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The IP multimedia subsystem is a key component of 3G networks. It enables the seamless provision of multimedia services to end users. Multimedia conferencing is an important category of multimedia services in IMS, serving as the basis for a wide range of applications including audio/video conferencing, multiparty gaming and distance learning. Floor control is an advanced feature of multimedia conferencing which manages the access to shared resources such as audio and video channels. This article extends the 3GPP architecture to include the design and implementation of advanced multimedia conferencing applications. It separates the floor control server and the media resource function processor, and it also opens up advanced conferencing capabilities. A SIP-based floor server control markup language and a multi-level abstraction API are proposed. The extensions are illustrated by a case study based on the design and implementation of a multiparty multimedia game. An important finding of this article is that tools and technologies for the design and implementation of multimedia conferencing applications in IMS are relatively immature. We therefore caution designers and implementers that they may need to develop additional tools and technologies to complete their projects, just as we did in the work presented in this article.