Upping the data rate

Lucent Technologies’ Bell Labs and FCI have demonstrated a data transmission rate of 25 Gbit/sec over an electrical backplane using a new signalling architecture and an optical connector.

Lucent Technologies’ Bell Labs and FCI have demonstrated a data transmission rate of 25 Gbit/sec over an electrical backplane using a new signalling architecture developed by Bell Labs and FCI’s AirMax VS optical connector system.

This is more than twice the highest transmission rates that have been demonstrated to date over electrical backplanes and is a significant step towards achieving 100 Gbit/sec Ethernet (4 x 25 Gbit/sec) over the same.

“100Gbit/sec Ethernet over electrical backplanes is important because it will allow us to further scale today’s data networks to support future high bandwidth applications such as movies on demand, telemedicine and military communications,” said Martin Zirngibl, director of Photonic Networks Research at Bell Labs.

The new application of duobinary signalling — a technique that uses three electrical signal levels to represent binary code in a communications transmission — to high-speed electrical backplanes was first documented by researchers Jeffrey Sinsky, Andrew Adamiecki and Marcus Duelk, of Bell Labs Photonic Networks Research group at the IEEE International Microwave Symposium in June 2004, and later in a paper published in the January 2005 issue of IEEE Transactions on Microwave Theory and Techniques.

“The group has demonstrated performance at 10 Gbit/sec over several different legacy backplanes, and is supporting a movement to use this technique as part of the IEEE 802.3ap standard for 10G PHY (physical layer) Ethernet over backplanes,” said Mary Mandich, a technical manager in Lucent’s Network Hardware Integration Research Department.

“In order to demonstrate 25Gbit/sec, we needed a connector with carefully controlled impedance characteristics, which prevents signals from bouncing back and forth, plus low crosstalk. We also need a signalling technique that can compress bandwidth without requiring excessively complex hardware. Our duobinary signalling technique combined with FCI’s AirMax VSR connector proved to be a successful recipe for achieving 25Gbit/sec transmission speeds,” said Jeffrey Sinsky, lead researcher on the duobinary effort at Bell Labs.

FCI’s AirMax VS connector achieves extremely low loss at frequencies up to 15GHz by using air as a dielectric, an insulator that resists electrical current.

“The connector’s novel design does not require metal shields, yet exhibits lower crosstalk than existing shielded high-speed connectors,” said Dana Bergey, manager of FCI’s Signal Integrity Group.

Lucent’s duobinary signalling architecture for electrical backplane systems takes advantage of the “natural rolloff” tendency of a backplane instead of fighting it like traditional approaches, and uses it to help shape the data bits and provide a clean signal at the receiver.

“With a small amount of additional pre-emphasis and equalization to correct the frequency response — technologies that already are commonly used with today’s communications transmitter and receivers – the output signal at the backplane requires only half the required bandwidth of traditional systems. And, we can convert the signal from duobinary format back to a traditional format using high-speed integrated circuit (IC) technology,” said Andrew Adamiecki, a researcher with Bell Labs.

The team achieved error-free performance at 25Gbit/sec over electrical traces as long as 24 inches using a backplane made with FR4 laminate — an industry standard material.

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