The ability to specify outage probability for cellular mobile radio networks is important in order to be able to assure a certain quality of service (QoS). In packet switched networks applying adaptive transmission, users have varying resource demands due to the adaptability. Furthermore, the medium, or a certain part of it, is not assigned exclusively to a user for the duration of a call or session. The determination of outage is therefore difficult. Time consuming simulations can be applied in order to determine a suitable network design capable of providing the required service. Alternatively, analytic or semi-analytic approaches can be applied. This paper presents an analytic model based on random variables (RVs) for the determination of the bandwidth demand of the downlink in packet switched cellular mobile radio networks applying adaptive transmission. The application of the model for determining outage probability is shown, an example of the application of the model in network design is given and the properties of the model are discussed. Comparisons with simulations are made in order to show the accuracy of the model.