The convergence of heterogeneous wireless networks, such as cellular mobile systems, wireless local area networks and other wireless broadband systems, leading to the so-called beyond 3G networks, introduces an increasingly strong trend, to margin monolithic approaches in favour of more flexible network architectures. Due to the diverse capabilities, in terms of bandwidth and delay characteristics, of each of the networks evolved in the formation of a beyond 3G system, the mobility functions (ie inter and intra-system handovers) present excessive complexity. It is apparent that new enhanced mobility mechanisms should be invented. In this direction, the typical system and user oriented handover scenarios, faced during the operation of such systems are described. The mobility mechanisms should be able to indicate the reasons (why) and the time instance (when) that a handover occurs, as well as its direction (where). Based on these scenarios and other QoS constraints, the generic requirements for mobility functionality are given. In order to strengthen more our arguments about the mobility complexity, we implemented an experimental B3G platform, studying this way various mobility scenarios. The analysis of the metrics tools used during the experiments