An Experimental Multimedia Communication System

ETHMICS is a testbed to investigate various aspects of multimedia communication in a high speed networked environment such as B-ISDN. With a view to realistic application experiments, a multimedia workstation has been developed, yielding new insights about workstation architecture and local access arrangements. In terms of software, ETHMICS is based on an object-oriented application framework. This framework has, e.g., allowed to develop a videophone application as one of its first derivatives. Hardware and software developments have been complemented by some fundamental studies on the impact of multimedia QOS requirements on the strategies for scheduling workstation and network resources. Basic Objectives and Initial Design Considerations With the ETHMICS project, we try to explore new forms of communication based on high speed networks such as B-ISDN. In particular, we investigate the impact of multimedia interpersonal communication on both end system and network architecture, aiming at a well balanced overall design reflecting high quality of service standards and attractive features for the user [1]. Our studies rely on a set of basic assumptions as follows: • Communication functions will be integrated into workstations and PCs, as these emerge to be the ubiquitous tool on the desk of most professionals and, ever more, for the residential user as well. • Although many applications may satisfactorily be restricted to voice, data and still pictures, there is a growing need for video, e.g., for desktop video conferences or display of computer animated graphics. • Most users will require that communication functions should not interfere with the performance of applications provided locally. Also, many applications call for more than just a single video window. Thus, in order to satisfy rather demanding performance requirements, we must depart from a workstation architecture derived from the classic Von Neumann machine to avoid critical bottlenecks such as the traditional workstation bus. • Expectations in terms of QOS, user friendliness and transmission quality (e.g., resolution, voice bandwidth, delay, etc.) call for very high performance hardware and consistency in user interface design across different applications. Object-oriented application frameworks have paved the way to meet the latter requirement, and they contribute towards better portability across different endsystem platforms. Figure 1: Framework for Studies on Multimedia Communication Our studies are performed in the framework illustrated by figure 1. Though this figure implies a top-down approach, it is obvious that many iteration cycles are necessary to reach an optimized design. In that sense, the present contribution concentrates on those items that are especially marked in this figure. The Architecture of a Multimedia Workstation Several dedicated functional units need to be integrated in a Multimedia Workstation in order to handle various media. A straightforward extension of present workstations could be realized by connecting several media interfaces to the system bus, as shown in figure 2. This would imply that all the real-time data flow would be routed through the workstation itself, on the system bus. A simple analysis of the data flow in terms of data rate, maximum latency and number of sources, clearly shows that any conventional system-bus would be overloaded, thus paralyzing the entire system. A more thorough analysis reveals that there are actually two bottlenecks: • the point where all the data streams are connected to the communications interface • the access of all the video data streams to the display unit. Figure 2: An Architecture with Media-Interfaces Directly Coupled to the System Bus We will elaborate on these two issues in the following sections. The Video Architecture A generic hardware architecture, which supports the handling of multiple video data streams [2], is shown in figure 3. Figure 3: The Video Hardware Architecture The analog to digital converters (ADC) generate the sampling clocks locked to each video source, implying that each camera and its corresponding ADC, as a source, can also be replaced by any video data stream transmitted asynchronously from a remote source. The scaling units perform the transformation of the picture size in real time to the format selected by the user. The entire scaling mechanism and the positioning are controlled through the window control unit. Window-Control and Synchronization are key elements to provide positioning, updating and synchronization of the multiple video-windows. application scenarios application services distributed application architecture application models quality of service network architecture network models performance evaluation information types / presentation source coding workstation architecture workstation models Video Source 1