A packet-based data network including: an audio/video network comprising: a packet-switched network over which data including audio and/or video data packets can be carried; at least one data source connected to the network and operable to assemble packetised data comprising audio/video data at a first resolution and at a second resolution greater than the first resolution, and to transmit data packets carrying multiple audio/video streams at the first resolution by multicast network transmission; at least one client connected to the network, being a data handling device for handling packetised audio/video data and being arranged to join the multicast group to receive the multiple audio/video streams at the first resolution, wherein the client is associated with a graphical user interface configured in conjunction with a processor to select an audio/video stream, and the processor is arranged to issue a unicast command to cause transmission of a selected audio/video stream at the second resolution from the source to the client.