models and of methodologies based on the SWN models supporting the design of systems. The practical interest of SWNs is witnessed by the several case studies present in the literature showing the applicability of the SWN formal-ism in different fields, e.g. control systems[2, 4], communication systems[13, 15], contact centers[12], fault tolerant systems[5, 8], etc., to study different aspects, from verification of properties, to performance and dependability analysis.
References
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[2] C. Anglano, S. Donatelli, G. Franceschinis and O. Botti, “Performance prediction of a reconfigurable high voltage substation simulator: a case study using SWN” In Proc. 7th International Workshop on Petri Nets and Performance Models, St. Malo, France , June 1997.
[3] P. Ballarini, C. Donatelli, and G. Franceschinis. Parametric stochastic well-formed nets and compositional modelling. In Proc. of the 21 th International Conference in Application Theory of Petri Nets (ICATPN 2000), volume 1825 of Lecture Notes in Computer Science, pages 43–62. Springer-Verlag, 2000.
[4] S. Bernardi, S. Donatelli, and A. Hor´ ath. Compositionality in the Great-SPN tool and its use to the modelling of industrial applications. Accepted for publication on Software Tools for Technology Transfer.
[5] A. Bobbio, G. Franceschinis, L. Portinale, and R. Gaeta, “Dependability Assessment of an Industrial Programmable Logic Controller via Parametric Fault-Tree and High level Petri Net” In Proc. 9th International Workshop on Petri Nets and Performance Models - PNPM01. IEEE Computer Soci-ety, 2001.
[6] L. Capra, C. Dutheillet, G. Franceschinis and J.M. Ilie, “Towards Per-formance Analysis with Partially Symmetrical SWN”, In Proc. 7th Inter-national Symposium on Modeling, Analysis and Simulation, College Park, MD, USA, October 1999.
[7] L. Capra, C. Dutheillet, G. Franceschinis and J.M. Ili` e, “Exploiting Partial Symmetries for Markov Chain Aggregation ”, In Proc. of First workshop on Models for Time-Critical Systems (MTCS 2000), Satellite workshop of CONCUR2000, Electronic Notes in Theoretical Computer Science, Volume 39, Issue 3
[8] L. Capra, R. Gaeta and O. Botti, “SWN Nets as a Framework for the
Spec-ification and the Analysis of FT Techniques Adopted in Electric Plant
Au-tomation”, In Lecture Notes in Computer Science, Vol. 1630: Application
and Theory of Petri Nets 1999, 20th International Conference, ICATPN’99, Williamsburg, Virginia, USA, pages 168-187. Springer-Verlag, June 1999.
[9] G. Chiola, C. Dutheillet, G. Franceschinis, and S. Haddad. “Stochastic well-formed coloured nets for symmetric modelling applications” IEEE Transactions on Computers, 42:1343–1360, 1993.
[10] G.Chiola, C.Dutheillet, G.Franceschinis, and S.Haddad, “A Symbolic Reachability Graph for Coloured Petri Nets”, Theoretical Computer Sci-ence B (Logic, semantics and theory of programming), Vol. 176, n. 1&2, April 1997, pp. 39-65.
[11] G. Chiola, G. Franceschinis, R. Gaeta, and M. Ribaudo. “GreatSPN 1.7:
GRaphical Editor and Analyzer for Timed and Stochastic Petri Nets” Per-formance Evaluation, 24:47–68, 1995.
[12] G. Franceschinis, C. Bertoncello, G. Bruno, G. Lungo Vaschetti, A. Pigozzi,
“SWN models of a contact center: a case study” In Proc. 9th International Workshop on Petri Nets and Performance Models - PNPM01. IEEE Com-puter Society, 2001.
[13] G. Franceschinis, A. Fumagalli and A. Silinguelli “Stochastic Colored Petri Net Models for Rainbow Optical Networks” Special issue of Advances of Petri Nets on Communication Network Applications, Lecture Notes in Computer Science, Springer Verlag, LNCS 1605, April 1999.
[14] R. Gaeta, “Efficient discrete-event simulation of colored Petri nets” IEEE Transaction on Software Engineering, 22(9), September 1996.
[15] R.Gaeta and M.Ajmone Marsan, “SWN Analysis and Simulation of Large Knockout ATM Switches” volume 1420 of LNCS, Proc. of 19th Inter-national Conference on Application and Theory of Petri Nets. Springer-Verlag, June 1998.
