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In this paper, simulation results are presented to demonstrate the effects on the performance of a TCP connection over a UMTS dedicated channel when the size of the Acknowledged Mode Radio Link Control (AM-RLC) data buffer at the Radio Network Controller is finite. The consequence of this restriction is that buffer overflow can occur. The TCP flavour (Tahoe, Reno, New Reno or SACK), FTP file size, RLC buffer size and DCH data rate are varied independently to map the effects of buffer size on the link throughput. The results are used to demonstrate how small changes in the RLC buffer size can have a significant impact on TCP throughput and that this impact varies considerably with TCP flavour. Explanations for the differing behaviours are examined in detail throughout the paper, before summarising with the overall conclusions.