Data Center Interconnects: Transmission Reach and Network Architecture

1. What Is Data Center Interconnect?

Data Center Interconnect refers to the network infrastructure that links two or more data center locations. It enables applications, storage systems, cloud platforms, and computing resources to communicate across separate facilities.

Unlike connections inside a single data center, DCI links often cover longer distances and may pass through metropolitan or regional fiber networks. They therefore require more careful planning of optical reach, latency, redundancy, and transport capacity.

The main objective is to provide sufficient bandwidth and reliable connectivity while controlling latency, optical transmission costs, and operational complexity.

2. DCI Transmission Distance and Application Scenarios

Transmission distance is one of the first factors considered during DCI design. A short campus connection may require only direct optical connectivity, while a metropolitan or regional link may need coherent transmission and optical amplification.

These are planning categories, not guaranteed distances for every optical module. A 10 km or 40 km Ethernet transceiver does not automatically support the same reach when connected through additional patch panels, multiplexers, or a different fiber route.

3. Common DCI Network Architectures

DCI can be deployed using different architectures depending on distance, traffic volume, fiber availability, and operational requirements. Three common approaches are direct point-to-point connectivity, DWDM-based transport, and routed optical networking.

Direct Point-to-Point Fiber

A direct point-to-point connection uses dedicated fiber strands between two data center locations. Each endpoint connects to the optical interface at the opposite facility.

This architecture is straightforward and can offer low operational complexity. It is suitable when dedicated fiber is available and the required reach fits the selected optical interface.

DWDM-Based DCI

Dense Wavelength Division Multiplexing allows multiple optical channels to share one fiber pair. Each channel operates at a different wavelength, increasing the capacity of existing fiber infrastructure.

DWDM is particularly useful when multiple high-capacity connections must operate over limited fiber resources. Optical amplifiers and wavelength management equipment may be added according to the link design.

Routed Optical Networking

In a routed optical architecture, coherent optical modules can be installed directly into compatible routers or switches. The electrical client traffic is mapped into a coherent optical signal that travels over the line system.

This approach can reduce the number of separate transponder devices and simplify the connection between packet networking and optical transport. However, platform support, coherent mode compatibility, optical power, and line-system requirements must be verified.

4. Optical Transceiver Types for DCI

Different optical transceiver technologies serve different distance and network requirements. The most important distinction is between conventional Ethernet client optics and coherent optical modules designed for transmission over longer fiber routes.

Conventional SR8, DR8, FR4, and LR4 modules should not be confused with coherent ZR or ZR+ optics. Their modulation, optical interfaces, reach characteristics, and line-system requirements are different.

5. 400G DCI: Coherent Transmission and Network Design

400G DCI is an important architecture for connecting high-capacity data centers. The introduction of coherent pluggable modules allows 400GbE client traffic to be transported over a single optical wavelength, reducing dependence on separate large transponder systems in supported deployments.

400ZR is an Optical Internetworking Forum (OIF) coherent interface designed for single-span, amplified DWDM links in the approximate 80–120 km application range. Its specifications define the coherent line interface and interoperability requirements.

For routes that exceed the baseline 400ZR application, 400ZR+ and OpenZR+ implementations can provide additional operating modes and reach flexibility. The achievable distance depends on the selected mode, optical line system, fiber characteristics, and engineering margin.

Typical 400G DCI design considerations include:

• Compatibility between the coherent module and the router or switch host.

• Supported modulation, FEC, baud rate, and line-side interface.

• DWDM channel spacing and wavelength configuration.

• Optical launch power and receiver operating range.

• Amplifier gain, noise figure, and optical signal-to-noise ratio.

• Fiber attenuation, chromatic dispersion, and polarization-related effects.

6. 800G DCI: Increasing Capacity per Wavelength

As AI training, distributed storage, and cloud infrastructure generate more traffic between data center facilities, 800G DCI provides a higher-capacity optical transport option.

800ZR is defined by an OIF Implementation Agreement for 800G coherent line transmission over amplified single-span DWDM links in the 80–120 km application range. It provides a standardized coherent interface for high-capacity DCI applications.

800ZR+ and other extended-reach coherent implementations target longer metro and regional networks. These implementations may support different data rates and reach modes depending on the platform, optical design, and network conditions.

The distinction between 800G client optics and 800G coherent optics is important. An 800G SR8 or DR8 module is designed for a specific parallel optical interface and short-reach application. An 800ZR module uses coherent transmission for a different type of optical line connection.

7. The Development of 1.6T Coherent DCI

As network traffic continues to grow, the next generation of DCI technology is moving toward higher capacity per wavelength. A 1.6T coherent line interface can potentially transport twice the capacity of an 800G wavelength, helping increase fiber utilization and reduce the number of individual optical channels required for a given traffic volume.

In September 2026, OIF announced its 1600ZR Implementation Agreement. The specification defines an interoperable 1.6T coherent line interface for single-span, amplified, point-to-point DWDM links up to 120 km.

This development provides a standards-based foundation for future 1.6T coherent DCI implementations. Commercial deployment still depends on module availability, host platform support, optical line-system compatibility, power consumption, and operational qualification.

