The performance of TCP based applications in a UMTS-HSDPA system is evaluated, using an analytical model. HSDPA (High Speed Downlink Packet Access) is an evolution of the UMTS standard over the air interface to achieve higher bit rates through the introduction of adaptive modulation and coding, Hybrid ARQ, fast scheduling, fast cell selection and MIMO. The proposed model is used to evaluate the effect of the TCP protocol on the bit rate of various data services (at 32, 64 and 128 kbps). The results show that the bit rate per flow decreases strongly if congestion in the wired network increases. The interaction between cell capacity and the TCP layer is also studied. A semi-analytical model to evaluate the cell capacity is also presented in this paper, and the correspondence between cell capacity and guaranteed TCP bit rate is reported in this paper for a Round Robin scheduler. The findings are supported by simulation results.