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An analytical model to evaluate the impact of TCP on the UMTS-HSDPA capacity is presented. A method to minimize the effect of TCP on wireless networks using shared channels is also proposed. HSDPA (high speed downlink packet access) achieves higher aggregate bit rates through the introduction of adaptive modulation and coding, hybrid ARQ, fast scheduling, fast cell selection and MIMO (space time coding and BLAST) techniques. The proposed model is used to evaluate the effect of the TCP protocol on the bit rate of various data services (at 64 and 128 Kbps). As expected, the bit rate per flow decreases strongly if congestion in the wired network increases. However, the capacity achieved by HSDPA is not as affected by TCP. Using this result, a method to maintain the bit rate per TCP flow at a given value without losing much cell capacity is proposed. The findings are supported by simulation results.