400ZR will signal coherent’s entry into the datacom world

• 400ZR will have a reach of 80km and a target power consumption of 15W 
• The coherent interface will be available as a pluggable module that will link data centre switches across sites    
• Huawei expects first modules to be available in the first half of 2020
• At OFC, Huawei announced its own 250km 400-gigabit single-wavelength coherent solution that is already being shipped to customers

Coherent optics will finally cross over into datacom with the advent of the 400ZR interface. So claims Maxim Kuschnerov, senior R&D manager at Huawei.

Maxim Kuschnerov400ZR is an interoperable 400-gigabit single-wavelength coherent interface being developed by the Optical Internetworking Forum (OIF).

The 400ZR will be available as a pluggable module and as on-board optics using the COBO specification. The IEEE is also considering a proposal to adopt the 400ZR specification, initially for the data-centre interconnect market. “Once coherent moves from the OIF to the IEEE, its impact in the marketplace will be multiplied,” says Kuschnerov. 

But developing a 400ZR pluggable represents a significant challenge for the industry. “Such interoperable coherent 16-QAM modules won’t happen easily,” says Kuschnerov. “Just look at the efforts of the industry to have PAM-4 interoperability, it is a tremendous step up from on-off keying.” 

Despite the challenges, 400ZR products are expected by the first half of 2020.

400ZR use cases 

The web-scale players want to use the 400ZR coherent interface to link multiple smaller buildings, up to 80km apart, across a metropolitan area to create one large virtual data centre. This is a more practical solution than trying to find a large enough location that is affordable and can be fed sufficient power.

Once coherent moves from the OIF to the IEEE, its impact in the marketplace will be multiplied

Given how servers, switches and pluggables in the data centre are interoperable, the attraction of the 400ZR is obvious, says Kuschnerov: “It would be a major bottleneck if you didn't have [coherent interface] interoperability at this scale.” 

Moreover, the advent of the 400ZR interface will signal the start of coherent in datacom. Higher-capacity interfaces are doubling every two years or so due to the webscale players, says Kuschnerov, and with the advent of 800-gigabit and 1.6-terabit interfaces, coherent will be used for ever-shorter distances, from 80km to 40km and even 10km. 

At 10km, volumes will be an order of magnitude greater than similar-reach dense wavelength-division multiplexing (DWDM) interfaces for telecom. “Datacom is a totally different experience, and it won’t work if you don’t have a stable supply base,” he says. “We see the ZR as the first step combining coherent technology and the datacom mindset.”

Data centre players will plug 400ZR modules into their switch-router platforms, avoiding the need to interface the switch-router to a modular, scalable DWDM platform used to link data centres.     

The 400ZR will also find use in telecom. One use case is backhauling residential traffic over a cable operator’s single spans that tend to be lossy. Here, ZR can be used at 200 gigabits - using 64 gigabaud signalling and QPSK modulation - to extend the reach over the high-loss spans. Similarly, the 400ZR can also be used for 5G mobile backhaul, aggregating multiple 25-gigabit streams. 

Another application is for enterprise connectivity over distances greater than 10km. Here, the 400ZR will compete with direct-detect 40km ER4 interfaces.

Having several use cases, not just data-centre interconnect, is vital for the success of the 400ZR. “Extending ZR to access and metro-regional provides the required diversity needed to have more confidence in the business case,” says Kuschnerov. 

The 400ZR will support 400 gigabits over a single wavelength with a reach of 80km, while the target power consumption is 15W.

The industry is still undecided as to which pluggable form factor to use for 400ZR. The two candidates are the QSFP-DD and the OSFP. The QSFP-DD provides backward compatibility with the QSFP+ and QSFP28, while the OSFP is a fresh design that is also larger. This simplifies the power management at the expense of module density; 32 OSFPs can fit on a 1-rack-unit faceplate compared to 36 QSFP-DD modules.

The choice of form factor reflects a broader industry debate concerning 400-gigabit interfaces. But 400ZR is a more challenging design than 400-gigabit client-side interfaces in terms of trying to cram optics and the coherent DSP within the two modules while meeting their power envelopes.

The OSFP is specified to support 15W while simulation results published at OFC 2018 suggest that the QSFP-DD will meet the 15W target. Meanwhile, the 15W power consumption will not be an issue for COBO on-board optics, given that the module sits on the line card and differs from pluggables in not being confined within a cage.

Kuschnerov says that even if it proves that only the OSFP of the two pluggables supports 400ZR, the interface will still be a success given that a pluggable module will exist that delivers the required face-plate density.

400G coherent

Huawei announced at OFC 2018 its own single-wavelength 400-gigabit coherent technology for use with its OptiX OSN 9800 optical and packet OTN platform, and it is already being supplied to customers.

The 400-gigabit design supports a variety of baud rates and modulation schemes. For a fixed-grid network, 34 gigabaud signalling enables 100 gigabits using QPSK, and 200 gigabits using 16-QAM, while at 45 gigabaud 200 gigabits using 8-QAM is possible. For flexible-grid networks, 64 gigabaud is used for 200-gigabit transmission using QPSK and 400 gigabits using 16-QAM.   

Huawei uses an algorithm called channel-matched shaping to improve optical performance in terms of data transmission and reach. This algorithm includes such techniques as pre-emphasis, faster-than-Nyquist, and Nyquist shaping. According to Kuschnerov, the goal is to squeeze as much capacity out of a network’s physical channel so that advanced coding techniques such as probabilistic constellation shaping can be used to the full. For Huawei’s first 400-gigabit wavelength solution, constellation shaping is not used but this will be added in its upcoming coherent designs.

Huawei has already demonstrated the transmission of 400 gigabits over 250km of fibre. “Current generation 400G-per-lambdas does not enable long-haul or regional transmission so the focus is on shorter reach metro or data-centre-interconnect environments,” says Kuschnerov.

When longer reaches are needed, Huawei can offer two line cards, each supporting 200 gigabits, or a single line card hosting two 200-gigabit modules. The 200-gigabits-per-wavelength is achieved using 64 gigabaud and QPSK modulation, resulting in a 2,500km reach.

Up till now, such long-haul distances have been served using 100-gigabitwavelengths. Now, says Kuschnerov, 200 gigabit at 64 gigabaud is becoming the new norm in many newly built networks while the 34 gigabaud 200 gigabit is being favoured in existing networks based on a 50GHz grid.