Optical Interconnects and Switching Reduce Power Consumption

You may not realize it but using the internet accounts for 5% of all the power we consume. Every byte that gets pumped through your computer is consuming power and that notebook is heating up your lap. Optical switching can lower that by more efficiently switching the data you transmit. Here we explain the problem and a solution to part of the problem.

The Internet has become an important business and entertainment tool for hundreds of millions of people worldwide. The data carried by the Internet doubles every 18 months and that rate seems likely to continue as we transition from email and audio downloads to high density graphic transmission and video on demand downloads1. The power consumed by the Internet is rising rapidly from around 4% of the world’s electricity today to greater than 10% of the world’s electricity in a few years2. For example, it is estimated that 40 Million transceivers are shipped per year. At ~1W power consumption, this corresponds to 350 Million kW-hrs per year, which costs around $70M per year, not including the cost of fans or cooling. Fig. 1 shows how the traffic crossing ATT’s network is growing, and that traffic has grown by a factor of 50 in 7 years. Fig. 2 shows how the Internet electricity usage is distributed, with 40% of the total consumed by core switching.

As Fig. 2 shows, about half of the power consumption is core switching and transmission. Fiber optic data transmission is very efficient because optical attenuation is more than a thousand times lower than electrical signal attenuation at 10 Gbit/s. Furthermore, the advent of dense wavelength division multiplexing (DWDM) has allowed upwards of 1 Tbit/s to be carried by one small optical fiber. Presently, those optical signals are detected and switched electrically and then new optical DWDM signals are generated. This requires a lot of power, typically more than 10 W/Gbit/s. It turns out that very efficient optical circuit switches can be made that require a minimum of power, typically one hundred times less, because MEMS (microelectrical mechanical systems) switches are basically capacitors with minimal switching energy. This difference is shown in Fig. 3, where it can also be seen that present MEMS optical switches could be another thousand times lower power. Achieving that is the goal of a lot of MEMS research.

Another important area of research at UCSB is to build optical routers that switch packets of information optically. The goal of the LASOR project at UCSB is to build an optical switch that can read the headers of each packet, determine where that information should be sent, and set the appropriate switches. Fig. 4 shows one element, the packet forwarding chip, of an all optical router. The capacity can be extremely high and this technology may be essential as data rates continue to climb4.

Other large sources of electricity consumption are the data centers and the computers that connect to them. About 1.5% of the electricity in the United States is used by data centers, and this is rising rapidly. Google has warned that the cost of electricity for its data centers will soon exceed the cost of the processors in the data center. As computers have gotten faster, the power consumption rises as the square of the clock frequency, and this has pushed IC thermal design to the limit. Fig. 5 shows how the power density has risen with each generation of processor5, and these power densities are a major roadblock to further advances. This has been the driver for multicore solutions since the power consumption rises linearly with the number of cores, rather than as the square of the clock frequency. However, it changes the problem to one of interconnecting the multiple cores with high capacity interconnects, which are often the new bottleneck, with 30% or more of the power going to drive interconnects. One solution that can greatly increase the energy efficiency is to replace the electrical wires interconnecting processors with optical interconnects, which have high capacity and don’t require the high power needed for equalization. UCSB has been a leader in optical interconnects and in demonstrating optical interconnect using low cost silicon photonic devices6. Fig. 6 shows an example of the kind of chip being investigated using silicon photonics. The goal is to solve the information bottleneck problem on processor chips, and to reduce the power required to drive electrical lines for IO.

References

  1. Keith Cambron, private communication.
  2. R. Tucker, Photonics in Switching, 2007.
  3. “Three-Dimensional MEMS Photonic Cross-Connect Switch Design and Performance,” X. Zheng, V. Kaman, S. Yuan, Y. Xu, O. Jerphagnon, A. Keating, R. C. Anderson, H. N. Poulsen, B. Liu, J. R. Sechrist, C. Pusarla, R. Helkey, D. J. Blumenthal, and J. E. Bowers, Journal of Selected Topics in Quantum Electronics, 9(2), 571-578, March (2003).
  4. LASOR
  5. S.Borkar, Intel.
  6. “Hybrid Silicon Evanescent Devices”, A. Fang, H. Park, Y.-H. Kuo, R. Jones, O. Cohen, D. Liang, O. Raday, M. Paniccia and J. E. Bowers, Materials Today, 10(7-8), July/August (2007).

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