This paper describes performance experiments for conventional networking protocols such as TCP-UDP/IP otter ATM. The results presented include application throughput, latency. CPU usage, and cell loss based on a local ATM network testbed consisting of DEC Alpha workstations running Windows NT 4.0 and a DEC GIGAswitch/ATM. We show that with well designed hardware and software implementations, Windows NT delivers TCP-UDP/IP throughput up to the theoretical limit of ATM operating at 155.52 Mbits/s signaling rate.