The OIF has started a project to combine Flexible Ethernet (FlexE) technology with the 400ZR coherent interface standard.
Karl Gass400ZR is designed to transmit a 400 Gigabit Ethernet (GbE) signal over at least 80km using coherent optical transmission.
Adding FlexE technology will enable 4x100GbE signals to also be transmitted.
Separately, the OIF has started a project to create a 96 gigabaud (GBd) coherent driver modulator specification. Such a symbol rate enables 800 gigabits of data to be sent using a single carrier.
400ZR and 400ZR+
The 400ZR was developed to create an interoperable, low-cost 400-gigabit coherent interface.
The 400ZR specification leaves open the type of optical module to be used but the industry is adopting such pluggable form factors as the QSFP-DD and OSFP.
“Pushed by our network operator members, the goal of 400ZR was that this would be a cost-driven project,” says Karl Gass, OIF physical and link layer working group, vice-chair optical. “It would have a specific function and everything would be optimised for that function.”
But those companies developing coherent digital signal processors (DSPs) say the 400ZR market alone will be insufficient to recoup their development costs. These players want to develop enhanced designs that extend the optical performance and the overall market, a module design category known as 400ZR+.
Optical performance can be enhanced in several ways for the 400ZR+. One way is to improve the performance of the forward error correction at the receiver to enhance the optical gain, even if by fractions of a decibel.
Another is to adopt several modulation schemes whereas the 400ZR uses 16-ary quadrature amplitude modulation (16QAM). The ability to send lower data rates than 400 gigabits will also enhance the transmission distances beyond ZR’s 80km.
The problem with 400ZR+ is that, unless you are coordinating with the other DSP vendors, you are not going to get interoperability
“The problem with 400ZR+ is that, unless you are coordinating with the other DSP vendors, you are not going to get interoperability,” says Gass.
The OIF did not take responsibility for coordinating 400ZR+ and, in retrospect, that was a lost opportunity, he says.
Combining FlexE with the 400ZR design is the OIF’s attempt to ‘reign in’ some of that ZR+ functionality.
“We’ve got a couple of things behind the scenes where we are saying: this is not ZR but in implementing ZR+, there are opportunities for interoperability,” says Gass.
The OIF is all for differentiation in the marketplace, stresses Gass, “but for 400ZR+, we left the opportunity for differentiation to be too big.”
The OIF wants the industry to get on the 400ZR bus
Flexible Ethernet
Adding FlexE will enable 4x100 Gigabit Ethernet (GbE) signals to be transmitted over a 400-gigabit wavelength, whereas 400ZR transmits a 400GbE signal only.
The OIF-defined FlexE has several attributes: it enables the bonding of Ethernet channels, allows sub-links where data can be sent over lower speed links and supports non-standard Ethernet client rates.
For the latest OIF project, FlexE will be used to bond the 100GbE channels. And four channel of 100GbE will not necessarily be the sole additional implementation.
“That will be one of the things covered but I don’t want to say that it will be the only thing covered,” says Gass. “But 4x100GbE is the specific example used for the project start.”
The FlexE can be added on the board alongside a 400ZR module or it can be added within the module, in which case it becomes a 400ZR+.
“If we get multiple people to sign up to that, it benefits the market by showing additional points of interoperability,” he says. But the issue is that there will be no guarantee that FlexE will be embraced in every 400ZR+ module.
The project’s outcome will be published by the OIF as a white paper.
Coherent driver modulator
The second OIF project started will specify a new generation of coherent driver modulator supporting a symbol rate of 96GBd.
The project follows the OIF’s Implementation Agreement published in late 2018 that specifies a 64GBd coherent driver modulator, work that began in October 2016.
Developing an Implementation Agreement ensures that there is a common standard for companies developing the higher baud rate coherent driver modulator.
“This project is different to some of our other coherent work where we are more focussed on size; this one is focussed on performance,” says Gass.
The OIF does not specify the modulator type used for the design and the assumption is that lithium niobate, indium-phosphide and silicon photonics technologies will all be used to implement the design. But each technology will need to meet the final specifications agreed upon.
“I don’t want to make it [the driver modulator project] sound easy but it is going to be the same people that worked two years ago on the RF bandwidth issues for the 64 gigabaud version,” says Gass.
Timescales
The higher baud rate coherent driver modulation Implementation Agreement is expected to be completed in 18 months.
The FlexE-400ZR white paper will be completed sooner as this is more pressing. “There are several implementations that are already in silicon,” says Gass.
The first 400ZR coherent pluggable modules are expected in the first half of 2020.