OpenZR+ : Guide to Coherent Optical Technology

Before diving into the OpenZR+ article it is imperative that we understand what is Coherent technology and what makes it superior to a Direct Connection.

Direct Connection has its limitations, to put it bluntly as the data rate goes up, the link length decreases exponentially for example : if you have a data rate of 1G the link length can go over 100km, however if you increase that same data rate up to 56G the link length drops to around 1km, where as Coherent technology not only uses amplitude to encode information it also uses other properties such as polarization and phase so by combining all of these techniques it allows us to increase the data rates while still maintaining long distances.”

How it works

Well like it was mentioned previously, coherent technology uses multiple encoding techniques:

• Amplitude Modulation – Controls the signal’s power level.
• Phase Modulation – Shifts the phase of the light wave to encode data.
• Polarization Multiplexing – Uses two polarization states (horizontal and vertical) to double data capacity.

What does it mean ?

This allows us to encode 3 bits in the same time it would take a Direct Connection to encode one bit. And this in return means that we have increased our data rates eightfold since we can encode eight different combinations.
We can detect the signal by using a coherent receiver which uses a reference signal that is generated by a local oscillator to compare it to the incoming signal and by mixing the two signals coherently we can reconstruct the amplitude and phase information per polarization, then these reconstructed signals are fed into the DSP (Digital Signal Processor) algorithms to compensate for transmission impairments and combine them into one high speed data signal.

So now that we understand what coherent is coherent technology we can dive into OpenZR+. It enhances the functionality of traditional transceivers by combining coherent optical technology and DWDM capabilities into compact, pluggable modules. Its primary functions are tailored for high-performance, scalable, and cost-efficient optical communication. let us dive right into the subject:

OpenZR+ is a Multi-Source Agreement (MSA) standard that extends the 400ZR standard to enable high-performance, cost-efficient, and interoperable coherent optical communications. It supports metro, regional, and long-haul networks with features like DWDM, tunability, and flexible modulation in compact pluggable modules.

• Multi-rate Support: Handles 100G–400G for flexible data rates.
• DWDM & Tunability: Supports up to 96 C-band channels for efficient bandwidth.
• Extended Reach: Operates over hundreds of kilometers with proper amplification.
• Advanced Modulation: Uses QPSK and 8QAM for optimized reach and efficiency.
• SD-FEC: Ensures robust transmission over long, noisy links.
• Compact Form Factor: Fits QSFP-DD/OSFP for easy integration with routers and switches.

Advantages of using OpenZR+

• Operational Simplicity: Plug directly into routers/switches, removing the need for transponders.
• Vendor Interoperability: MSA ensures seamless multi-vendor compatibility.
• Cost Savings: Reduces CAPEX and OPEX by consolidating equipment.
• Future-proof: Offers tunability, multiple rates, and extended reach for scalability.

Well it’s really simple, the evolution of coherent optical technology and industry needs for scalable networking solutions made the same technology you’re reading about. But to dive deeper we have to go back to the 2000’s when coherent optical transmission began to more often and emerged as a transformative technology enabling long distance data transmission with high spectral efficiency, however this technology was initially only implemented in large, standalone transponders which were used in carrier networks.

Then around the 2010 advances in silicon photonics, DSP (short for : digital signal processing) and miniaturization allowed coherent optics to be integrated into pluggable modules, bringing the technology ever so slightly closer to switches and routers. And then at around 2016 OIF (short for : Optical Internetworking Forum ) began the development of the 400ZR standard to address the need for coherent optical solutions for data center interconnect (DCI). Their goal was to be able to create a pluggable module in the smaller form factors like QSFP-DD and QSFP, it supported a fixed data rate of 400G and distances of up to ~120km, suitable for the demands of that time. So in 2019 the 400ZR standard was finalized, marking a milestone in the industry by enabling coherent optics to operate in compact pluggable form factors. But its limitations became evident, it’s distance was at a fixed rate of ~120km restricted access for for bigger projects, the lack of multi-wavelength operation and tunability reduced its efficiency in DWDM (Dense wavelength-division multiplexing) systems, and the 400ZR’s focus on a single data rate really showed how limited its usefulness was in networks requiring diverse bandwidths. Thus the demand for a new solution became evident, something that would retain the cost-effectiveness and its simplicity of 400ZR, but extend its capabilities.

And then in 2020 the industry, recognizing the gaps in 400ZR, came together and established the OpenZR+ Multi Source Agreement (MSA) it aimed to define a new standard, one that would extend the reach and feature set of 400ZR while maintaining its interoperability and cost efficiency, the name OpenZR+ reflects its roots in the 400ZR standard, with the + signifying additional features.

Use cases of OpenZR+

OpenZR+ modules are supported on platforms designed to accommodate pluggable coherent optics. Such as routers and switches with QSFP-DD or OSFP interfaces and software capabilities to manage coherent optical links.

