Following the development of CPO, NPO, and LPO, the optical communications industry now welcomes a new "star": XPO (eXtra-dense Pluggable Optics). Arista Networks, in collaboration with over 45 industry partners, introduced XPO in a white paper. This pluggable optical module solution is specifically designed to meet the extreme bandwidth demands of modern networks, particularly AI clusters and hyperscale data centers. But what exactly is XPO? How does it differ from CPO and NPO, and what unique advantages does it bring?
What Is XPO Transceiver?
XPO represents a major leap in data throughput. XPO module features 64 high-speed electrical lanes using 200Gbps PAM4 signaling, resulting in an astounding total bandwidth of 12.8Tbps per module, which is eight times that of a conventional 1.6Tbps OSFP module. Looking ahead, XPO's roadmap already includes 400Gbps signaling, which will double the bandwidth to 25.6Tbps. XPO module measures 60.8mm × 111.8mm × 21.3mm, roughly 2.7 times wider than a standard OSFP module, but the performance gains far exceed the increase in size, achieving four times the front-panel bandwidth density.
Figure 1: Front and back view of a XPO transceiver module with a yellow release handle
For customers building large-scale Intelligent Computing Centers (AIGC centers), XPO allows for a 75% reduction in the number of switch cabinets. This leads to a massive decrease in physical floor space, lower real estate overhead, and reduced infrastructure costs (including power distribution, cabling, and installation).
Example: A 400MW 100,000-Card Scale Center (128,000 GPUs)
- Using OSFP: Requires approximately 1,408 switch racks.
- Using XPO: Requires only 352 switch racks.
Figure 2: Illustration of the reduction in the switch rack footprint by 75% between OSFP and XPO
Higher port density facilitates a "flatter" network architecture with fewer layers. This streamlined Scale-Out approach reduces the number of data transmission hops, significantly lowering latency—a critical factor for synchronized AI training workloads.
XPO Architectural Features
How does XPO achieve such high integration density? The answer lies in its innovative structural design.
Liquid Cooling
Unlike traditional modules that rely on a single PCB, XPO uses two independent 32-channel PCBs (paddle cards). These are sandwiched around a central liquid-cooled cold plate. This arrangement allows for simultaneous cooling of both PCBs through direct contact.
The Hot Side (Internal): High-power, heat-generating components such as the Digital Signal Processor (DSP), laser drivers, and transmit circuitry are mounted on the inner sides of the PCBs, directly facing the cold plate.
The Cold Side (External): Low-power components, including receive circuitry and control logic, are positioned on the outer surfaces of the PCBs.
Ejection Lever (Release Pull-Tab)
A very prominent handle is visible on the XPO module: the mechanical ejection lever. Because XPO features an incredibly high density of high-speed electrical contacts, the force required for insertion and extraction is substantial.
This lever design provides a 1:11 mechanical advantage, allowing operators to plug or unplug the XPO module manually without specialized tools. This ensures a secure and reliable electrical connection across hundreds of signal pins.
High Voltage, Low Current
XPO also introduces a significant improvement in power delivery.
The Traditional Problem: Conventional pluggable optical modules use a 3.3V DC input. For high-power modules, this results in extreme current (Power = Voltage × Current). For instance, a 400W module at 3.3V would require over 120A of current. This necessitates massive power connectors and heavy copper traces, creating significant design constraints.
The XPO Solution: XPO pulls 48–50V DC (nominal) directly from the rack busbar. The voltage conversion from 48V to 3.3V is then handled internally on the module's paddle cards. For the same 400W module, the required current drops to less than 10A.
By drastically reducing the current, the power connectors can be smaller, and bulky "worst-case" voltage regulators are no longer needed on the motherboard. Moving the voltage conversion inside the XPO module also increases system-level reliability—if a regulator fails, it only affects a single module rather than interrupting the entire switch.
Clean Linear Channels
The XPO architecture is meticulously designed to eliminate signal crosstalk. It strictly separates transmit (TX) and receive (RX) signals onto opposite sides of the paddle cards, creating what is known as "Clean Linear Channels." This layout minimizes interference and maximizes signal integrity.
High-speed signals are physically isolated from power and control signals (such as I2C/I3C, Reset, and Interrupts). Low-speed signals utilize independent, dedicated edge connectors to prevent power supply noise from bleeding into high-speed data paths.
