In this section, we will show and explain the unusual architecture of the Pegasus system. The system consists of workstations and servers, interconnected by an ATM network. We use an ATM network as it can provide high bandwidth and low latency. ATM networks can scale gracefully to large sizes and link bandwidths and very large aggregate bandwidths.
Multimedia systems need special hardware for input and output of digital audio and video. Once digitized, video and audio streams must be transported to where they are processed, stored or rendered. Video requires substantial, but not staggering bandwidths: using frame-by-frame compression, for instance with JPEG, a video stream requires no more than a megabyte per second. Modern networks can easily provide this bandwidth. Using compression methods that compress groups of frames, such as MPEG, much higher compression can be reached, albeit at the cost of higher end-to-end latency. Audio has modest bandwidth requirements compared to video, but is much more susceptible to jitter, that is, the irregularities in the transport and processing times.
For smooth and efficient handling of interactive digital audio and video, the paths between origin and destination must be as short as possible. Gratuitous processing and transportation increase the end-to-end latency and hence decrease the quality. Thus, it is desirable that audio and video data are not handled by operating-system and application code except when application-specific processing is being carried out.
Figure 1 shows an important aspect of the Pegasus architecture -- the target end-system architecture. The figure shows a conventional workstation and its network interface connected to an ATM switch. However, also connected to the switch we see a camera device, a display device, an audio device, and then the rest of the ATM network. The important point is that the switch is under control of the workstation, that is, all connections through the switch are managed by the workstation, so that the workstation is also in control of the multimedia devices. This setup is much like that of the Desk-Area Network [.hayter 1991.]. However, in a real DAN, an ATM switch fabric actually forms the central backbone of the workstation itself; CPU, memory and devices all communicate via the switch. The Pegasus project, partly because of its time frame of only three years, uses a conventional bus-based architecture for its processor devices, but uses the DAN mechanism for connecting multimedia devices.
In this architecture, when video flows from a camera in one system to a display in another -- as is the case in video-phone and video-conferencing applications -- no processors need to process any video data. This goes for the audio data too, of course. Hence the processors in the workstations, at both the camera and display, only need to manage the connections and devices.