In a Wednesday keynote here, John D'Ambrosia, chief Ethernet evangelist for Dell Inc., updated the status of the 400Gbit/s standards effort and defended the Institute of Electrical and Electronics Engineers Inc. (IEEE) decision not to work on a 1Tbit/s standard yet.

"In some cases, technology has to catch up in advance. I think that's a key point to why terabit isn't happening yet," D'Ambrosia said.

The 400Gbit/s effort starts officially in May, when the study group meets for the first time. (D'Ambrosia is acting chairman but hasn't been officially named chairman; they'll need to actually meet in order for that to happen.)

The IEEE 100Gbit/s standard took four years to ratify, starting from the first study group meeting, so it's reasonable to guess that a 400Gbit/s standard might be finalized in 2017.

Why not chase Terabit Ethernet instead? Some people have suggested that, but the economics just don't work out yet, D'Ambrosia said. "The question, ultimately, is doing it at the right cost so the cost per bit continues to fall."

Cost is also going to dictate some of the details of 400Gbit/s. For example, a lot of investment has gone into interfaces that divide signals into 16 lanes -- the 300-pin transponder and certain optical connectors being examples. Those 16-lane formats are likely to be used for 400Gbit/s, partly because they already exist, and partly because the math divides out to 25Gbit/s per lane, which is the channel speed that electronics are starting to reach.

Terabit Ethernet wouldn't enjoy the same advantage. Even when 50Gbit/s electronics exist, that would require a 20-lane interface -- which, while possible, would add to the cost of developing 1Tbit/s devices.

The International Telecommunication Union, Standardization Sector (ITU-T) also appears to be leaning towards 400Gbit/s rather than 1Tbit/s for the next optical speed, but D'Ambrosia cautioned against confusing that effort with the IEEE's Ethernet standard. They're not the same thing.

The groups are cooperating, though. "We've put a stake in the ground now for the ITU-T to build on. In previous generations it was kind of a moving target," he said.

D'Ambrosia also took a moment to invite silicon photonics vendors to come to the IEEE meetings. "Make your case there" instead of in the press, he said.

Silicon photonics doesn't have to be regarded in the 400Gbit/s standard, since the IEEE doesn't standardize particular implementations of technology, but D'Ambrosia's point was that the silicon photonics camp seems to think it's got something to contribute right now. "For those people who think it should, I extend the invitation to get involved," he said.

For more

• Google Wants Variable-Rate Ethernet
• OIF Takes an Early Look at 400G
• Ethernet Group Starts to Talk Terabit

— Craig Matsumoto, Managing Editor, Light Reading