Part 2: Coherent developments
The Optical Internetworking Forum’s (OIF) group tasked with developing two styles of 400-gigabit coherent interface is now concentrating its efforts on one of the two.
When first announced last November, the 400ZR project planned to define a dense wavelength-division multiplexing (DWDM) 400-gigabit interface and a single wavelength one. Now the work is concentrating on the DWDM interface, with the single-channel interface deemed secondary.
Karl Gass"It [the single channel] appears to be a very small percentage of what the fielded units would be,” says Karl Gass of Qorvo and the OIF Physical and Link Layer working group vice chair, optical, the group responsible for the 400ZR work.
The likelihood is that the resulting optical module will serve both applications. “Realistically, probably both [interfaces] will use a tunable laser because the goal is to have the same hardware,” says Gass.
The resulting module may also only have a reach of 80km, shorter than the original goal of up to 120km, due to the challenging optical link budget.
Origins and status
The 400ZR project began after Microsoft and other large-scale data centre players such as Google and Facebook approached the OIF to develop an interoperable 400-gigabit coherent interface they could then buy from multiple optical module makers.
The internet content providers’ interest in an 80km-plus link is to connect premises across the metro. “Eighty kilometres is the magic number from a latency standpoint so that multiple buildings can look like a single mega data centre,” says Nathan Tracy of TE Connectivity and the OIF’s vice president of marketing.
Since then, traditional service providers have shown an interest in 400ZR for their metro needs. The telcos’ requirements are different to the data centre players, causing the group to tweak the channel requirements. This is the current focus of the work, with the OIF collaborating with the ITU.
The catch is how much can we strip everything down and still meet a large percentage of the use cases
“The ITU does a lot of work on channels and they have a channel measurement methodology,” says Gass. “They are working with us as we try to do some division of labour.”
The group will choose a forward error correction (FEC) scheme once there is common agreement on the channel. “Imagine all those [coherent] DSP makers in the same room, each one recommending a different FEC,” says Gass. “We are all trying to figure out how to compare the FEC schemes on a level playing field.”
Meeting the link budget is challenging, says Gass, which is why the link might end up being 80km only. “The catch is how much can we strip everything down and still meet a large percentage of the use cases.”
The cloud is the biggest voice in the universe
400ZR form factors
Once the FEC is chosen, the power envelope will be fine-tuned and then the discussion will move to form factors. The OIF says it is still too early to discuss whether the project will select a particular form factor. Potential candidates include the OSFP MSA and the CFP8.
Nathan TracyThe industry assumption is that the 80km-plus 400ZR digital coherent optics module will consume around 15W, requiring a very low-power coherent DSP that will be made using 7nm CMOS.
“There is strong support across the industry for this project, evidenced by the fact that project calls are happening more frequently to make the progress happen,” says Tracy.
Why the urgency? “The cloud is the biggest voice in the universe,” says Tracy. To support the move of data and applications to the cloud, the infrastructure has to evolve, leading to the data centre players linking smaller locations spread across the metro.
“At the same time, the next-gen speed that is going to be used in these data centres - and therefore outside the data centres - is 400 gigabit,” says Tracy.