My 100G transceivers show "link down" even though everything is connected. What should I check first?

Start with these most common checks: Clean the connectors Check FEC settings Verify fiber type Check DDM(TX/RX power) readings

How do I know if my received optical power is too high or too low?

Check the Digital Diagnostics Monitoring (DDM) data using your switch's CLI, there you will find the RX actual readings, usually the alarms for various states are in affect if the RX is either low, too low, high or too high. Additionally you can compare the redings to the acceptable limits listed in our product datasheets. If power is too high on short links, add optical attenuators or the laser may burn down. If too low, clean connectors, check for fiber damage, or verify the link distance is within specifications.

I've tried everything but my 100G link still won't come up. What's a systematic approach to isolate the problem?

You can fallow these steps to determine the root cause of the failure you are experiencing Loopback test - Guide for this test can be found here! Known-good swap - Swap the faulty module with a one that you know works without issues and check if the link comes up Cable swap - Swap the patch cables used Port swap - Test faulty modules in different ports on the same switch or in another one Configuration double-check - Verify switch port configurations if they support the module used If all these steps fail, collect the DDM data showing temperature, voltage, current, and optical power readings, then contact our technical support team with this information here.

Finned-Top and Flat-Top OSFP explained

Posted at November 19, 2025 in Useful Information by Gints Golubs

9 Likes

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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