OpenZR+ modules enable various capabilities, making them highly versatile for networking use cases such as:

• • 1. Datacenter Interconnect (DCI):
    • Application: Interconnect hyperscale datacenters across metro or regional distances.
    • Advantage: Provides high-capacity and low-latency links directly from switch/router ports.
  • 2. Metro and Regional Transport:
    • Application: Connect metro aggregation points or regional offices to core networks.
    • Advantage: Enhanced reach compared to 400ZR makes it suitable for multi-span links.
  • 3. IP over DWDM (IPoDWDM):
    • Application: Replace dedicated transport hardware with a converged IP+optical layer by directly plugging OpenZR+ modules into routers and switches
    • Advantage: Simplifies network architecture and reduces cost.
  • 4. Scalable Bandwidth Management:
    • Application: Deploy different data rates (100G, 200G, etc.) on the same network infrastructure as per demand.
    • Advantage: Enables granular capacity planning and traffic engineering.
  • 5. Cost-effective Network Upgrades:
    • Application: Add coherent optical capabilities to existing network platforms without replacing them.
    • Advantage: Cost-effective migration to higher-speed, longer-distance networking.

Switches that do support 400G DCO modules:

• 1. Cisco Nexus 9300/9500 Series
• 2. Arista 7800R3 and 7500R3 Series
• 3. Juniper PTX10008 and PTX10016
• 4. Nokia 7750 Service Router (SR-1 and SR-7)
• 5. Edgecore AS7926-40XKFB
• 6. Broadcom Tomahawk 4 Platforms
• 7. FS N9510-64D
• 8. Huawei CloudEngine 8800 Series

Switches supporting 400G DCO are typically found in platforms built for scalable, high-capacity networks and include brands such as Cisco, Arista, Juniper, Nokia. Nevertheless simply having a QSFP-DD port on a switch or a router does not guarantee full functionality. There are also other factors that come in-to play if you want to get the most out of your transceivers.

At up most importance we have the firmware and software support, since coherent optics require specialized drivers and management tools for features like – error correction, tunability and modulation control it is necessary to make sure you have everything up to date and that everything is compatible.

Then we have power and thermal design, QSFP-DD ports must be capable of handling the higher power draw (usually between 10 to 15 watts) of the 400G coherent modules, reason being is that coherent optics consume more power than standard optics, and bad thermal management can lead to overheating thus, making the modules fail. Just make sure that the device has sufficient cooling and power headroom for the DCO module.

Network and optical layer configuration since coherent optics often require wavelength tuning and specific modulation formats this step is also very important when talking about 400G DCO. These settings may vary from DWDM channel plans and distance requirements for your network.

Line Card and Port Support it is important that you check beforehand that your router’s or switches line card explicitly support coherent pluggable optics. Some line cards may be limited to client optics and not support coherent functionality so to be safe confirm with the vendor before if and whether the QSFP-DD port really does support coherent DCO modules. But even then there are a lot of factors that go into play, things that just may prevent you from full functionality so to be sure, check and do your research.

Challenges

The main challenges of OpenZR+ include power consumption, integration complexity, initial costs, wavelength management, and vendor interoperability. However, with proper planning, training, and investment in complementary technologies (e.g., SDN tools and advanced monitoring systems), these challenges can be mitigated to fully realize the benefits of OpenZR+.

• • • Power Consumption: High power usage strains device cooling and power.
      • Solution: Optimize hardware for heat dissipation.
    • Integration Complexity: Needs firmware updates and optical tools for smooth deployment.
      • Solution: Test extensively before deployment.
    • High Initial Costs: Expensive to deploy compatible devices.
      • Solution: Gradual adoption based on needs.
    • Wavelength Management: Managing DWDM channels can be complex.
      • Solution: Use SDN tools and centralized management.
    • Reach vs. Capacity: Balancing data rate and distance is challenging.
      • Solution: Plan modulation formats carefully.
    • Vendor Interoperability: Modules may not work seamlessly across vendors.
      • Solution: Ensure compliance and testing.
    • Operational Complexity: IPoDWDM simplifies hardware but complicates operations.
      • Solution: Train staff and use advanced monitoring.
    • Fiber Quality: Old fibers may limit performance.
      • Solution: Upgrade fiber and amplification.
    • Competing Standards: OpenZR+ competes with alternatives like Open ROADM.
      • Solution: Choose based on scalability and compatibility needs.

Conclusions

In conclusion no matter what your thoughts are on OpenZR+ it does offer a lot of benefits, such as cost efficient coherent optics, simplified architectures, and scalability but it also requires a lot, for e.g. you need to address previously mentioned draw backs, like power consumption, integration complexity and vendor interoperability. Never the less, with strategic planning, training, complementary technologies like SDN and advanced monitoring system, OpenZR+ truly can do wonders, unlocking the full potential of modern IP and optical networks. So if you can balance the strengths against its challenges OpenZR+ can improve your networks with ease.
And just incase you’re interested you can check out all the modules that we provide with OpenZR+:

FAQ