This paper presents and describes the real-time testbed for all-IP Beyond 3G heterogeneous wireless networks that has been developed in the framework of the European IST AROMA project. The main objective of the AROMA testbed is to provide an advanced and realistic framework where the benefits of the algorithms developed within the AROMA project for both the radio access network and core network parts, as well as for the management of the end-to-edge quality of service, can be demonstrated. In particular, this paper focuses on the radio access part of the testbed, providing an in-depth description of its implementation and showing performance results with numerous supporting data that demonstrate the potentials and capabilities of the developed tool for the real-time evaluation of Radio Access Technology (RAT) selection policies under realistic scenarios. To this end, the behavior of two innovative RAT selection algorithms recently proposed in the literature has been assessed, analyzed, and compared in order to demonstrate the applicability of the tool.