The paper systematically explores the social dimension of external interruptions of human activities. Interruptions and interruption handling are key issues in human-computer interaction (HCI) and computer-supported cooperative work (CSCW) research. However, existing research has almost exclusively dealt with effects of interruptions on individual tasks. In this paper we call for expanding the scope of analysis by including the effect of interruptions on the social context. We identify four facets of the social 'ripple effect' of interruptions: location, communication, collaboration, and interpersonal relation. We discuss the advantages of extending the notion of interruptions and its implications for future research.

This paper presents two case studies of informal group communication using multimedia conferencing that supports various media including video, audio and chat. The studies provide a comparison of audio and chat as communication medium and present data on usage patterns, user preferences and attitudes. The quantitative and qualitative data collected suggest that chat does have advantages in some situations when used for informal communication along with video. The results provide evidence against the hypothesis that chat is a low bandwidth alternative only used when audio communication is unavailable. This suggests that video mediated chat deserves further attention from designers and the research community, since it is often ignored as a "useful" scenario.