Finned-Top and Flat-Top OSFP explained
When browsing and searching for OSFP optical transceivers you might have come across two types: finned and flat-top. Two different housing versions for two different use cases, so which to choose? In this article we will explain what each of the versions are, how do they differ and which to choose for your applications!
First things first
The OSFP transceiver. The finned-top and flat-top transceivers are the two caging versions for a Octal Small Form-factor Pluggable also known as OSFP. It is designed to support very high-speed data links in modern data centers and telecom networks.
The term “Octal” refers to the modules eight electrical data lanes, each capable of carrying 50Gbps, 100Gbps, or even 200Gbps per lane, depending on modulation (PAM4). This architecture enables total link capacities of 400G, 800G and even 1.6T.
OSFP modules are engineered to handle power consumption up to 25W or higher, depending on the optics type. This makes the form factor suitable for advanced modulation schemes, coherent optics, and longer-reach links. And as the link speeds rise and module power increases, OSFP offers a larger thermal envelope and flexible cooling options.
Finned-top transceivers
A finned-top OSFP module has an integrated heat sink (metal fins) built into the top of the transceiver caging. These fins increase surface area and improve heat dissipation. Such transceivers are ideally suited for 800G optical modules. These high power modules can generate significant heat, that’s up to 15–20 W or more, so the fins here help to maintain acceptable temperatures by increasing airflow contact.
The finned-top caging as well has two variants: closed finned-top design and a open finned-top design.
A closed finned-top OSFP uses a sealed or partially enclosed fin structure. On the other hand a open finned-top OSFP has exposed, spaced fins with direct airflow channels through or between them. The difference between the two is not just the design itself, but how each manages airflow and heat transfer. The open finned-top allows air to move through the fins for more efficient cooling, while the closed finned-top relies on air flowing over the surface, offering better protection and structural strength in lower airflow environments.
Flat-top transceivers
The flat-top module design refers to optical transceivers where the top structure of the optic is flat. This design is usually used for lower power modules, for example, the 400G. The colling for this type of transceiver need to be done externally.
The flat-top transceiver is also called as a RHS, which means Riding Heat Sink. This means that the heat sink is a part of the host system. When an OSFP transceiver is inserted in the cage of the host device the heat sink is on top of the transceiver providing efficient heat dissipation. That’s where the name comes from – riding heat sink.
Typical use cases – which one to choose
When choosing the appropriate design it of course mainly differs on the systems cooling design, power class, and mechanical clearance.
Finned-Top OSFP is best suited for:
- •High-power modules (>15 W), typically 800G and above
- •Air-cooled systems where strong front-to-back airflow is available
- •Switches or line cards without cage-mounted heatsinks (the riding heat sinks) – from just a visual perspective there is not a possibility to insert a finned-top transceiver into a host device meant for RHS
Flat-Top OSFP is best suited for:
- •Lower-power modules (≤15 W), typically 400G
- •Systems with cage-integrated or chassis-level cooling (RHS or liquid cooling)
- •High-density platforms where space and airflow paths are tightly managed
So in short 800G transceivers have the finned-top design and are meant for usage in systems where module must cool itself using airflow through its own fins and the 400G OSFP have a flat-top design and are meant to be used in host devices where the cooling is done externally.
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400G OSFP & OSFP112 Optical Transceivers
400G OSFP and OSFP112 transceivers deliver 400 Gigabit Ethernet connectivity in the OSFP form factor, built for data center spine-leaf fabrics, campus backbones, and metro links. Both variants share the same cage and the same optics. The difference sits entirely on the host side, in how many electrical lanes the module expects from the connected switch or NIC. OSFP modules use a 400GAUI-8 electrical interface: eight lanes at 53.125 Gbps PAM4, converted by an internal gearbox to four 100G-class optical lanes. These suit host platforms built on 50G-class SerDes. OSFP112 modules use 400GAUI-4: four lanes at 106.25 Gbps PAM4 mapped one-to-one onto four optical lanes with no gearbox, for platforms built on 100G-class SerDes. Because the optical side is identical, an OSFP DR4 and an OSFP112 DR4 can interoperate across the fiber when the PMD and link parameters match at both ends. The form factor choice is dictated by what the host equipment presents electrically, not by what sits at the far end of the link. Our portfolio spans the full range of reach requirements. Short-reach VR4 and SR4 modules handle intra-rack connections over multi-mode fiber at 50m and 100m, while DR4 and DR4+ variants extend parallel single-mode connectivity to 500 meters and 2km. For cross-building and metro links, FR4 and LR4 modules provide CWDM4-based transmission reaching 2km and 10km respectively. The OSFP112 modules in this range use a flat-top (RHS) design, relying on a riding heat sink integrated into the host cage, while the OSFP modules are finned-top and cool through their own fin stack. Some cages accept only one style, so confirm the exact port specification before ordering. All modules support DDM/DOM diagnostics and comply with OSFP MSA standards. Confirm the electrical interface and heatsink variant the platform expects before ordering. The module reports the same identifier for the finned-top and flat-top heatsink variants, so it will not flag a heatsink mismatch on its own.
800G OSFP800 Optical Transceivers
800G OSFP800 transceivers represent the cutting edge of data center optical connectivity, purpose-built for AI/ML clusters, GPU interconnects, and hyperscale spine-leaf architectures demanding maximum bandwidth density. The OSFP800 form factor supports eight 100G electrical lanes using PAM4 modulation, delivering 800Gbps aggregate throughput per port. Our portfolio spans the full range of data center reach requirements. Short-reach SR8 and 2×SR4 modules handle intra-rack connections over multi-mode fiber, while DR8 and 2×DR4 variants extend parallel single-mode connectivity to 500 meters. For cross-building links, 2×FR4 and 2×LR4 modules provide CWDM4-based transmission reaching 2km and 10km respectively. For latency-sensitive workloads, our Linear Pluggable Optical (LPO) series eliminates DSP processing entirely, reducing power consumption and minimizing signal delay—critical for distributed AI training where microseconds matter. All modules comply with 800GAUI-8 electrical interface specifications and OSFP MSA standards for seamless interoperability across leading switch and NIC platforms.