[16] S. Haddad, J-M Ilie, and K. Ajami, A Model Checking Method for Partially Symmetric Systems, Proc. of of FORTE XIII, Pisa, Italy, October 2000.
[17] S. Haddad, J-M Ilie, M. Taghelit, and B. Zouari, “Symbolic marking graph and partial symmetries”, In Proc. of 16th Int. Conference on Application and Theory of Petri Nets, ICATPN ’95, pp. 238-257, Torino, Italy, June 1995.
[18] S. Haddad and P. Moreaux, “Evaluation of High Level Petri Nets by Means of Aggregation and Decomposition” In Proc. of 6th International Workshop on Petri Nets and Performance Models, N. Carolina, USA, pages 11-20.
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Sys-tems. PhD thesis, University of Edinburgh, 1997.
LinLiuandJonathanBillington
ComputerSystemsEngineering Centre
UniversityofSouthAustralia
SCTBuilding,MawsonLakesCampus,MawsonLakes,SA5095,Australia
liuly002@students.unisa.edu .au, jonathan.billington@unisa.edu. au
Abstract. Thispaperanalysespartof ITU-TrecommendationH.245,\Control protocolfor
multimedia communication". This is a pilot study of an ongoing project on modelling and
analysing Internet multimedia communication standards with Coloured Petri Nets (CPNs).
TheCapabilityExchangeSignalling(CES)protocolofH.245ismodelledwithCPNs.Analysis
ofthemodelsshowsthatthisprotocolperformswellingeneral,butsomeinadequaciesalsohave
been found.Firstly,this protocolcould failifthe wrappingofthe sequencenumbersusedby
theprotocolcanhappen,nomatterwhethertheunderlyingmediumofthisprotocolisreliable
or not.Secondly,if theproblem withsequencenumberwrap canbeavoided,then,whenthe
transportmediumisunreliable,thisprotocolmaybeineÆcient.
1 Introduction
RecommendationH.245 [8]is thecontrol protocolfor multimedia communication developed by the
TelecommunicationStandardizationSector ofInternationalTelecommunicationUnion(ITU-T) and
has been used by a series of ITU-T multimedia system standards.These include recommendation
H.310[3]forbroadbandaudiovisualcommunication,H.324[4]forlowbit-ratemultimedia
communi-cation,andH.323[6]forpacket-basedmultimediacommunication.
With therapidgrowthoftheInternet,techniques andstandardsformultimedia communications
overpacket-basednetworksareof increasingimportance.Atpresent,H.323isthekey
recommenda-tionformultimediaapplications,e.g.IP(InternetProtocol)telephonyandmultimedia conferencing,
overIP-basednetworks,includingtheInternet[9].H.323isaseriesofrecommendationscomprising,
besides H.323itself, H.225.0 (callsetup and admission control) [5],H.245 (mediachanneland
con-ferencecontrol)[8],andotherrelatedprotocols[15].TheH.323seriesofrecommendationsdescribes
systems,logicalcomponents,messagesand procedures that enablereal-timemultimediacalls to be
establishedbetweentwoormorepartiesonapacketnetwork.Thepacket networkisnotrequiredto
provideaguaranteedQualityofService(QoS).TheH.323seriesalsospeciestheinteroperationwith
multimediasystemsoverdierentnetworks,e.g.theIntegratedServicesDigitalNetwork(ISDN)and
thePublicSwitched TelephoneNetwork (PSTN).
Theresearchpresentedhereispartofaresearchproject[11]onmodellingandanalysingtheH.323
standardwith Coloured Petri Nets(CPNs) [10]. Previouswork on modelling and analysing
multi-media communicationsystems using Petri net techniques has concentrated on multimedia streams
synchronization[1,14] and QoS issues [16]. Little work has been done on protocols such as H.323
whicharerelatedtothegeneralarchitecture,controlandproceduresofmultimediasystemsoverthe
Internet.ThisproposedresearchisexpectedtoexpandtheapplicationdomainofPetrinettechniques
into the Internetmultimedia communication areaand to investigate theanalysis techniques which
aremostappropriateforthevericationofInternetmultimedia communicationsystems.
H.245 is chosen as the rst standard to be modelled and analysed in this research, and some
initial results have been presented in [13]. Although there is some ambiguity in the denition of
H.245,comparedwithother recommendationsoftheH.323series,H.245ismorematureandbetter
described.Moreover,H.245isoneofthecoreprotocolsoftheH.323series.Hencewerstinvestigate
H.245tolaythefoundationsforthestudy ofother H.323protocols.
