.Weproposedemploying aprediction-

basedCFDAMA techniqueto improvetheaccessoverthesatellitemedium.It is obviousby

intuition thattheuseof predictionwill improvethedelayperformanceof aDAMA technique

astherequests(demands)alwaysreportfutureneeds.This meansthat the roundtripdelay

of the reservation periodshouldnot contribute to the total delayexperiencedby the data

traffic. However, thelevel of improvementdependson theutilized predictionalgorithm;its

accuracy andprocessingspeed,which is outof thescopeof our thesis.

In non-predictionCFDAMA, thebuffer sizemustbeat leastequivalentto theroundtrip

delay(20 time frames)becausedatatraffic will usuallybe queuedfor aroundthat period

beforethey canutilize theassignmentsrespondingto their demands.Oneof theinteresting

aspectsthat we would like to studyaboutthe prediction-basedCFDAMA is the effect of

buffer sizeon thetotal delayandthecell lossrate.However, aswe have alreadydiscussed

85

in thelastchapter, SDL cannotbeusedto satisfysuchanobjective. In this chapter, we aim

to provide a simplifiedprediction-basedCFDAMA performancemodelto demonstratethe

effect of buffer sizeon the total delayandcell lossrateof the system. We usethe Opti-

mizedNetwork EngineeringTools (OPNET)[19], which is widely usedin simulationand

evaluationof telecommunicationssystems.Resultsobtainedonly serve for demonstration

purposes.We alsoconsiderthe devisedperformancemodelasa nucleusfor possiblefu-

ture researchinterestedin designingoptimumpredictionandschedulingalgorithmsto be

utilized in theBSA system.

5.1 CFDAMA Protocol

For simplicity, wehavedevelopedaprimitiveCFDAMA protocol.As previouslydescribed,

in CFDAMA, theschedulerrespondsto demandsfrom differentterminalsdynamicallyeach

timeframe.Theextracapacityis distributedoverterminalsaccordingto aspecialalgorithm.

For scheduling,we have utilized a first comefirst serve policy. For thefreedataallocation

scheme,wehaveassumeda fair distribution,whereunallocatedcapacitywill bedistributed

oneby oneover registeredterminalsin a rotatingRoundRobinmanner(i.e., a typical free

allocationsequenceto registeredterminalswouldbe2,6,10,7,5,2,6,etc.).For requestoppor-

tunities,we have utilized the following algorithmbasedon theactivity of terminals:Each

registeredterminalhasaninformationrow in alookuptableinsidethescheduler. Thesched-

uler usesthetableto identify theperiodicityof requestopportunitiesaswell asnumberof

time framesleft (usinga time stamp)beforethe next requestopportunity. Whenever the

schedulerreceivesa requestfor bandwidth(indicatingterminalactivity) from a terminal,it

resetstheperiodicityof requestsbackto one(i.e.,allocatesa requestin thenext frame).On

theotherhand,eachtimeframewith norequestsreceivedfrom acertainterminal,thesched-

86

ulerwill decreaseits stampby one.If therequestperiodexpires(stampbecomeszero)with

still no requestsreceived,theschedulerwould increasetherequestperiodicityby one.This

processwould continueaslong asthe terminalremainsdormant(no requestsarereceived

by thescheduler)until a certainperiodicity is reachedafterwhich it becomesconstant.As

specifiedin previouschapters,arequestmightbeexplicit (requestopportunities)or implicit

(piggybacking).Whenever theschedulergrantscapacityfor a terminal,it expectsthat ter-

minal will reportany demandsthroughpiggybackingandremovesthat terminalfrom the

requestopportunitylist (if it is scheduledfor a requestopportunitynext frame).

5.2 CFDAMA ProtocolOPNET Model

We initially extracteda subsetof theSDL modeldescribedin Chapter4. Themodelspeci-

fiesaprediction-basedCFDAMA protocoloperatingunderthenormaltransfermode.Next,

wemanuallytranslatedtheSDL modelinto anequivalentOPNETperformancemodelwith

the necessarymodificationsto introducethe describedschedulingandfree assignmental-

gorithms1. The translationprocesswasa simple direct syntaxtranslation. Figures5.1

and5.2 depictthestructureof thedatamanagingprocessin theSDL andOPNETmodels

respectively.

5.2.1 Model Assumptions

ThedevelopedOPNETmodelis shown in Figure5.3. In modeloperation,we assumethe

following:1As discussedin Chapter4, the SDL modeldefinessimplified schedulingandfree assignmentbehavior

throughemploying randomdecisions.