Ecosystem Reuse
XPO holds a distinct advantage in terms of its ecosystem: it is compatible with existing silicon photonics and optical components. There is no need to re-develop chips from scratch, which facilitates rapid industrialization.
The large surface area of the XPO adapter card provides ample design space. It can support virtually any optical module solution currently in existence or under development, including 1600G-DR, FR, LR, SR, ZR, ZR+, Coherent-Lite, and more.
Technical Challenges of XPO
While XPO offers numerous advantages, it also faces significant hurdles that the industry must address.
Extreme Power Consumption
The power demand for XPO is substantial. To achieve a 12.8T throughput, the power consumption of a single XPO module has soared to approximately 400W. By comparison, a mainstream 1.6T module consumes about 25W. Proportional to bandwidth—which is 8x higher—XPO's total power is 16x higher, meaning the power-per-bit is effectively doubled. In contrast, Co-Packaged Optics (CPO) can reduce power consumption by as much as 70% compared to traditional pluggable solutions.
Dependency on Liquid Cooling
XPO relies natively on integrated liquid cooling systems. This mandates specific data center deployment conditions and introduces complex challenges regarding design, long-term reliability, and maintenance costs. Transitioning an air-cooled facility to support liquid-cooled XPO modules requires significant infrastructure investment.
Signal Integrity and Path Loss
Compared to NPO and CPO, XPO places the optical engine at the edge of the PCB (on the front panel), further away from the switch ASIC.
- Longer Electrical Paths: The increased distance leads to higher signal attenuation.
- Increased SerDes Load: To maintain signal quality over these longer paths, the SerDes (Serializer/Deserializer) must work harder, which further drives up power consumption.
Stringent PCB Requirements
The demand for high-speed signal integrity over longer distances places immense pressure on PCB engineering:
- Material Costs: XPO requires ultra-low-loss PCB materials to drive high-speed signals over long distances, which significantly increases manufacturing expenses.
- Structural Integrity: As the PCB surface area grows, engineers must account for board strength and rigidity to prevent sagging or damage.
- Precision Engineering: The "gold finger" contact area is incredibly dense. Managing precision tolerances, lateral warping, and the Coefficient of Thermal Expansion (CTE) of different materials is critical to ensuring a reliable electrical connection.
Outlook for XPO
In summary, the core advantage of XPO lies in its ability to achieve ultra-high density and massive bandwidth while maintaining a pluggable form factor.
Through clever architectural design, it effectively addresses heat dissipation and reliability issues. It also holds a distinct edge in terms of industrial ecosystem readiness and engineering feasibility.
For a long time, the industry consensus was that only CPO (where the optical engine and switch chip are co-packaged and non-pluggable) could achieve the necessary balance of "increased bandwidth + manageable power consumption."
However, operations and maintenance (O&M) teams at major cloud providers have been hesitant to embrace CPO. Their primary concern is flexibility: if a single optical component fails in a CPO setup, the entire motherboard might need to be replaced. This is both labor-intensive and prohibitively expensive.
XPO is essentially a product of compromise. Through innovative design improvements, it manages to satisfy the demand for higher bandwidth while remaining pluggable—even if the trade-off is higher power consumption.
XPO's commitment to an open technical roadmap is a strategic win. It facilitates the rapid formation of an ecosystem, accelerating technological maturity and large-scale adoption while preventing any single company from monopolizing the technology.
While traditional pluggable modules are reaching their physical limits at the 800G and 1.6T levels—and theoretical models suggest a shift to CPO is necessary for 3.2T—CPO faces its own hurdles. Co-packaging requires higher manufacturing precision, faces mass-production difficulties, suffers from lower yields, and carries higher costs.
Conclusion
In the short to medium term, using XPO or NPO as a transitionary technology is a highly viable strategy. The pluggable architecture of XPO allows cloud providers to rapidly scale per-port bandwidth in data centers while keeping costs in check.
Currently, the prevailing industry view is that CPO is a long-term inevitability, but XPO/NPO is a mid-term practical necessity. Whether XPO can achieve dominant market acceptance and how long its lifecycle will last remains to be seen.
Article Source: Understanding of XPO Transceiver
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