Coherent optical transmission has been crucial in addressing network operator problems for the last decade. In this time, coherent technology has expanded from a solution reserved only for premium long-distance links to one that impacts data center interconnects, metro, and access networks…
Coherent optical transmission has been crucial in addressing network operator problems for the last decade. In this time, coherent technology has expanded from a solution reserved only for premium long-distance links to one that impacts data center interconnects, metro, and access networks, as explained in the video below.
The development of the 400ZR standard by the Optical Internetworking Forum (OIF) proved to be a milestone in this regard. It was the result of several years of progress in electronic and photonic integration that enabled the miniaturization of 400G coherent systems into smaller pluggable form factors. With small enough modules to pack a router faceplate densely, the datacom sector could profit from an ideal solution for high-capacity data center interconnects. Telecom operators wanted to implement a similar solution in their metro links, so they combined the Open ROADM standardization initiative with the 400ZR initiative to develop the OpenZR+ agreement that better fits their use cases.
This article will elaborate on these standardization projects—400ZR, OpenROADM, and OpenZR+—and explain what use cases each was designed to tackle.
What Is 400G ZR?
To cope with growing bandwidth demands, providers wanted to implement the concept of IP over DWDM (IPoDWDM), in which tunable optics are integrated into the router. This integration eliminates the optical transponder shelf and the optics between the routers and DWDM systems, reducing the network capital expenditure (CAPEX). This is shown in the figure below.
However, widely deploying IPoDWDM with coherent optics forced providers to face a router faceplate trade-off. Since DWDM modules have traditionally been much larger than the client optics, plugging DWDM modules into the router required sacrificing roughly half of the costly router faceplate capacity. This was unacceptable for datacom and telecom providers, who approached their suppliers and the Optical Internetworking Forum (OIF) about the need to develop a compact and interoperable coherent solution that addressed this trade-off.
These discussions and technology development led the OIF to publish the 400ZR implementation agreement in 2020. 400ZR specifies a (relatively) low-cost and interoperable 400 Gigabit coherent interface for a link with a single optical wavelength, using a double-polarization, 16-state quadrature amplitude modulation scheme (DP-16QAM). This modulation scheme uses sixteen different constellation points that arise from combining four different intensity levels and four phases. It doubles the usual 16-QAM transmission capacity by encoding information in two different polarizations of light.
The agreement specified a link reach of 80 kilometers without amplification and 120 kilometers with amplification. For forward error correction (FEC), the 400ZR standard supports a concatenated FEC (CFEC) method. CFEC combines inner and outer FEC codes to enhance the performance compared to a standard FEC code.
The 400ZR agreement does not specify a particular size or type of module, but its specifications targeted a footprint and power consumption that could fit in smaller modules such as the Quad Small Form-Factor Pluggable Double-Density (QSFP-DD) and Octal-Small Form-Factor Pluggable (OSFP). These form factors are small enough to provide the faceplate density that telecom and especially datacom operators need in their system architectures. So even if we often associate the 400ZR standard with QSFP-DD, other form factors, such as CFP2, can be used.
What Is Open ROADM?
In parallel with the 400ZR standardization efforts, telecom network operators had a different ongoing discussion.
Reconfigurable Optical Add-Drop Multiplexers (ROADMs) were a game-changer for optical communications when they entered the market in the 2000s. Before this technology, optical networks featured inefficient fixed routes and could not adapt to changes in traffic and demand. ROADMs allowed operators to remotely provision and manage their wavelength channels and bandwidth without redesigning the physical network infrastructure.
However, ROADMs were proprietary hardware with proprietary software. Changing the proprietary ROADM platform needed extensive testing and a lengthy integration process, so operators were usually reluctant to look for other platform alternatives. Besides, ROADMs still had several fixed, pre-defined elements that could have been configurable through open interfaces. This environment led to reduced competition and innovation in the ROADM space.