87

process DATA_MANAGER

DCL ARRIVAL BOOLEAN;DCL CURRENT_ARRIVAL INTEGER;DCL PIGGYBACKING BOOLEAN:=TRUE;DCL DATA_GRANT INTEGER;DCL SENT_DATA INTEGER;DCL ARRIVED_PACKETS QUEUED_DATA_PACKETS;DCL TERMINAL_QUEUE DATA_QUEUE;SYNONYM FRAME_PERIOD=1;

TIMER FRAME_TIMER;

SET (NOW+FRAME_PERIOD,FRAME_TIMER)

WAIT_FOR_MAP_TRIGS

REQ_TRIG

ARRIVAL

FALSE TRUE

REQ(CURRENT_ARRIVAL)

PIGGYBACKING:=FALSE

WAIT_FOR_MAP_TRIG

DATA_TRIG(DATA_GRANT)

TERMINAL_QUEUE

ELSE

SENT_DATA:=SERVE(TERMINAL_QUEUE,DATA_GRANT),TERMINAL_QUEUE:=REFRESH(TERMINAL_QUEUE,DATA_GRANT)

SATISFY AS MUCH ASPOSSIBLE FROMDATA_QUEUE ANDUPDATE

PIGGYBACKING

TRUE

DATA(SENT_DATA,TRUE,CURRENT_ARRIVAL)

FALSE

DATA(SENT_DATA,FALSE,0)

WAIT_FOR_MAP_TRIG

EMPTY

WAIT_FOR_MAP_TRIG

FRAME_TIMER

TERMINAL_QUEUE

EMPTY ELSE

TERMINAL_QUEUE:=UPDATE(TERMINAL_QUEUE) INCREMENT DELAY VALUESAND DROP LONG DELAYED PACKETS

PIGGYBACKING:=TRUE

SET (NOW+FRAME_PERIOD,FRAME_TIMER)

ANYARE THERE ARRIVALSTHIS FRAME

ARRIVAL:=FALSE

PIGGYBACKING:=FALSE

ARRIVAL:=TRUE

ARRIVED_PACKETS:={QUEUED_PERIOD 0,QUEUED_AMOUNT CURRENT_ARRIVAL}

TERMINAL_QUEUE

ELSE

TERMINAL_QUEUE:=TERMINAL_QUEUE//MKSTRING(ARRIVED_PACKETS)

EMPTY

TERMINAL_QUEUE:=MKSTRING(ARRIVED_PACKETS)

WAIT_FOR_MAP_TRIG

Figure5.1: CFDAMA DataManagerfor Terminalsin theSDL Model

1. Successfulregistrationfor apopulationof fiveterminals.Weconductthestudyonthe

initial populationwithout any terminalsjoining or leaving thenetwork.

2. Roundtrip propagationdelayover the satellitenetwork is constantand is equalto

twentytimestheframeperiod.

3. Thetotal bandwidthcapacitypertime frameis equalto 50 slots.

4. Input traffic is distributedaccordingto aPoissondistribution2 .

5. More thanonerequestmessagecanfit into a singulardatatime slot. We assumethat

eachdatatimeslot canfit four requestmessages.

6. Terminalscanonly queuedatapacketsfor N timeframesafterwhichthey aredropped.

2Thechoiceof Poissonis dueto its simplicity andis only usedfor thedemonstrationpurposes.

88

Figure5.2: CFDAMA DataManagerfor Terminalsin theOPNETModel

89

5.2.2 Model Operation

As shown in Figure5.3, thesystemconsistsof a centralschedulerconnectedto five termi-

nalsin a starconfiguration.Eachterminalis representedby instancesof a Map Analyzer

andaDataManager. In orderto initialize thesystem,wehaveimplementedaNew Terminal

processthatfeedstheschedulerwith addressesof registeredterminalsandconsequently, the

schedulerwould initialize eachterminalwith its correspondingaddress.Theschedulerrep-

resentstheMCSfunctionalityandregularlybroadcastsMAP frameseachtime frame.Each

terminalanalyzesthe MAP andaccordingly, transmitstheir requestsandgrantswherever

possible.

For demonstrationpurposes,we assumean input traffic accordingto a Poissondistri-

bution. We have abstractedthepredictionbehavior aspredictionalgorithmsareout of the

scopeof this thesis.In orderto simulatepredictionbehavior, wegeneratetheexpectedtraf-

fic arrivalsearlyby takingasampleof thedistribution. Following that,we introduceanoise

(alsofor simplicity basedonanoisefunctiondistributedaccordingto aPoissondistribution)

to the samplevaluewith the resultbeingthe prediction. The resultingpredictionvalueis

transmittedasa requestwhenever possibleaccordingto normalmodeoperation.Thedis-

tribution sampleon the otherhand,is delayedand is actuallyaddedinto the dataqueues

after20 frames(oneroundtrip propagationdelay)to simulatetraffic arrival. If we assume

that thetraffic distribution hasaninstantaneousvalue�

with a mean�

andthat thenoise

function(usedto generatethesamplenoisevalue)hasaninstantaneousvalue � with amean

� , thereforetheinstantaneousnoisevaluewill be � � = � - � andthepredictedvaluewill

be � =�

+ � ���

90

Figure5.3: CFDAMA SystemConfigurationin OPNETModel

91

5.2.3 PerformanceMeasurements

We have describeda systemwherefive subscriberssharea total capacityof 50 slots. This

impliesamaximumloadperuserof 10slots(i.e., themaximumaveragetraffic asubscriber

cangenerateper time frameis 10). Numerousimportantperformancemeasurescanbeat-

tainedusing this model,suchas,queuingdelay, cell lossrate,bandwidthutilization, etc.

