It is well documented that the effective throughput of TCP can suffer on plain ATM networks. Several research efforts have aimed at developing additions to ATM networks like Early Packet Discard that avoid TCP throughput degradation. This paper instead investigates improvements to TCP that allow it to perform well on ATM networks without switch-level enhancements, thus avoiding additional complexity and loss of layer transparency in ATM switches. We present an enhanced version of TCP Vegas and show that it performs nearly as well on plain ATM networks as on packet-oriented networks. Moreover, like the original TCP Vegas, our version achieves a significant increase in throughput over the BSD 4.4-Lite TCP. We also present an analysis of TCP dynamics over ATMnetworks, and a simulation based study of the various enhancements in our TCPimplementation and their impact on TCP performance over ATM and packet-oriented networks.