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By employing more than one antenna at the transmitter and by properly coding data across the transmit antennas, physical layers (PHYs) with space-time block codes (STBCs) promise increased data rates with minimal decoding complexity at the receiver. This paper presents a comprehensive study of how the STBC gains at the PHY translate to significant network performance improvement in 802.11a wireless local area networks. We base our study on a detailed, across-all-layers, simulation of an 802.11a system. We have extended the network simulator with an implementation of the 802.11a PHY, which allows us to assess the impact of STBC not only at the PHY layer, but at the higher layers as well. An extensive set of simulations illustrates the merits of transmit diversity (in the form of STBC) and sheds light on how performance can be improved for transmission control protocol (TCP) traffic. Essentially, STBC presents to TCP a "smoother" wireless channel; this is corroborated by a brief theoretical analysis as well.