As data center networks evolve from 400G to 800G and toward 1.6T and beyond, increasing the speed of individual optical ports is no longer enough to solve every networking challenge. The rapid growth of AI workloads is putting pressure on the entire infrastructure, including bandwidth density, power consumption, thermal management, physical space, and maintenance.
For years, the evolution of optical networking was relatively straightforward: higher-speed optical modules provided more bandwidth while maintaining the familiar pluggable architecture. However, AI clusters are changing the requirements. Thousands of GPUs or accelerators may need to exchange enormous amounts of data simultaneously, creating a network environment where the density and efficiency of the overall system can become just as important as the bandwidth of a single port.
This has led the industry to explore new optical interconnect architectures. Co-Packaged Optics (CPO) represents one of the most highly integrated approaches, placing optical engines much closer to the switching silicon. At the same time, the industry is also looking for ways to push the traditional pluggable concept further. XPO (eXtra-dense Pluggable Optics) is one such approach, focusing on significantly higher optical density, advanced thermal management, and system-level power delivery while retaining the basic advantages of a pluggable architecture.
Why Are Traditional Pluggable Optics Facing New Challenges?
The development of optical modules from 100G to 400G and 800G has delivered enormous improvements in network capacity. However, the challenges associated with AI infrastructure are not simply a matter of insufficient bandwidth per port.
Large-scale AI training involves continuous communication between large numbers of accelerators. Parameters, gradients, and other data must be exchanged between computing nodes at extremely high rates. As the number of accelerators increases, the network must handle a massive amount of east-west traffic, making aggregate bandwidth and network density increasingly important.
At the same time, the number of optical connections in a large AI cluster can reach tens of thousands or more. Even when individual optical links have relatively high reliability, a sufficiently large deployment inevitably creates more opportunities for failures. This makes serviceability and replacement an important part of optical network architecture.
Thermal management is another major constraint. High-performance switching ASICs, accelerators, and optical modules all contribute to rack-level power consumption. As rack power densities increase, conventional air cooling becomes increasingly difficult to scale, and liquid cooling is being adopted more widely in high-density computing environments.
This creates an interesting problem for optical interconnects. A conventional pluggable module may be easy to replace, but increasing its bandwidth and power consumption also makes its thermal and mechanical requirements more demanding. Simply making the module faster does not necessarily make the entire system more efficient.
The result is a shift in design thinking—from optimizing the performance of an individual optical module toward optimizing bandwidth density, power efficiency, cooling, and serviceability at the system level.
What Is XPO?
XPO, or eXtra-dense Pluggable Optics, is an emerging high-density optical interconnect architecture designed for next-generation data center and AI networking applications.
The basic idea is not to abandon pluggable optics, but to significantly expand what a pluggable optical unit can provide. Instead of treating the optical module as a relatively small, independent transceiver, XPO takes a more system-oriented approach to bandwidth, electrical connectivity, power delivery, and thermal management.
This makes XPO fundamentally different from simply developing a higher-speed version of an existing OSFP or QSFP module.
An XPO architecture can combine a much larger number of high-speed electrical and optical channels within a single high-density unit while incorporating a dedicated thermal solution. The approach is particularly relevant to AI infrastructure, where large numbers of high-speed links need to be concentrated into a limited amount of rack space.
One of the most notable aspects of the XPO concept is its emphasis on density. Instead of focusing exclusively on increasing the bandwidth of one optical lane or one traditional module, XPO attempts to increase the amount of network bandwidth that can be delivered within a given physical area.
This distinction is important. In a conventional networking architecture, increasing aggregate bandwidth often means increasing the number of ports, switches, or modules. XPO aims to achieve greater aggregate capacity without increasing physical infrastructure at the same rate.
How Does XPO Work?
The XPO concept involves several major architectural changes compared with conventional pluggable optics. These include a high-density multi-channel electrical architecture, a compact multi-PCB mechanical structure, integrated thermal management, and a higher-voltage power supply approach.
One of the key structural ideas is the use of a dual-PCB “belly-to-belly” configuration. Rather than placing all high-speed circuitry on a single PCB, two PCBs can be arranged back-to-back within the optical unit. This creates a much denser architecture while providing space for thermal management between the two boards.
The arrangement also allows different functional and thermal regions to be organized more deliberately. High-power components can be positioned close to the central cooling structure, shortening the thermal path between heat sources and the cooling system.
This is particularly important as the power consumption of high-density optical interconnects increases. In a conventional air-cooled module, heat must eventually be transferred into the surrounding air. In an XPO architecture designed for liquid cooling, the cooling structure can instead remove heat directly from the module through a dedicated cold plate.
Integrated Liquid Cooling
Liquid cooling is one of the most important differences between the XPO concept and conventional pluggable optical modules.
Instead of relying exclusively on airflow around the module, a liquid-cooled XPO architecture can place a cold plate directly within the optical unit. The cooling structure can be positioned between the two PCBs so that heat generated on both sides has a relatively short path to the coolant.
