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Coming soon: 25Gbit/s backplanes

Posted: 09 Feb 2009 ?? ?Print Version ?Bookmark and Share

Keywords:Ethernet? 25Gbit/s backplane? Serdes?

Experts at a DesignCon panel agree that Ethernet will shift up to 40- and 100Gbit/s data rates using 10Gbit/s and eventually 25Gbit/s serial channels in as few as three years. But the road to those new speed grades has plenty of bumps ahead, they noted.

Engineers will need to make significant stepwise advances in areas ranging from Serdes design to memory interfaces and board materials. But if those advances emerge as expected some 40- and 100G Ethernet products could begin to ship as early as 2010, they said.

David Stauffer, a senior technical staff member at IBM Corp., expressed confidence the industry will be able to shift up from today's 10Gbit/s serial backplanes to 25Gbits/s ones while maintaining acceptable board yields and costs.

"In the last month or two, I've seen simulations that suggest it's achievable at reasonable costs," he said. "A couple months ago I hadn't even seem simulations that approached this level."

The shift is needed to avoid "an unmanageable number of 10G serial links in the backplane," he added.

Separately, two researchers at Penn State University showed the technical feasibility of sending high-speed Ethernet signals over Category 7A copper cables. In a DesignCon paper they described a simulation of 40Gbit/s transmissions over 50 meters and 100Gbits/s over 15m using advanced equalization and coding.

However, the PCBs used in 40- and 100G systems will require a suite of new materials and tailored designs beyond the FR4 used today.

"FR4 is definitely out for 25G and even quite a bit of 10G backplanes," said Joel Goergen, chief scientist at Force10 Networks Inc.

Different designs will call for different board materials. Engineers will also have to employ news shapes of traces and vias to avoid air bubbles that could obstruct the fast signals, he said.

"It's really becoming a complex matrix of parameters," he added.

Prepping 25G era
On the silicon front, non-return-to-zero signaling schemes for Serdes chips show the most promise for the 25G generation. However, engineers will need to develop new techniques to handle rising crosstalk while staying within a 1.5x factor of power consumption over today's 10G Serdes, Stauffer said.

"I think we could see 25G Serdes in a year or two if the need is there," he added.

Adam Healey, a systems architect at LSI Corp., said engineers need to define a superset Serdes design that can tackle all four of the copper and optical interfaces defined by the IEEE 802.3ba group drafting the 40- and 100G Ethernet spec. The Optical Internetworking Forum is defining the 25G Serdes now, he said.

The network processors and traffic manager chips processing packets in the 40- and 100G systems will need upgraded memory interfaces, said Goergen of Force10. They will need to carry as much as 100GbBps, up from 10- to 40Gbyte today.

The Jedec group is beginning to address that need, although its main focus is on memory interfaces for videogame consoles.

"We expect to see a lot of serial memory interfaces from Jedec over the next two to four years," he said. "We need them to have much more efficient coding algorithms than the scrambling mechanisms we have today," he added.

Several factors are driving the need for speed, said Ilango Ganga, a server communications architect at Intel. They include the rise of multicore processors, virtualized CPUs and I/O and converged storage and networking traffic on Ethernet, he said.

Dense blade server designs came out late last year using 10Gbit/s backplanes. Many of them were designed to accommodate four channels for 40Gbit/s capabilities in the future.

"We believe 40G adoption will come to servers in 2013," he said. "We thought originally 2012 was a transition time frame, but with the economic downturn we now see it in 2013," he added.

"Obviously everyone is scaling back whether its 1 or 16 percent," said Goergen. "Despite the recession, I have great faith the telecom sector will not be hit quite as hard as the rest of the world because we have a product people demandInternet access."

The ramp of 10Gbit/s Ethernet is just beginning, noted Seamus Crehan, VP of market watcher Dell' Oro Group. About three million 10G ports shipped in 2008, most of them in network switches. That's up from about two million in 2007, he said.

- Rick Merritt
EE Times

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