ATOMIC is an inexpensive high-speed LAN built by USC/ISI. It is based upon Mosaic technology developed for fine-grain, message-passing, massively paraIiel computation. Each Mosaic processor is capable of routing variable length packets, while providing added value through simultaneous computing and buffering. ATOMIC adds a general routing capability to the native Mosaic wormhole routing through store-and-forward and path concatenation. ATOMIC aggregate bandwidth scales as the number of nodes increases, and it has a small interface cost. Each ATOMIC channel has a data carrying capacity of 480 Mb/s. A prototype ATOMIC LAN has been constructed along with host interfaces and software that provides full TCP/IP compatibility. Using ATOMIC, 1500-byte packets have been exchanged between hosts at an aggregate transfer rate of more than 1 Gb/s. Other tests have demonstrated throughput of 5.25 million packets per second over a single Mosaic channel. The architecture is flexible both in topology and functionality. This paper describes the architecture and performance of ATOMIC.