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Serial interfaces beat a path to 56Gbit/s

Posted: 14 May 2014  Print Version  Bookmark and Share

Keywords:Ethernet  Optical Internetworking Forum  56Gbit/s 

The Optical Internetworking Forum (OIF) will carry out separate initiatives to push Ethernet speeds to 400Gbit/s today, stretching it further to 1Tbit/s in the future. Engineers with proposals to create links that transmit data at up to 56Gbit/s are expected to come forward at an assembly in Berlin next week.

Today's most advanced networks are using 25G serial interfaces now emerging from the lab. A lively debate has sprung up over whether 40G or 50G links will be the next big building block for Ethernet and other network technologies.

"I don't think there's enough material in the public domain to make those decisions yet," said David Stauffer, chairman of the physical and link layer working group at the OIF, a group of communications specialists who chart the course in high performance interfaces. "Some companies have opinions, but there is no industry consensus yet."

The demand for faster data rates is clear. Datacenters and telecom operators have been calling for terabit rates—something comms engineers say are not yet practical.

Working at the bleeding edge, engineers in Norfolk, Va., also will meet next week to start formal work on a 400GE standard, IEEE 802.3bs. They are expected to hammer out an initial spec based on 16 25G serial lanes

"That's a lot of wires and pins," said Stauffer. "To get to 400G, you really need to get to 50G lanes."

The OIF initially will work on four flavours of 56G. They include a spec that covers half a meter of board traces and one connector, a chip-to-module project, and two very short-reach proposals. Both the very short-reach projects will link SerDes and switch chips, one across about 5cm of board space and another over just a centimetre on an interposer in a 2.5D chip stack.

"These interposer solutions will become more prevalent to get to the integration levels people need—switch chips just don't have room for all the SerDes they need on board anymore," said Stauffer.

Delivering a 56G backplane specification may require a whole new signalling technology. The widely used nonreturn to zero method will not stretch 56G signals the meter of board space backplanes need, Stauffer said.

The OIF will require all the 56G proposals to work down to 40G speeds. Whether 40G or 50G becomes the new baseline will depend on how much cost and power the technology requires.

"People are looking for the fastest path forward to get faster throughput and still stay in the current power profiles—power is everything these days," he said.

Stauffer's employer, the start-up Kandou Bus, expects to make multiple proposals at the Berlin meeting. But just what concepts get traction at the forum remains to be seen, he said.

Meanwhile, engineers continue to push 25G links to longer reaches. The IEEE 802.3bj spec for backplanes added forward correction to the OIF's 30" spec to extend it to 40" for backplanes.

"Any of these could go to a cable if the signal loss is consistent with the backplane," Stauffer said.

"You might get somewhat longer than a meter on a cable because a cable is more controlled. I am sure there are systems vendors doing that, but they wouldn't have to tell anyone about it."

- Rick Merritt
  EE Times





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