Comparison of optical and electrical data interconnections at the board and backplane levels

Abstract

The benefits of optical interconnections over distances of hundreds of metres or more are well established. However, the increasing complexity of computers and other electronic equipment places severn demands on interconnection capabilities over much shorter distances where optics can also offer potential advantages. A comparison of optical and electrical interconnects at the board and backplane levels of modem mainframe computers has been performed using the interconnect criteria appropriate at these levels. For example, within the CPU (central processor unit) the most important criterion is latency. In electrical interconnections this arises from a combination of the propagation time together with signal distortion caused by the frequency dependent attenuation characteristic of the transmission line and reflections from terminations, vias and other discontinuities. This gives latencies which increase supralinearly with increasing line length. The optical signal, however, propagates faster and has no frequency dependent delay, but a length independent delay is incurred in the electrical/optical interfaces, setting a lower limit on the distance over which optics offers an advantage. This trade-off is examined. Power consumption and interconnect density are also compared. Optical backplane geometries allowing unrestricted bi-directional connections (ie. any point to any other point) are less well defined, but a comparison has been made of one particular geometry based on a wideband bus consisting of up to eight nodes, each operating at 32 Gb/s.