8. Transmission Reach: What Determines the Actual Distance?

Optical transmission reach is determined by the combined performance of the transceiver and the complete optical path. The nominal distance printed on a module specification is based on defined operating conditions. Real network routes may introduce additional losses and signal impairments.

Fiber Attenuation

Fiber attenuation reduces optical power as the signal travels along the link. The total loss depends on fiber length, wavelength, fiber characteristics, and the number of optical connections.

Connector and Splice Loss

Patch panels, adapters, splices, and intermediate distribution points introduce additional loss. These values must be included in the optical link budget.

Chromatic Dispersion

Chromatic dispersion causes different spectral components of an optical signal to propagate at different speeds. It can affect signal quality over longer transmission distances, especially when the system has limited dispersion tolerance.

Polarization Effects

Polarization mode dispersion and other polarization-related effects can affect high-speed optical signals. Coherent receivers and DSP techniques help compensate for selected impairments, but the overall system still has performance limits.

Optical Signal-to-Noise Ratio

Optical amplifiers increase signal power but also introduce noise. In amplified DWDM systems, OSNR is an important factor in determining whether the receiver can maintain the required signal quality.

9. DWDM, Optical Amplifiers, and Line Systems

DWDM is widely used in DCI networks because it allows multiple high-capacity channels to operate over the same fiber pair. Each channel uses a designated wavelength or frequency slot, and a multiplexer combines the channels onto the transmission fiber.

At the receiving end, a demultiplexer separates the wavelengths and directs each channel to its corresponding optical receiver.

For longer routes, optical amplifiers may be required to compensate for fiber attenuation. Their placement and configuration depend on the span loss, required output power, channel count, and overall line-system design.

Common optical transport components include:

• DWDM multiplexers and demultiplexers: Combine and separate optical wavelengths.

• EDFA amplifiers: Compensate for optical power loss in supported C-band or other designed optical systems.

• ROADM: Allows selected wavelengths to be added, dropped, or routed through an optical network.

• Optical Supervisory Channel: Supports monitoring and management functions in compatible line systems.

• Optical monitoring equipment: Helps observe channel power, wavelength, and transmission conditions.

The use of an amplifier does not automatically guarantee a longer reach. Amplifier noise, gain flatness, channel loading, receiver sensitivity, and nonlinear effects must also be considered.

10. DCI Network Redundancy and Availability

Data center connectivity often carries critical applications, storage replication, and business services. A single optical route may be insufficient when network availability is a major requirement.

Redundancy can be implemented at the fiber, optical transport, and packet networking layers. The correct design depends on the required availability target, route diversity, recovery time, and cost.

Redundant links should be evaluated for shared-risk link groups. Two fiber pairs that follow the same physical duct may still be affected by the same construction damage or facility incident.

11. DCI Latency and Bandwidth Planning

DCI performance is not determined by bandwidth alone. Latency becomes especially important for distributed databases, storage replication, synchronous services, and applications that communicate frequently between facilities.

Fiber propagation introduces distance-related delay. Equipment processing, optical transport, switching, routing, and network congestion add further latency.

For standard single-mode fiber, a useful planning approximation is about 5 microseconds of one-way propagation delay per kilometer. Actual values depend on the fiber route and physical propagation characteristics.

These figures represent approximate fiber propagation delay only. They exclude switching, routing, FEC processing, optical equipment, and other network overhead. The actual route length may also be longer than the straight-line distance between two data centers.

Bandwidth planning should consider both the current traffic demand and future growth. A DCI system may need to support storage replication, virtual machine migration, backup traffic, AI dataset movement, and inter-site application communication simultaneously.

12. Selecting the Right DCI Optical Solution

Choosing an optical solution requires matching the transmission technology to the actual network environment. A short campus link and a regional DWDM route have different requirements even if both carry the same Ethernet client rate.

For a short DCI connection, conventional single-mode Ethernet optics may provide a simpler and more economical design. For amplified metro DCI, coherent pluggables can provide high capacity over a single wavelength. Longer regional routes may require enhanced coherent modes and a dedicated optical transport architecture.

13. C-LIGHT Optical Transceiver Solutions for DCI Infrastructure

C-LIGHT provides high-speed optical connectivity products for data center, AI infrastructure, and enterprise networking applications. Different product families support different parts of the optical network, from short-reach data center connections to selected single-mode links and high-bandwidth interconnect platforms.

For coherent DCI applications, customers should specifically confirm whether the required module is a 400ZR, 400ZR+, 800ZR, or another coherent implementation. A conventional 400G or 800G client optical module should not be assumed to support coherent DWDM transmission simply because its data rate matches the DCI capacity.

14. Conclusion

Data Center Interconnect is a key part of modern cloud, AI, and enterprise network infrastructure. Its design requires a balance between transmission reach, bandwidth, latency, optical performance, fiber utilization, and network availability.

Direct fiber connections can provide a straightforward solution for short-distance links, while DWDM and coherent optical technologies support higher-capacity metro and regional connectivity. The development of 400ZR, 800ZR, and 1600ZR continues to expand the role of pluggable coherent optics in data center transport.

Successful DCI deployment depends on matching the optical module to the complete network architecture. Fiber characteristics, optical budget, amplification, host compatibility, redundancy, and operational monitoring must all be considered during planning and qualification.

15. Frequently Asked Questions