In[13],aninitialCPNmodeloftheCapabilityExchangeSignalling(CES)protocolofH.245was
createdandanalysed,anditwasassumedthatthetransportlayeroftheCESprotocolisreliable.The
majorpartof[13],however,isonthemethodologyformodellingandanalysisofInternetmultimedia
aremadeto theinitialmodelpresentedin[13],which isre-analysedwhen theunderlyingtransport
medium is reliable. Moreover, the CES protocol is also analysed under the assumption that the
underlyingtransport mediummaybeunreliable.Thepropertiesofthisprotocolareinvestigatedby
statespaceanalysis of theCPN models andsomeinterestingresultsareobtained.The models and
theanalysisarediscussedindetail.
This paperisorganisedasfollows.Section2reviewstheCESprotocol.Sections 3and4present
theCPN modelsoftheprotocolandtheiranalysisresultswhentheunderlyingtransportmedium is
reliableandunreliable respectively.Finally,Section 5 summarisesthe resultsandpointsoutfuture
directionsofresearch.
2 The CES Protocol
Sincedierentparties involvedin amultimedia call mayhavedierenttransmitand/or receive
ca-pabilities, theyneed to maketheir capabilities known to each other. Thus themultimedia streams
sent by an end can be understood appropriately by its peer end(s). The CES protocol is used by
amultimedia communication end to inform a peer end of its capabilities. It should be made clear
that, although this protocol is named as capability exchange signallingprotocol,it is not used by
twoendstoexchangeand/ornegotiatetheircapabilities.Theinitiatorofthesignallingsendsoutits
capabilities and expects an acknowledgmentfrom the responder side. Once the responder receives
thecapabilities, itisonlyexpectedtoacknowledgetheinitiatorwhether itcanacceptthemornot.
Meanwhile,therespondersidedoesnotsenditsowncapabilitiestotheinitiatorside.Tobeconsistent
with[8],however,westillusetheterminologyexchangeinthefollowing.
As dened in [8], the CES protocol consists of aset of capability exchange messages and
pro-cedures,and the CES Entities (CESEs). There is an outgoingCESE and an incoming CESE. For
aparticular capabilityexchange, anoutgoing CESEis active atthe initiator sideand an incoming
CESEisactiveat theresponderside.
TheH.245protocols,includingtheCESprotocol,aredesignedtobeindependentoftheunderlying
transport medium [8], which means that they can operate over either a reliable or an unreliable
transportlayer.WhentheseprotocolsareusedinanH.323system,however,theyarerequiredtorun
aboveareliabletransportlayer,e.g.aTCP(TransmissionControl Protocol)connection.
This section introducestheCES protocol and clariesthe inconsistencies foundin theprotocol
denition.Sections 2.1 and 2.2 describe theCES protocol as it is dened in H.245 [8]. Weuse the
sameheadings for these two sections (and their lower level headings) asthose used in [8]. Section
2.3liststheinconsistenciesfoundin theCESprotocoldenition.Allofthetablesandguresinthis
sectionaretakenfrom [8].
2.1 Communicationbetween CESE and CESEUser
2.1.1 Primitive Denition
Table1denestheCESprimitivesandtheirparameters.TheCESEandtheCESEusercommunicate
using these primitives.The four TRANSFER primitivesare used to transfercapabilities. Thetwo
REJECTprimitivesareusedtorejectacapabilitydescriptorentry,andtoterminateacurrent
capabil-itytransfer.PrimitivesTRANSFER.requestand TRANSFER.indicationhavethesameparameters.
PROTOIDspeciestheversionoftherecommendationinuse.MUXCAPindicates themultiplexing
capabilities ofthe outgoingend, and themultimedia receiveand transmitcapabilities are given by
CAPTABLE and CAPDESCRIPTORS. The CAUSEparameter of a REJECT primitive indicates
thereasonforrejectingaCAPTABLEorCAPDESCRIPTORSparameter.TheSOURCEparameter
oftheREJECT.indicationindicatesthesourceoftherejection,eitherUSERorPROTOCOL.
2.1.2 CESE States
Therearetwostatesdened foranoutgoingCESE:IDLE (readytoinitiate acapabilityexchange);
andAWAITINGRESPONSE(waitingforaresponsefromthepeerCESE).Similarly,twostatesare
denedforanincomingCESE:IDLE;andAWAITINGRESPONSE,whichindicatesthattheCESE
iswaiting foraresponsefrom itsuser.