The Architecture and The Transformation Approach

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:

1. the attributes of MMOs, including
   • the size of each MMO
   • the type of each MMO
   • the structure of MMOs
2. the hardware specification that describes
   • the type of the machine
   • the capability to display graphic objects
   • the capability to display image objects
   • the capability to playback audio objects
   • the capability to playback video objects
   • the type of display attached to the machine
   • the type of audio system of the machine
   • the capacity of the hard drives of the machine
   • the bit rate of the communication network
   • the quality of service (QOS) [2, 8, 20] of the communication network and the overall system. A request for transmission can be accepted only when the transmission can be guaranteed under the current specification of the QOS.
   • the IP address of the user
3. the transmission scenario specified by the user, such as
   • which objects in the MMOs are to be transmitted
   • the timing requirement
   • the quality of service requested
   • the heuristics to be invoked in order to negotiate with the QOS manager. For example, in case the network cannot guarantee the transmission due to a bandwidth constraint, progressive transmission [26] for objects having types ``image" or ``video" might be employed to transmit a low-resolution image/video first, with a smaller size in comparison with the original object, and then the refined image/video progressively.

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.

User Interface

User Interface module deals with the domain-specific applications of DMS. Possible applications could be: delayed teleconferencing [11] for the virtual office, virtual library and virtual laboratory. User Interface module is closely related to applications. There are two kinds of user interfaces: Generic user interface and application specific user interface. Generic user interface is application independent, serving as a user interface with the kernel DMS. Application users may not have access to this interface if an extra application specific user interface and application layer are built upon it. Four basic functions are provided in the generic user interface: MMO editing, MMO browsing, knowledge generation, and MMO presentation.

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.

Object Exchange Manager

Object Exchange Manager(OEM) maintains an Object Exchange Format(OEF) which contains all information of an MMO. It is a key module in the DMS system which interacts with all other modules of the system. OEM will be discussed in depth in Section 6.

Integration and Synchronization

Integration and Synchronization module looks into the overall properties and behaviors of an MMO. It deals with spatial as well as temporal relations within an MMO. It also takes into account the communication and synchronization requirements. It performs MSS to MDS, and MDS to MCS transformations.

Network Management

Network Management module consists of a set of network primitives carrying out MMO transmission. The supported network primitives are: connection establishment, connection closing, message sending, message receiving, and synchronization. It provides the network service as the result of negotiation with the underlying transport service provider.

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.