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Initiatives to add mobility to the Internet and packet data services in third-generation (3G) cellular systems are being considered as candidate solutions for the delivery or IP data to mobile users. The Transport Control Protocol (TCP) will continue to be the most widely used transport protocol for a certain duration in 3G cellular Internet networks. Initially designed for high-speed wireline networks where random packet loss can be negligible, TCP can yield a significant throughput deterioration when applied to lossy links without modification since the primary cause of packet loss may not be network congestion. Two new types of loss can dominate TCP's performance: one is dropped packets because of route disconnection from frequent handoffs; the other is relatively high bit error rate (BER) of radio propagation channels due to multipath fading. Unlike much or the early TCP studies that largely relied on simulations, this paper presents a comprehensive analysis by modeling typical TCP congestion control algorithms. We adopt finite-state Markov channel (FSMC) model as the basis of link error definition since in typical cases it is not adequate to model a radio communication channel as a two-state Gilbert-Elliott channel when the channel quality varies dramatically. To enhance TCP performance in cellular networks, we adopt the idea of active-mobile-host to constrain the negative effect of handoff on TCP throughput in a minimized range. The accuracy of our analytical model is verified through the comparison with measurements using UCB/LBNL Network Simulator (ns-2).