The design of a multimedia interactive conferencing system based on widearea networks as a communication infrastructure is discussed. Co-operation activities take place by sharing application packages installed in remote servers and workstations. The necessary upgrades to the existing communication protocols which make them support multicast connections are described in detail.

The DUAL network is based on multi-microprocessor nodes mounted on a Multibus chassis and interlinked by means of 140 Mbit/s fiber optics trunks. The DUAL ring is installed in a 160-hectare campus and serves as backbone network to a number of Ethernet TM, ISO 8802.X LANs and X25 networks. As well as acting as backbone network to other existing networks, DUAL itself provides LAN, X25 and other advanced services. Indeed it has been designed with the ambition to offer not only conventional network services (datagram and connections) but also unique multicast (radios and conferences) and synchronization primitives (semaphores) in support of distributed and parallel processing on a large scale.

The Virtual Terminal approach has been used by first by the ARPANET in the U.S. In the last few years, the problem of defining Virtual Terminal Protocols has been approached by several international organizations such as EIN, Euronet, IFIP and by national organizations such as the Belgian University Networks, the German PIX Network, the French Cyclades Network and INFORREP (french' user association'). Asociated manufacturers have also presented a proposal. Till now, an internationally accepted Virtual Terminal Protocol has not yet emerged.