Grid computing offers the prospect of harnessing huge amounts of computational resources. However, it is being argued that such potential cannot be fully exploited due to the nature of the Internet architecture which is not suitable for high-speed communication of large volumes of data. This has motivated EC-GIN, a European project which aims to exploit the network in a way that better suits the needs of Grid applications. In order to reach such a goal, the network requirements of Grid applications first need to be understood. We have conducted a survey to investigate the requirements and characteristics of a number of Grid applications used in scientific research. Among other things, the survey results have revealed the diversity of Grid traffic, suggesting that there is more to Grid traffic than just transfers of huge bulks and tiny control signals. In this paper, we present these results and identify different classes of traffic behaviour that have been observed within the results. We then validate our findings by looking in detail at two of the applications that we have surveyed.