These trends drove the launch of the Open ROADM project in 2016 and the release of their first Multi-Source Agreement in 2017. The project aimed to disaggregate and open up these traditionally proprietary ROADM systems and make their provisioning and control more centralized through technologies such as software-defined networks (SDNs, explained in the diagram below).
The Open ROADM project defined three disaggregated functions (pluggable optics, transponder, and ROADM), all controlled through an open standards-based API that could be accessed through an SDN controller. It defined 100G-400G interfaces for both Ethernet and Optical Transport Networking (OTN) protocols with a link reach of up to 500km. It also defined a stronger FEC algorithm called open FEC (oFEC) to support this reach. oFEC provides a greater enhancement than CFEC at the cost of more overhead and energy.
What Is OpenZR+?
The 400ZR agreement was primarily focused on addressing the needs of large-scale data center operators and their suppliers.
While it had some usefulness for telecom network operators, their transport network links usually span several hundreds of kilometers, so the interface and module power consumption defined in the 400ZR agreement could not handle such an extended reach. Besides, network operators needed extra flexibility when defining the transmission rate and the modulation type of their links.
Therefore, soon after the publication of the 400ZR agreement, the OpenZR+ Multi-Source Agreement (MSA) was published in September 2020. As the diagram below explains, this agreement can be seen as a combination of the 400ZR and Open ROADM standardization efforts.
To better fit the telecom use cases of regional and long-haul transport links, OpenZR+ added a few changes to improve the link reach and flexibility over 400ZR:
- Using the more powerful oFEC defined by the Open ROADM standard.
- Multi-rate Ethernet that enables the multiplexing of 100G and 200G signals. This provides more options to optimize traffic in transport links.
- Support for 100G, 200G, 300G, or 400G transport links using different modulation types (QPSK, 8QAM, or 16QAM). This enables further reach and capacity optimization for fiber links.
- Higher dispersion compensation to make the fiber link more robust.
These changes allow QSFP-DD and OSFP modules to reach link lengths of up to 480 km (with optical amplifiers) at a 400G data rate. However, the FEC and dispersion compensation improvements that enable this extended reach come at the price of increased energy consumption. While the 400ZR standard targets a power consumption of 15 Watts, OpenZR+ standards aim for a power consumption of up to 20 Watts.
If operators need more performance, distances above 500 km, and support for OTN traffic (400ZR and OpenZR+ only support Ethernet), they must still use proprietary solutions, which are informally called 400ZR+. These 400ZR+ solutions feature larger module sizes (CFP2), higher performance proprietary FEC, and higher launch powers to achieve longer reach. These more powerful features come at the cost of even more power consumption, which can go up to 25 Watts.
The following table summarizes the use cases and characteristics of the approaches discussed in the article: 400ZR, Open ROADM, OpenZR+, and 400ZR+.
|Edge Data Center Interconnects
|Carrier ROADM Mesh Networks
and Data Center Interconnects
|Target Reach @ 400G
|120 km (amplified)
|500 km (amplified)
|1000 km (amplified)
|Target Power Consumption
|Up to 15 W
|Up to 25 W
|Up to 20 W
|Up to 25W
|Typical Module Option
|100-400G Ethernet and OTN
|100-400G Ethernet (Multi-rate)
|100-400G Ethernet and OTN
|QPSK, 8QAM, 16QAM
|QPSK, 8QAM, 16QAM
|QPSK, 8QAM, 16QAM
|Forward Error Correction
|Standards / MSA
|Open ROADM MSA
400ZR is an agreement primarily focused on the needs of data center interconnects across distances of 80 – 120 kilometers. On the other hand, OpenROADM and OpenZR+ focused on addressing the needs of telecom carrier links, supporting link lengths of up to 500 km. These differences in reach are also reflected in the power consumption specs and the module form factors typically used. The 400ZR and OpenZR+ standards can only handle Ethernet traffic, while the Open ROADM and 400ZR+ solutions can handle both Ethernet and OTN traffic.Tags: 400ZR, Data center, demand, disaggregation, extend, fiber, interoperable, Open ROADM, power, size, ZR+