Recent advances in high speed communication networks, mass storage, digital video, data compression, as well as the advocacy of the Information Superhighway by the government, have stimulated the research and development of distributed multimedia systems (DMSs). One of the challenges in the design of a distributed multimedia system is to devise a suitable specification model for various schemas in different levels of the system. Another important research issue is the integration and synchronization of heterogeneous multimedia objects (MMOs).
Related works regarding the timing specification, i.e., the synchronization, of multimedia objects for the purpose of presentation can be found in [1, 9, 12, 15, 16, 25]. The synchronization and integration of multimedia objects in the communication network layer of multimedia systems has also been discussed in [15, 16, 17, 18, 25]. In order to satisfy the requirements, for example real-time delivery, of some multimedia applications, the quality of service (QOS) support of the communication network and of the operating systems has been an important research topic [2, 17, 19, 20, 23]. The quality of service is also considered in the presentation layer of the multimedia systems [8, 24].
In our view, there are three different multimedia schemas in the DMS. The Multimedia Static Schema (MSS) specifies the static structure of the composite multimedia objects, including the temporal and spatial relations among the objects (see Figure for an example). The Multimedia Data Schema (MDS) specifies the properties of MMOs, the properties of a set of objects as a whole, and the temporal relations between the objects, thus supporting the integration and synchronization of MMOs (see Figure for an example). The Multimedia Communications Schema (MCS) is derived from MDS by adding further communication control and synchronization requirements in order to satisfy the capacity constraint of processors, bandwidth constraint of the communication network, as well as to effectively utilize the service of the communication network (see Figure for an example). Finally, the network primitives are derived from MCS to perform the transmission of MMOs [13, 14].
A hypergraph model is suitable to specify the structure of composite MMOs  and is employed to specify the MSS in this paper. The G-Net model [6, 7], a Petri net  based model, is intended for the design and specification of complex information systems . Petri net based models have been proposed to serve as a suitable model for the unified framework to specify different levels of the DMSs [16, 29] and the G-Net model is adopted in this paper as the model for both MDS and MCS. Thus, the transformation from an MDS to its corresponding MCS can be formulated as the G-Net transformation from the G-Net description of MDS to that of MCS [13, 14]. The special hierarchical feature of the G-Net [6, 7] is utilized in the transformation.
A key module in a distributed multimedia system is the Object Exchange Manager (OEM). In order to exchange multimedia objects in a distributed system, a uniform representation, such as MHEG (Multimedia and Hypermedia Information Object Expert Group) , is needed to maintain all information of an MMO. The Object Exchange Manager maintains and manages the uniform representation and interacts with other system modules.
Our approach is neither to design a communication network protocol to support QOS, nor to develop a powerful multimedia authoring tool. What we propose is a novel idea of the feasibility of the transformation between multimedia schemas, and the transformation will provide a unified framework to support a link from application layer, synchronization and integration in the presentation layer, down to the communication network layer. The paper is organized as follows. In Section 2, the architecture of a DMS and the transformation approach are presented. The models to specify multimedia schemas and the transformation algorithms are described in Section 3. An example that illustrates the transformation algorithms is presented in Section 4. Section 5 describes the transformation from the MCS to the corresponding network primitives that will accomplish the transmission of the MMOs. The design and implementation of the OEM is described in Section 6. Section 7 describes the implementation of our experimental multimedia system, based on the transformation approach and the object exchange manager. Section 8 presents the conclusions and discusses the future research issues.