The VuNet hardware has been adapted to the needs of the VuSystem programming model. The following issues motivated the design of the hardware infrastructure:
Since the network peripherals are highly programmable, all peripherals on the network can be easily shared between all hosts connected to the network. Traffic such as video can be directed to compression servers which could be programmed to deliver the data directly to a network based storage device. Video data could be channelled to the host workstation in compressed or uncompressed form.
Sophisticated functions such as ATM adaptation layers, link table setup, and flow control are pushed to the edges of the network and into the clients. This simplifies the design of the network hardware and allows flexibility in implementing such functions in host software. The hardware can then be easily designed to efficiently handle the multimedia streams. Multiple video windows can be displayed by simply starting another video process on the host.
An easy to use client interface is important so that workstation-network communication is efficient. Clients can be designed with ease; in combination with software flexibilty, clients can be designed to be simple and highly flexible.
Network communication should be as transparent as possible to the application. All classes of traffic (video, audio, file transfer, etc.) are treated in a similar manner until they reach the specific target application. Multiple video streams are easily supported since hardware limitations are not placed on the number of video streams.
Interactive applications based on the ViewStation programming model process data during scheduler time slices. Therefore their consumption and generation of media data is inherently bursty. Accordingly, sources and sinks of video and audio information must be able to handle data in bursts and the VuNet and its hosts must support bursty traffic. This model, in which the network and devices accomodate the bursty patterns of applications and their users, differs substantially from other models in which the application is expected to adapt to continuous media streams.
A key aspect of the ViewStation programming environment is the support for graceful degradation of audio and video processing as workstation resources become scarce. One such example of this is a temporary decrease in frame rate in a video viewer application as disk reads occur. This requirement is much more of an issue for video due to the data rates involved. Therefore, VuNet-based video sources must support a mechanism for throttling their frame rate and the VuNet must allow for efficient cross-network control of video sources.
One important benefit of our software-intensive approach is the ability to port our system to higher performance workstations easily. The life cycle for designing a dedicated hardware multimedia system is longer than that of the workstation model cycle. Hence, we wanted a system in which we could easily take advantage of faster workstations when they became available.
An ATM platform provides the ability to interface to a variety of other systems. Providing a seamless interface between the local and wide area allows any other systems compatable with ATM to run all ViewStation applications.
