T High-bandwidth Distributed Computing Parallel Fiber-optic Sci Links

he drive to include more and more processors and I/O devices in parallel processor complexes has created a need for box-to-box interconnection links capable of delivering bandwidth comparable to system backplanes. Point-to-point links overcome the speed and cost constraints of shared buses. However, copper-based point-to-point links can reliably carry high-speed data only a limited distance. Parallel fiber-optic links substantially increase transmission distance and surpass copper's bandwidth capability. Our research addresses the integration of parallel fiber-optic data links into system designs that require a high-bandwidth link over moderate distances. To demonstrate the feasibility of using parallel fiber-optic technology as a fundamental building block in large-scale commercial and parallel machines, we constructed a link testbed. This testbed integrates Optoelectronics Technology Consortium's (OETC) parallel fiber-optic technology and IBM's high-band-width Scalable Coherent Interface-Link (SCI-Link) technology. (OETC is an ARPA-sponsored industrial consortium of AT&T, IBM, Honeywell, and Lockheed Martin, chartered to develop optical bus technology.) OETC's prototype transmitter and receiver modules comprise an optical link with a 2-Gbytes-per-second transmission rate and transmission distances of up to 100 meters. Based on the IEEE 1596-1992 SCI standard , 1 IBM SCI-Link is a general-purpose communications protocol for multiple system functions, including message passing, I/O bus, and various forms of shared memory. Like the OETC link, SCI-Link has a transmission rate of 500 Mbits per second per line. To improve data integrity, IBM modified the SCI standard protocol by including in the SCI-Link protocol a 32-bit CRC (cyclic redundancy code), hardware packet retry, duplicate packet suppression, and end-to-end acknowledgments. IBM designed the SCI-Link protocol for either copper or parallel fiber-optic medium; hence, the same SCI-Link module design has been successful in both copper and parallel optical test-beds. The OETC/SCI-Link testbed, which began operating in September 1994, is, to our knowledge, the first functional demonstration of the reliable framing and transmission of SCI packets over a parallel optical bus. It has demonstrated low bit-error rates (less than 10 −15) over distances 10 times that of a copper bus with microcoaxial cables, using a fiber-ribbon cable 10 times smaller than the copper cable. The testbed not only demonstrates this emerging technology's feasibility but promises computer system designers new layout flexibility. Some forms of distributed computing require high-bandwidth links, supporting distances of tens of meters. In the context of our work, high bandwidth means 1.0 Gbyte per second and higher. Obviously, this rate will increase as the technology develops. High-bandwidth links …