This paper addresses the impact of telephone transmission channels on automatic speech recognition (ASR) performance. A real-time simulation model is described and implemented, which allows impairments that are encountered in traditional as well as modern (mobile, IP-based) networks to be flexibly and efficiently generated. The model is based on input parameters which are known to telephone network planners; thus, it can be applied without measuring specific network characteristics. It can be used for an analytic assessment of the impact of channel impairments on ASR performance, for producing training material with defined transmission characteristics, or for testing spoken dialogue systems in realistic network environments. In the present paper, we present an investigation of the first point. Two speech recognizers which are integrated into a spoken dialogue system for information retrieval are assessed in relation to controlled amounts of transmission degradations. The measured ASR performance degradation is compared to speech quality degradation in human-human communication. It turns out that different behavior can be expected for some impairments. This fact has to be taken into account in both telephone network planning as well as in speech and language technology development.