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The ISM frequency bands (902~918 MHz (region 2 only), 2.4 GHz ~ 2.483 GHz, and 5.725~5.875 GHz) are unlicensed bands available for use by commercial technologies. In particular, the 2.4 GHz ~ 2.483 GHz band is very popular and is currently used by WiFi, Zigbee, Bluetooth, and RFID technologies. Moreover, microwave ovens, hand held phones, and other wireless devices operate in the same frequency range, often interfering with each other. The effects of the coexistence of different standards are complicated and significant. However, network simulators do not model the effect of interference from co-located devices using different technologies. In particular, they model external noise, including interference from technologies outside the primary network being simulated, as a flat Gaussian component. In this paper, we propose a methodology of modeling external interference, by considering the realistic characteristics interference generated by co-located wireless standards. The traditional view that noise is flat in space and time, is inaccurate when one considers interference from co-located technologies. The noise signals are generated from their sources, attenuate over distance and experience fading. In addition, external interference exists only when its sources are active. Based on these characteristics, we propose a realistic and configurable noise model that captures such behavior. We demonstrate that this model can be well-fitted to real noise data. Moreover, we also demonstrate the impact of the model on the performance of network protocols as estimated by simulation.