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Traditionally, IP Network planning and design is mostly based on average delay or loss constraints which can often be easily calculated. Jitter, on the other hand, is much more difficult to evaluate but this is particularly important to manage the QoS of real-time and interactive services such as VoIP and streaming video. In this paper, we propose a simple model for the delay jitter that is fast enough to be used in networks design algorithms. It takes into account the packets delay correlation and also the correlation of tandem queues that have a significant impact on the end-to-end jitter. We present simple formulas for the jitter of Poisson traffic in a single queue that can be calculated quickly. We then extend them to the end-to-end jitter of a tagged stream based on a tandem queueing network. The results given by the model are then compared with event-driven simulations. We find that they are very accurate for Poisson traffic over a wide range of traffic loads and more importantly, that they yield conservative values for the jitter so that they can be used in network design procedures.