TCP performance over 3G links can be enhanced by means of Active Queue Management (AQM) techniques at the downlink buffer of the mobile network link layer (RLC). In this paper we describe Slope Based Discard (SBD), a novel deterministic buffer management technique whose discarding policy is based on the observation of the filling rate of the buffer. By means of extensive simulation experiments we compare the performance of TCP over three different RLC buffer management schemes: drop-tail (no AQM), SBD and Random Early Detection (RED). The simulation scenarios are configured to evaluate how the end-to-end goodput and delay are influenced by the number of multiplexed flows, the TCP flavor, the buffer size and the wireless channel error rate.