To provide seamless handoffs is an important task of cellular systems. A user of a real-time conversation on a mobile terminal should not notice when moving from one base station to another one. In this paper we address handoff procedures in a scenario where the radio access network is assumed to be IP-based, i.e., IP is used up to the base stations, and the mobile terminal runs a Mobile IP client. First we will motivate the need for differentiation of fast handoffs and seamless handoffs. Then we will survey some previously proposed micro-mobility extensions; thereby we will address the question of what degree of micro-mobility support is needed in the typical structure of a radio access network. The main part of this paper then discusses network-initiated/assisted handoffs in combination with Mobile IP. Here, we aim to bring together ideas of 2G/3G systems and of IP-based approaches.
3rd Generation networks as proposed by 3GPP claim to follow the path towards fixed-mobile convergence and full support of Internet services. Although the providers have obviously recognised the dynamics of the Internet, their attempt to provide IP-services over the system has led to a circuit switched architecture. This forthcoming infrastructure will be a sophisticated, complicated, and quite expensive network, with some IP-equipment in the middle (core-network). From an IETF-biased engineers view, some parts of this network and protocols could be dropped, except that they are probably needed for backward compatibility. But since backward compatibility and saving of investment is a major concern of the standardising bodies, the evolving architectures carry a big burden.