Boost Your Bandwidth 400ZR and Open ZR+!

400G ZR and ZR+

Bandwidth! You can never have enough! Network operators face the ever-persistent challenge to support increasing data traffic while balancing capital and operating expenditures. To do so, we often rely on technology advances to increase transport capacity and drive down the cost per bit.

Some of these advances come in the form of incremental improvements. Leveraging Moore’s law and the associated die shrinks and integration to go from XFP to SFP+ form-factor is a good example of this. Other advances involve a fundamental architectural change, such as DWDM-based networks, to help drive throughput.

A while back in our Breaking Down Buzzwords series, we touched on coherent optical transmission and how coherent optics will be leading the way for 100G-400G transmission, not just for long-haul or sub-sea links, but more and more in the metro, access, and datacenter interconnect (DCI) realms. Achievements in coherent optical technology over the past 10 years have been significant, but they have been primarily based on proprietary, bookended implementations.

The challenge, as is so often the case, is to evolve from bulky discrete, line-card based solutions into a client-side optical form-factor, something that can plug directly into a router platform and bypass the need for an external transport system. This is where 400ZR and OpenZR+ come in.

So, what’s the difference?


The Optical Interoperability Forum (OIF) really started looking at standardizing an interoperable coherent optical interface back in 2016 and released the initial 400ZR specification in March of 2020. The goal was to define specific performance limitations to meet the cost, size, and power restrictions expected of a client-side, pluggable transceiver, such as QSFP-DD or OSFP. The 400ZR specification was targeted for point-to-point 400G DCI links primarily in the 80-120km range.

The key benefit of the OIF 400ZR implementation agreement (IA) is that a network operator can now mix and match coherent, DWDM interfaces from different OEMs throughout their network by defining the specific signaling (baud) rate, modulation format, and forward error correction (FEC) algorithms. By focusing on a specific application (single-span 400G ethernet DCI) in which certain performance criteria could be limited, and relying on technological advancements (eg, CMOS die shrink and integration) for the on-board DSP, 400ZR provides a cost-effective solution within a 15W module power target.


The OpenZR+ multi-source agreement (MSA) leveraged off the initial success of 400ZR’s demonstrated interoperability to take pluggable coherents a step further.

Still targeting a pluggable form-factor such as QSFP-DD or OSFP, and opening the module power budget to 25W, OpenZR+ offers increased functionality and performance over 400ZR. This increased performance provides interoperable 100G, 200G, 300G, and 400G line rates over metro and long-haul networks, as well as supporting Ethernet and OTN.

Whereas the OIF 400ZR focuses on one modulation format (16 QAM) and a specific line rate (400G Ethernet), OpenZR+ provides additional flexibility to support 100G to 400G line rates, with optical reaches (amplified) 500km and beyond. DCI is still the primary target application for OpenZR+, but with higher performance FEC (open FEC or oFEC) and the additional flexibility of various modulation formats (16QAM, 8QAM, DP-QPSK), a network operator has a few more tools at their disposal for longer reach or multi-span interconnects.

The Future of Coherent Optical Transmission

Just as 10G DWDM evolved from its long-haul transport roots into the high-volume metro access space we are seeing today, coherent optical transmission is making its way into the pluggable, interoperable world so coveted by network operators.