S/NET: A High-Speed Interconnect for Multiple Computers

This paper describes S/NET (symmetric network), a high-speed small area interconnect that supports effective multiprocessing using message-based communication. This interconnect provides low latency, bounded contention time, and high throughput. It further provides hardware support for low level flow control and signaling. The interconnect is a star network with an active switch. The computers connect to the switch through full duplex fiber links. The S/NET provides a simple memory addressable interface to the processors and appears as a logical bus interconnect. The switch provides fast, fair, and deterministic contention resolution. It further supports high priority signals to be sent unimpeded in presence of data traffic (this can viewed as equivalent to interrupts on a conventional memory bus). The initial implementation supports a mix of VAX computers and Motorola 68000 based single board computers up to a maximum of 12. The switch throughput is 80 Mbits/s and the fiber links operate at a data rate of 10 Mbits/s. The kernel-to-kernel latency is only 100. We present a description of the architecture and discuss the performance of current systems.