This approach changes the thermal design philosophy from module-level air cooling to direct thermal management through the system’s liquid-cooling infrastructure.
The cold plate itself can incorporate internal flow channels designed to move coolant through areas with higher heat concentration. A dedicated inlet and outlet provide the connection to the broader liquid-cooling system.
Such an architecture is particularly relevant to AI and HPC environments, where liquid cooling is increasingly being considered or deployed to support higher rack-level power densities.
However, liquid cooling also introduces new engineering requirements. The system needs reliable fluid connections, leak prevention, thermal interfaces, coolant management, and appropriate service procedures. Therefore, XPO’s liquid-cooling capability should be viewed as part of a broader system architecture rather than simply an additional feature of an optical module.
High-Density Electrical Architecture
XPO also changes the electrical side of the optical interconnect.
The architecture is designed around a large number of high-speed electrical channels, enabling a single optical unit to aggregate significantly more bandwidth than conventional pluggable modules.
A representative XPO implementation can use 64 electrical channels operating at 200G PAM4, providing an aggregate electrical bandwidth of up to 12.8 Tbps. Future generations can potentially move toward higher per-channel speeds, creating a path toward even greater aggregate bandwidth.
At these data rates, signal integrity becomes a major engineering challenge. Channel layout, crosstalk, insertion loss, return loss, and power delivery all become increasingly important as the number of high-speed lanes increases.
The separation of high-speed data paths from low-speed management and control signals is another important aspect of the architecture. Interfaces used for functions such as management, monitoring, reset, and alarms can be physically separated from the high-speed data channels, helping simplify the electrical environment and reduce potential interference.
XPO also introduces a different approach to power delivery.
Traditional optical modules commonly operate from a low-voltage supply such as 3.3V. As module power increases, delivering hundreds of watts through a low-voltage architecture requires extremely high current. This increases conductor losses, connector requirements, and power-delivery complexity.
A higher-voltage DC input architecture can significantly reduce the current required for the same power level. For example, an XPO design using a rack-level DC bus around the 48–50V range can deliver high power with substantially lower current than a conventional low-voltage input.
This makes the approach more compatible with the broader trend toward higher-voltage DC power distribution in modern data centers.
XPO vs. Traditional OSFP
The difference between XPO and conventional OSFP is therefore much greater than simply increasing the number of optical lanes.
A traditional OSFP module remains a relatively compact pluggable transceiver. It is designed to fit into a standardized cage and operate as an individual replaceable component. Its thermal management is generally based on an integrated heat sink and the host system’s airflow.
XPO takes a much more system-oriented approach. The physical unit is larger, contains many more channels, requires a more sophisticated power architecture, and can incorporate direct liquid cooling.
This allows XPO to target a different level of network density.
| Feature | Conventional OSFP | XPO |
|---|---|---|
| Design Philosophy | High-speed pluggable module | Extra-dense pluggable optical unit |
| Primary Focus | Port-level bandwidth | System-level bandwidth density |
| Channel Density | Relatively limited | Significantly higher |
| Thermal Management | Primarily air cooling | Designed to support advanced liquid cooling |
| Power Architecture | Typically low-voltage | Can use higher-voltage DC input |
| Mechanical Structure | Compact single-module architecture | High-density multi-PCB architecture |
| Serviceability | Individual module replacement | Pluggable architecture with higher-density unit |
| Target Applications | General data centers, Ethernet, InfiniBand | AI/HPC and high-density data centers |
The important point is that XPO is not necessarily intended to replace every conventional optical module. Traditional OSFP remains highly practical for many networking applications where standardization, compatibility, and simple field replacement are more important than extreme density.
XPO instead addresses situations where the physical and thermal limits of conventional pluggable architectures are becoming significant constraints.
XPO vs. CPO: Two Different Approaches
The comparison between XPO and CPO is particularly interesting because both technologies are attempting to address the same fundamental problem: how to provide dramatically higher optical bandwidth and better energy efficiency as switch and AI system requirements increase.
However, they approach the problem from opposite directions.
CPO integrates optics much closer to the switching ASIC. Instead of placing the optical engine in a conventional pluggable module at the front panel, the optical components are co-packaged with or positioned extremely close to the switching silicon.
The major advantage is the extremely short electrical path between the switch ASIC and optical engine. Shorter electrical connections can reduce signal loss and potentially improve power efficiency at very high data rates. CPO therefore represents a highly integrated architecture with the potential for very high bandwidth density.
The challenge is that this integration also changes maintenance and system design.
When the optical engine is closely integrated with the switching ASIC, replacing a failed optical component is no longer as straightforward as removing a conventional pluggable transceiver. Manufacturing processes, optical alignment, thermal design, packaging, testing, and supply-chain coordination also become more demanding.
XPO takes a different approach.
Rather than eliminating the pluggable architecture, XPO attempts to push it to a much higher level of density and integration. It combines multiple high-speed channels, advanced power delivery, and liquid cooling within a pluggable optical unit.