However, asour aim hereis to studytheeffect of buffer sizeon theaveragequeuingdelay

perslot andconsequentcell lossratesat variableloadsof a singleterminal. We have con-

sideredtwo queuedroppingscenarios,whereterminalscanpossiblyqueuepacketsfor upto

5 and10 time frames.Simulationwasrun for a total periodof 40,000time frames.Results

arepresentedin Figure5.4.As indicatedin thefigure,increasingthedropperiod,decreases

thedelaysignificantly. However, eventhoughthelossrateis higher, differencesbetweenthe

two scenariosis minimum. Hence,an importantobjective on thedesignlevel would be to

chooseanoptimumdropperiodto achieve minimumpossibledelaywith leastpossiblede-

teriorationin thecell lossperformance.This resultandconsequentoptimizationprocedures

cannotberealizedusingthebehavioral model.It is thereforeobviouslyessentialto develop

a performancemodel to attain robust specifications.As we have pointedout in previous

chapters,a lot of researchis currentlybeingconductedin orderto integratebehavior and

performance,especially, for real-timetelecommunicationssystems.

92

0

1

2

3

4

5

6

7

8

9

100% 95% 90% 85%

Terminal Load

Ave

rag

e D

elay

per

Slo

t

Max Queue Delay = 5

Max Queue Delay = 10

0

0.005

0.01

0.015

0.02

0.025

100% 95% 90% 85%Term ina l Load

Ave

rag

e S

lot

Lo

ss R

ate

Max Queue Delay = 5

Max Queue Delay = 10

Figure5.4: AverageDelay per Slot andAverageCell LossRatefor OneTerminalunderDif ferentLoads

93

Chapter 6

Conclusion

In this thesis,we have suggestedthe expectedpopularityof future residentialmultimedia

servicesover accessnetworks. We have highlightedthe essenceof developinga new ef-

ficient MAC layer to contribute to successfuldeploymentof theseresidentialapplications

over a satelliteaccessnetwork. Accordingly, we have constructeda systemarchitecture

to satisfysucha requirement.We have presentedprotocolstacksanddescribedprotocol

behavior. Thesystemarchitecturedictatesa simpledistributedschedulingscheme.It also

supportsdynamiccapacityallocation(DCA) over differentialservicecategories. We have

also justified the useof a CFDAMA-basedaccesstechniqueand proposedpossiblefur-

therenhancementby employing prediction.Furthermore,we have alsodevelopeda formal

modelthat canrenderfast implementation.Verificationandvalidationof the modelwere

successfulin confirmingsystemcorrectness.However, theformalmodellackedany capabil-

ity of measuringperformanceattributes.Hence,wehavepresentedanOPNETperformance

model for a simplified prediction-basedCFDAMA protocolanddemonstratedthe effects

of buffer sizeon the performance.The OPNETmodelcanserve asa platform for future

studiesinvolving specificpredictionalgorithms.

94

The field of researchinvolving satelliteaccessis very broadandour work canonly fit

asa singlepartof thecompletepicture. Theresearchissuesarevastandrangefrom phys-

ical layer transmissiontechniquesto upperlayer TCP extensions.Researchon the MAC

layer level alsostill requiresfurtherengineeringstudies.In orderto effectively utilize the

proposeddevisedsystem,accurateyet optimumpredictionalgorithmsmustbedeveloped.

An optimumpredictionalgorithmwill achieve bestaccuracy in minimumprocessingtime.

Anotherfield of possibleresearchinvolvesstandardizationof thedifferentialservicecate-

gories. Minimum differentialcategoriesthat canconvey all possibleupperlayersconnec-

tionsaredesired.Thenumberof differentialcategoriesrepresentsthemaximumnumberof

connectionswith theschedulerperterminal. Thesmallerthenumberof categoriesdefined

(while maintainingtherequiredQoSof all upperlayers’connections),theeasierandfaster

schedulerprocessingwill be. Finally, we canidentify traffic admissionandpolicy control

algorithmsaspossiblecandidatesfor researchthat canalsocontribute to enhancesystem

operationandefficiently utilize resources.

95

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