Performance comparison between electrical and optical backplanes

Abstract

The exponential growing of the requested bandwidth capacity for high performance computing systems requires the development of novel backplane solutions within the rack as well as for the rack-to-rack fabric interconnections [1]. The solutions currently used are based on copper interconnections via backplane, driven by a suitable transceivers subsystem. The evolution of the transmitting and receiving interfaces, of the connectors, and of the printed board (PB) technology has allowed to increase the single interface bandwidth, limiting the power consumption and increasing the integration. While the consumption measured in W per Gb/s has had an exponential decrease, the consumption of the interface including serializer/deserializer, clock data recovery, preemphasis and equalization is growing. Furthermore, the complexity and density increasing is supported by evolving silicon technology able to manage more W/mm 2 . The realization of the transmission lines in PB even if based on the most sophisticated materials available on the market, shows very significant losses at very high working frequency. Also connectors, even if now very optimized, contribute to increase loss and crosstalk. We believe that the electrical backplane is close to reach its limitation. Hence, alternative solutions have to be explored. Due to the fact that optical technologies [2-4] are able to provide higher capacity over longer distances than electrical transmission systems, a natural answer to the limitations shown by the Cu-based interconnections is to exploit fiber optical backplane [5].In this paper we simulate by means of HyperLynx® tool the performance of electrical backplane configurations based on copper interconnects, considering the best dielectric materials available on the market and their related roughness, the best Cu stripline width and dielectric thickness (in Fig. 1, on the left the parameters used in the simulations; on the right a result of the simulations). Electrical backplane remains limited by line loss: high frequency is possible, paying in terms of complexity and costs of interfaces. A possible solution based on fiber optics backplane is carried out in order to demonstrate its capabilities, in terms of capacity, power budget and consumption. We consider high-performance 25-Gbps VCSEL source [6] at 850 nm as optical source, together with a commercial photodiode. Unlike the electrical backplane, the optical solution is not limited in transmitted frequency by the backplane implementation, but only by the chosen bandwidth of the exploited TX/RX devices, saving more than half of consumption power. Fig. 1 On the left: the parameters used in the simulations of the electrical backplane. On the right: simulated power analysis of the best electrical backplane in case of horizontal coupled striplines with and without equalization. In the inset typical interconnection via backplane.