This creates an important middle ground between traditional pluggable optics and fully integrated CPO.
| Aspect | XPO | CPO |
|---|---|---|
| Architecture | Extra-dense pluggable | Co-packaged / highly integrated |
| Optical Engine Location | Within pluggable optical unit | Closely integrated with switching ASIC |
| Electrical Path | Shortened through high-density architecture | Extremely short |
| Thermal Design | Can incorporate liquid cooling | System/package-level thermal management |
| Replaceability | Maintains pluggable concept | More difficult at individual optical-engine level |
| Upgrade Flexibility | Relatively high | Lower |
| System Integration | High | Very high |
| Main Advantage | Balance between density and serviceability | Maximum integration and potential efficiency |
| Main Challenge | Mechanical, power, and thermal complexity | Manufacturing, maintenance, and integration complexity |
This makes XPO and CPO less like direct competitors and more like two different points on the evolution of optical interconnect architecture.
Why XPO Matters for AI Data Centers
The value of XPO becomes clearer when considering the physical constraints of large AI clusters.
AI infrastructure is increasingly constrained by the amount of computing power, network bandwidth, power delivery, and cooling that can be installed within a fixed amount of rack space.
Suppose a network architecture requires a very large aggregate switching capacity. With conventional pluggable modules, achieving that capacity may require a large number of optical ports and therefore more switch hardware, racks, cabling, power, and cooling infrastructure.
Increasing bandwidth density can change that equation.
If significantly more network capacity can be concentrated into a smaller physical footprint, the number of network racks required for a given amount of computing capacity may potentially be reduced. This can also reduce the associated requirements for power distribution, cabling, cooling infrastructure, and physical space.
For AI data centers, this is particularly important because networking is no longer an isolated infrastructure layer. The network must scale together with the compute layer.
The objective is therefore not simply to make one optical connection faster. The broader objective is to create a network architecture in which compute density, network density, power density, and cooling capacity can scale together.
Is XPO the Future of Optical Networking?
It is too early to say that XPO will replace conventional pluggable optics or become the dominant alternative to CPO.
The architecture introduces significant advantages, but it also creates new engineering challenges. High-density electrical connections require extremely careful signal-integrity design. High-power optical units require advanced thermal management. Liquid cooling introduces fluid connections and service considerations. The larger and denser mechanical structure also changes how technicians interact with the optical interface.
These factors mean that XPO is likely to be most relevant where its advantages justify the additional system complexity.
AI training clusters and high-performance computing environments are obvious candidates because they place unusually high demands on bandwidth density and thermal efficiency. In more conventional enterprise or cloud networking environments, standard 400G, 800G, and future 1.6T pluggable modules may remain more attractive because of their mature ecosystem and simpler deployment model.
Conclusion
The evolution from traditional pluggable optics to XPO and CPO reflects a broader change in how data center networks are designed.
Traditional optical modules primarily focus on increasing the bandwidth of individual ports. As AI clusters become larger and more power-dense, however, the industry must consider a much wider set of constraints: bandwidth density, power delivery, thermal management, physical space, reliability, and serviceability.
CPO addresses these challenges through extreme integration, bringing optical engines much closer to the switching silicon. XPO takes a different path by maintaining the fundamental pluggable concept while dramatically increasing channel density and integrating advanced power and cooling technologies.
This makes XPO an interesting middle ground between conventional pluggable optics and fully integrated optical architectures. It does not attempt to solve every problem through maximum integration. Instead, it focuses on achieving a practical balance between high bandwidth density, advanced thermal management, system efficiency, and maintainability.
As 800G, 1.6T, and future high-speed networking technologies continue to develop, optical interconnect design will increasingly be evaluated at the system level rather than simply by the specifications of an individual transceiver.
Frequently Asked Questions
What does XPO stand for?
XPO stands for eXtra-dense Pluggable Optics. It describes an emerging high-density pluggable optical architecture designed for next-generation AI and data center networks.
How is XPO different from conventional OSFP?
XPO uses a much denser architecture with more high-speed channels, advanced power delivery, and support for system-level liquid cooling. Conventional OSFP modules are generally smaller, individually pluggable transceivers designed around more traditional electrical and air-cooling architectures.
Is XPO the same as CPO?
No. CPO integrates optical engines closely with the switching ASIC, while XPO retains a pluggable optical architecture and increases density through multi-channel, multi-PCB, advanced thermal, and power designs.
Why does XPO use liquid cooling?
As optical bandwidth and module power increase, removing heat through conventional airflow becomes increasingly challenging. Integrating a cold plate into the XPO architecture provides a shorter and more direct thermal path and allows the optical interconnect to work with liquid-cooled data center infrastructure.
Will XPO replace traditional optical modules?
Not necessarily. XPO is primarily intended for extremely high-density applications where conventional pluggable optics face bandwidth, power, or thermal limitations. Standard optical modules will likely remain important across many conventional data center and networking applications.
Why is XPO relevant to AI data centers?
AI clusters require extremely high aggregate network bandwidth while operating under strict power, space, and cooling constraints. XPO’s focus on bandwidth density and system-level thermal management makes it particularly relevant to these high-performance environments.
