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This paper considers a discrete-time queueing model with infinite storage capacity and one single output line. Users can start and end sessions during which they are active and send packets to the queueing system. Each active user generates a random but strictly positive number of packets per time slot: this results in a session-based arrival process of packets. This model can for instance be applied to study the traffic of a file server, where one file download by a user corresponds to one session. The steady-state probability generating functions of the number of active sessions, buffer occupancy (number of packets stored in the buffer) and packet delay are derived. We also derive an approximation for the tail probabilities of the buffer occupancy. This allows us to study the influence of the different system parameters: some examples are presented.