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In this paper, we propose a new version of TCP to improve (1) efficiency in wired-wireless combined networks with nonnegligible random packet losses, and also (2) friendliness to existing protocols, such as TCP-Reno. TCP-Westwood (TCPW) was proposed to improve efficiency in such networks; however, it is shown to be unfriendly to existing protocols under certain RTT and/or router buffer capacities. Since friendliness to existing protocols is one of the most important issues in a real network environment where different protocols coexist, we propose TCPW-BBE (TCPW with buffer and bandwidth estimation) to ensure the friendliness even under varying effective buffer capacities. Based on buffer capacity estimation mechanism, TCPW-BBE reacts more appropriately to a packet loss event, whether the loss is due to congestion or link errors. Simulation results show that TCPW-BBE maintains friendliness to TCP-Reno in networks for a broad range of buffer capacities, RTT, with/without RED routers, yet retaining the efficiency of the original TCPW.