Network emulators are used in many contexts of communication networks for the design and the development of network management and routing strategies as well as for the tuning of multimedia services as Voice Over IP, video streaming, TV on-demand, to cite a few. These devices are generally used for modifying, in a controlled way, data traffic flows by changing, in real time, several critical parameters as delay, packet loss percentage, throughput, and so on. Due to very attractive features as high versatility and configurability and low cost, the solutions based on general purpose hardware platforms and free/open-source software are the most ones adopted in the practice for implementing network emulators. Nevertheless, in such architectures the complex interaction of software and hardware sections should affect the accuracy and repeatability of such systems in correctly emulating the desired network behaviors. Consequently, a suitable pre-characterization stage of such kind of network emulators should be performed before they are used. In this framework, the paper describes a methodological approach for designing suitable test-bed and measurement procedure able to reliably characterize the performance of such systems. The final aim of the research activity is to provide a suitable uncertainty model and a confidence level for the parameters provided by network emulators, which can drive the final users in more reliably analyzing the experimental results coming from their test campaigns and which involve the network emulators.