Mobile ad-hoc networks (MANETs) have many advantages, self-adaptivity for instance, that make them attractive for multiple applications. Most of these networks use the MAC 802.11b protocol for the medium access control. As all nodes use the same transmission frequency, interferences can occur compromising the quality of service (QoS) provided to flows. In this report, we show how to evaluate the bandwidth required at the MAC level for a flow whose characteristics are known at the application level. The bandwidth evaluation on a node accounts for the interferences due to the flow itself and the other flows. This evaluation has been validated by confrontation with results obtained by NS-2 simulations. We then propose an efficient admission control based on this evaluation. An example illustrates the proposed solution and highlights the accuracy of the evaluation.