The architecture of a distributed multimedia system can be divided into three layers: the application layer, the system layer, and the communication layer. Figure depicts the three layers and the relation among the layers. When a user invokes the application, such as a multimedia message/mail system, and initiates an MMO instance, an MSS is created, for example, by a multimedia editor. The MSS is then transformed into the corresponding MDS to support the presentation of MMOs and to serve as the input for transformation in the next phase. Subsequently, the transformation rules are applied to transform the MDS to its corresponding MCS, and then to the network primitives to conduct the transmission of the MMOs according to the specification [13, 14].
Figure: Transformation between multimedia schemas in a distributed multimedia system.
Due to the heterogeneous characteristics of MMOs [25] and various platforms which invoke multimedia applications, the specification of the transmission of MMOs involves the following factors:
The block diagram of the architecture of a distributed multimedia system and
Figure: Architecture of a distributed multimedia system.
transformations between multimedia schemas in the system is depicted in Figure .
This architecture features a unified approach for the modeling of multimedia application, presentation and communication, as a structure consisting of the G-Net specification, attributes, procedures and relations. The system consists of the following modules: User Interface, Object Exchange Manager, Integration and Synchronization, and Network Management.
MMO editor provides facilities for editing an MMO hypergraph structure. By providing a set of drawing functions, it enables users to create, add, and delete nodes or links in the MMO hypergraph structure. It is a user interface that gets the MMO hypergraph structure from the user. The obtained hypergraph structure is stored in an internal C structure and can then be translated into the Object Exchange Format (OEF) in ASCII (see Appendix III).
MMO browser lets users walk through the MMO hypergraph structure created by MMO editor. If one is interested in some node or link, he/she simply selects(clicks on) that node or link. All attributes of that node or link are then displayed.
Generator is a knowledge acquisition facility. By providing users with a user interface, it gets knowledge associated with an MMO instance and converts it into a script of rule set. Knowledge is represented as a (condition, event, action) triple internally. But users need not be bothered to understand this internal representation.
The presentation module presents (displays or plays) the MMO by following the specification in MDS and accessing the MMO hypergraph structure. The MDS and the hypergraph structure of the MMO, i.e., MSS, are obtained from the Object Exchange Format, which in turn may come from a remote site via network.
As shown in the figure, a user can invoke a Multimedia Editor/Browser to compose MMOs. After the composition, MSS of the composed objects is generated and sent to the Integration and Synchronization Module for transformations into MDS and MCS. The Multimedia Viewer can then utilize the MDS to view the presentation of the MMOs just created. If the QOS manager cannot guarantee the transmission based on the MCS, the Integration and Synchronization Module has to modify the MCS to negotiate with the QOS manager. After the negotiation is achieved, the MCS will be transformed into network primitives to perform the transmission. By employing the concept of object exchange [5], the Object Exchange Manager is devised to maintain and manage the Object Exchange Format [28] and to interact with the other modules in the system.