Common usage of the term ``multimedia'' is to describe systems which incorporate both traditional computer data forms, such as text and graphics, and data forms such as audio and video. This presents new problems due to both the isochronous requirements for audio and video and the significantly higher bandwidth that is needed to transport video. The wish to link machines which use this mix of data types is one of the driving forces behind the development of high speed networks based on the Asynchronous Transfer Mode (ATM); these aim to provide the necessary performance and guarantees about the timeliness of data transfers.
The Asynchronous Transfer Mode makes use of small fixed size cells to carry data through the network. It is connection oriented and each cell contains a label to identify to which of the multiplexed communications the cell belongs. The emerging standards [\bf\protect\citenameATMUNI93][\bf\protect\citenameCCITT90] subdivide the label into a virtual path and a virtual channel; within this paper, this subdivision is not significant and so the term circuit identifier is used to mean the whole of the label. As scheduling of both links and switches is performed on a per cell basis, the small size enables preemption to be performed at a fine granularity - such techniques are also common in many modern high speed workstations, where buses are designed to enable cache access to preempt long DMA transfers. This property is used in the network to offer a range of Qualities of Service (QoS) and support a spectrum of traffic, from isochronous to very bursty.
Designers of multimedia workstations must address many of the same problems. In particular there is a large amount of data, which is often sensitive to jitter, moving between the peripherals of the end-system and the network interface. This phenomena has been recognised for many years and has been termed ``Intensive I/O'' [\bf\protect\citenamePasquale92]. Currently multimedia systems concentrate on the presentation of the data to a human user, therefore requiring tight timing constraints. However, the ever increasing CPU power available in a desktop machine also enables applications which process such data in real time; while some of these may be more tolerant to jitter than the human eye, they can only serve to increase the bandwidth requirements.
A workstation architecture which aims to effectively support these forms of applications should therefore enable multimedia data to be delivered directly to peripheral end-points with minimal or no interaction by the CPU, while continuing to permit processing of the data by the CPU where required.
The approach that we have developed is to extend the use of ATM to the device and processor interconnect within a multimedia workstation [\bf\protect\citenameHayter91]. This interconnect, which we call a Desk Area Network (DAN), uses ATM cells as the basic unit of data transfer. The aim is to provide improved system performance by the use of a device and processor interconnect which:
Similar work is in progress in the ViewStation project [\bf\protect\citenameAdam93], and [\bf\protect\citenamePfeifer93], where again ATM is used as the basis for a multimedia machine. The assumption that future high speed networks will be packet based [\bf\protect\citenameCohen92] has led to an approach like the DAN but using packet switching internally [\bf\protect\citenameFinn91].
This paper presents the architecture, describes the demonstrator that has been built and presents results from the initial experiments performed using the system.