||The emergence of high-speed networks and distributed multimedia applications have been driving the development of new network transport systems. Of the existing work in designing high-speed multimedia services and protocols, only a few address the issue of scalability support for the development of multiparty interactive multimedia (MIM) applications such as video-conferencing. In this thesis, we propose a new communication abstraction known as the Group Channel which facilitates and supports the implementation of MIM applications. The Group Channel is a high level abstraction for group communication. The Credit Scheme and the dynamic bandwidth calibration scheme are provided as an integral part of the Group Channel services. The Credit Scheme allows users to allocate network bandwidth (expressed in terms of credits) among the participants associated with the Group Channel dynamically, while the calibration scheme automatically determines the maximum amount of credits appropriate for the system as participants join and leave the Group Channel. The Multimedia Transport Protocol (MTP) is proposed as a realization of the Group Channel services in the ATM network. Its prototype implementation and a simple multiparty video-conferencing application built on top of the MTP prototype are described in this thesis. An evaluation study and comparison with a commercial video-conferencing product has been performed. Our results show that the Group Channel is capable of guaranteeing performance of MIM applications irrespective of the group size and differences in workstation speeds. We conclude that dynamic resource sharing, allocation and calibration are important features needed to support performance guarantee for large-scale MIM applications.