Hypermedia Synchronization Modeling: A Case Study 1

Abstract: The semantics of hypermedia documents depends mainly on the satisfaction of bothlogical and temporal synchronization constraints. Therefore, the specification and verificationof temporal and logical properties of hypermedia systems is a fundamental stage that must bedone before implementation. In this paper we propose a formal model, called HTSPN (Hierar-chical Time Stream Petri Net), that allows a complete and accurate formal specification of bothtemporal and logical synchronization within hypermedia distributed systems to be done. More-over, this model provides means for the simulation and analysis of hypermedia systemsbehaviour. In this paper we show, through a case study, the contributions of the HTSPN formalmodel for hypermedia system design. 1 Introduction Hypertext is a non-linearly organized information structure presented for browsing in a quickly-accessed,electronic form. In such organization, data are structured as a network of nodes connected by links. Nodes containinformation units, made up text and pictures. Links define relationships between nodes. Hypermedia is a naturalextension of hypertext where nodes contain multimedia information (e.g. text, audio, video, graphics, images).Nowadays, there are several available on-line systems and laboratory prototypes of hypermedia systems. Thelatter are based on different concepts and models. To solve this problem, several multimedia/hypermedia modelsand interchange standards are being proposed; they define syntactic and semantic concepts in order to implementdistributed open systems. The widely accepted Dexter hypertext reference model is an attempt to capture, bothformally and informally, important abstraction found in a wide range of existing and future hypertext systems[Halasz&Schwartz, 1994]. In parallel, international standards such as HyTime [Newcomb et al.,1991] and MHEG[MHEG, 1994] are being developed for representing and interchanging multimedia/hypermedia information.Because of their complex information space, hypermedia systems are source of «users disorientation andinformation overload» [Conklin, 1987]. The complexity in hypermedia systems design increases with the need ofsynchronization mechanisms, to ensure the correct temporal playing of dynamic multimedia scenarios. Specify-ing hypermedia systems by means of informal or semi formal models may be a misunderstanding source and donot allow the use of standard analysis tools to ensure both the logical and the temporal consistency of hypermediasystems. Conversely, formal models allow the construction of an accurate and unambiguous semantics to hyper-media. Moreover, formal models allow powerful verification, simulation and analysis methods to be appliedbefore implementation. Therefore, formal specification techniques are able to bring important contributions to theemerging and still not very steady concept of hypermedia systems.The first part of this paper introduces the Hierarchical Time Stream Petri Net formal model (HTSPN)intended to provide a complete and accurate specification of both temporal and logical synchronization withinhypermedia distributed weakly synchronous systems [Senac & al., 1995a]. This new model is a hypermediaextension of the TSPN (Time Stream Petri Nets) model [Senac & Diaz, 1994], already used for the specificationand verification of weakly synchronous distributed multimedia systems. The HTSPN model provides an easy andconcise modeling of asynchronous events in hypermedia systems with both hierarchical design capabilities and