OSFP Thermal Management: Why 800G Optics Need “Finned” Tops and Riding Heatsinks
In the race to 800G and 1.6T speeds, network engineers are facing a new enemy. It isn’t latency, and it isn’t fibre attenuation.
It’s heat.
As data centres pack more density into 1U switches, the physics of optics has changed. While legacy 100G modules were cool enough to touch, modern 800G OSFP modules run hot enough to burn you.
To combat this, the OSFP (Octal Small Form-factor Pluggable) form factor introduced radical changes to how transceivers look and function. If you’ve ever wondered why some modules look like car radiators or why your new network card requires a “Flat Top” module, this guide is for you.
Quick Summary:
- The Challenge: 800G modules consume ~25W+ of power, generating massive heat.
- The Solution: OSFP uses an “Integrated Heatsink” design, unlike QSFP-DD.
- The Variations: You must choose between Finned Top (for switches) and Flat Top (for NICs/Servers).
The Heat Problem: Why is 800G So Hot?
To understand OSFP thermal management, you have to look at the power draw.
- Legacy 100G (QSFP28): Typically consumes 3.5W to 5W.
- Modern 800G (OSFP): Typically consumes 20W to 30W.
This 6x increase in power density is due to the advanced DSPs (Digital Signal Processors) required to calculate PAM4 modulation.
OSFP’s Secret Weapon: The Integrated Heatsink
This is where the design philosophy of OSFP shines.
The competing form factor, QSFP-DD, maintains backward compatibility by keeping a flat top. It relies on the *switch chassis* to have a heatsink that presses down on the module (a “Riding Heatsink”).
OSFP took a different approach. The designers realised that relying on the switch cage wasn’t efficient enough for 25W+ loads. Instead, they built the thermal solution directly onto the module.
When you look at a standard OSFP module, the “nose” is covered in aluminium fins. This increases the surface area significantly, allowing the switch fans to blow air directly through the module’s nose, wicking heat away from the DSP and lasers much faster than a flat surface could.
Finned Top vs. Flat Top (RHS): Which Do You Need?
Recently, a new confusion has entered the market. You might see OSFP Flat Top or OSFP RHS modules listed alongside standard ones.
These were created because the standard “Finned” OSFP is physically tall. While it fits perfectly in a switch, it is often too bulky to fit inside a sleek Network Interface Card (NIC) or a DPU (Data Processing Unit) inside a server.
To solve this, the industry created two physical variants:
| Feature | OSFP “Finned Top” (Integrated) | OSFP “Flat Top” (RHS) |
|---|---|---|
| Physical Look | Has distinct metal fins on top. | Smooth, flat metal top. |
| Cooling Method | Airflow passes through the module’s own fins. | Relies on a Riding Heatsink (RHS) built into the server/NIC cage. |
| Primary Use Case | Data Centre Switches (Spine/Leaf) | Server NICs & DPUs (NVIDIA ConnectX-7, etc.) |
| Compatibility | Standard OSFP Ports | OSFP-RHS Ports (mechanically keyed) |
The Selection Rule:
If you are plugging into a Switch (Arista, Cisco, Juniper), you almost always need Finned Top.
If you are plugging into a Server SmartNIC (NVIDIA, Broadcom), you likely need Flat Top / RHS.
Future Proofing: Ready for Liquid Cooling
The OSFP form factor isn’t just solving today’s heat problems; it’s looking at the 1.6T future.
Because the OSFP shell is robust and designed for thermal transfer, it is the preferred form factor for immersion cooling (dunking the entire server rack in non-conductive fluid) and cold-plate liquid cooling. The “Flat Top” design, in particular, creates a perfect surface for a liquid cooling plate to make contact, enabling power envelopes of 40W or 50W per module in the future.
Conclusion
Thermal management is no longer an afterthought—it is the defining constraint of 800G networking. The OSFP form factor has won the favor of hyperscalers largely because of this superior thermal architecture.
Whether you need the airflow efficiency of a Finned Top or the compact versatility of a Flat Top, ensuring you match the module type to your host device is critical for network stability.
