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Quality of Service impairments in wireless and wired access networks for IP-based next generation networks (NGNs) differ fundamentally from the ones which we typically encounter in core networks. Examples include but are not limited to high delay, low bandwidth, and high jitter. Even though most NGN architectures claim to be access-agnostic, applications deployed on NGNs have requirements which may not be met by specific access technologies. This raises the need to test NGN application prototypes on user acceptance for a huge number of possible access technology combinations. Unfortunately, tests relying on real access network implementations are expensive and time-consuming while simulation at physical or logical link layer are subject to complex parameter configuration. In this paper we present a distinct approach which bases on the sound theoretical background of the IETF IP Performance Metric (IPPM) framework. We argue that careful analysis of performance metrics in existing public access networks can capture access network properties which are sufficiently accurate to model "typical" access links at IP-layer level using either emulation or simulation. We propose a three-step model which starts with metric and methodology selection, continues with parameter acquisition by means of measurements in existing access networks, and ends with emulation or simulation of this access network. The goal of our process is to provide NGN application developers with means to automatically test their applications from the very beginning on the compatibility with various access network technologies, covering both, signaling and data streaming aspects of access networks. We illustrate the procedure using performance measurement results of a public UMTS Release 5 network and UMTS access network emulation by an extended open-source delay emulator.