SCIENCE

Fujitsu Laboratories and Fujitsu Laboratories of America has announced the development of a high-speed multi-channel transceiver circuit that operates at 10 Gbps and extends in-server data-transmission distances by a factor of 1.7 over existing technology. This technology would be put to use in backplanes(1) that serve as communication channels in multi-processor servers, which combine multiple CPUs for higher performance. The new technology can compensate for distortions in data signals that would become evident when channels are lengthened, and allows these channels to be extended from the current distance of 70 cm to 120 cm(2). Fujitsu’s new technology will enable the realization of large-scale high-performance server systems that can connect a great number of more processors than previously possible.

Details of the technology were presented at the IEEE International Solid-State Circuit Conference 2011 (ISSCC 2011), being held in San Francisco from February 20 – 24.
Background

Datacenters that support modern cloud-computing systems rely on high-performance, high-density server systems, and typically use either blade servers or large-scale multi-processor servers. Both types of servers use a circuit board called a backplane, on which other circuit boards containing processors and data-communication chips are densely installed. The backplane is designed to enable communications between these other circuit boards.

In order to achieve systems with higher performance in the future, these backplanes will need to be able to pass data signals at higher speeds, and to connect more processors into larger systems.

Technological Challenges

With backplanes operating at transfer speeds of 10 Gbps, transmission losses result in signal distortions, making it impossible to transmit the data correctly. Furthermore, if the signal distortions become too large, it becomes difficult to accurately recover the clock signal components to detect either “0″ or “1″ data values.

These signal distortions increase with higher transmission speeds and longer transmission channels, so that in existing multichannel transceivers, the distance limit for 10 Gbps communications has been 70 cm(3). This makes it difficult to speed up communications in large-scale servers – which measure 85 cm wide – and pose a challenge to extending the transmission distance for high-speed transceivers.
Newly-developed Technology

Fujitsu Laboratories developed a new signal-processing algorithm that compensates for the amplitude distortion(4) and phase distortion(5) that emerges over long backplane channels, so that signal distortions resulting from a maximum of 41 dB of loss at 10 Gbps can still be fully corrected. The result is that backplane channels can be lengthened by a factor of 1.7, to 120 cm.

Key characteristics of the new technology are as follows.
1. Phase-distortion compensation

Fujitsu Laboratories developed a technology that watches for and detects phase distortions that emerge over long backplane signal channels, and that can accurately correct data through applied control of signal-compensation circuits (equalizer circuits) on both the sending and receiving side.
2. Precise clock-signal generation

To minimize the negative impact of noise components remaining after compensating for distortions arising from long signal channels, Fujitsu Laboratories developed a circuit that generates a more accurate clock, making it possible to select parts of the transmitted data that are relatively unaffected by the noise. Extracting an accurate clock signal makes it possible to distinguish “1″s and “0″s in the data with accuracy.

Results

Employing this high-speed transceiver circuitry for integrated circuits made it possible to maintain high-speed communications over longer transmission channels, paving the way for large-scale, high-performance server systems featuring more processors. Also, because this circuitry was able to reduce signal distortions to below current levels, circuit boards that were thus far made with expensive materials in order to minimize losses, could be made more inexpensively of materials that result in relatively high losses, while still achieving 10 Gbps throughput.
Future Developments

Fujitsu Laboratories plans to extend this technology to the latest process technologies and apply it to high-performance server systems.
Glossary and Notes

1 Backplane:
A circuit board with high-density connectors used to connect multiple other circuit boards, especially in a server. The primary communications channel in high-performance, high-density server systems. Also sometimes referred to as a “midplane.”
2 120-cm transmission channel:
Established using standard circuit-board material (FR-4) and via two connectors.
3 70-cm limit on transmission channels at 10 Gbps:
Established using standard circuit-board material (FR-4) and via two connectors.
4 Amplitude distortion:
A waveform distortion attributable to changes in a signal’s amplitude that vary depending on the frequency.
5 Phase distortion:
A waveform distortion attributable to changes in the signal’s propagation delay that vary depending on the frequency.