alt.hn

3/22/2026 at 11:44:33 PM

Unlocking 25 Gigabit/S on 10 GbE Direct Attach Copper

https://kohlschuetter.github.io/blog/posts/2026/03/22/unlock25/

by kohlschuetter

3/23/2026 at 11:53:44 AM

Very cool! Also cool to see inside. It really is directly attached with nothing in between, I honestly wasn't expecting there to be NOTHING in the signal path.

> In a sense, it’s a revolutionary way of how to network — it separates the “what” from the “how”. To be fair: that revolution already happened over 20 years ago

How quickly we have forgotten about AUI (Attachment Unit Interface)

by kalleboo

3/23/2026 at 1:22:49 PM

AUI was still very much alive with XAUI[0] during the 10Gbps era!

If you're willing to be a little bit more flexible, the MAC-to-PHY distinction never really went away with the various MII generations: every speed from 10Mbps to 10Gbps is available using more-or-less the same interface.

[0]: https://en.wikipedia.org/wiki/XAUI

[1]: https://en.wikipedia.org/wiki/Media-independent_interface

by crote

3/23/2026 at 12:23:56 PM

Thanks!

AUI: So that's what that port next to the BNC connector on the 3COM 3C509B was for! We've come a long way.

by kohlschuetter

3/24/2026 at 2:59:37 AM

At least DACs work out of the box even with an Intel NIC...

I tried to do something crazy. I had a 4-port XMC 10Gb NIC in the flight computer for a balloon experiment. Two ports of which were used for 10 Gigabit links to an FPGA for data acquisition. I wanted one port to be 100-BaseFX. Turns out that it was not possible to get a compatible SFP module (ok, it's sort of crazy to use a 10 Gb NIC for a 100Mb connection, but the copper RJ45 transceivers all work fine!).

by cozzyd

3/23/2026 at 12:33:12 PM

That was a very eye-opening post, full of very interesting details on how modern wired networks work, between NIC and the physical layer... so many mysteries about SFP black magic just falled into place in my head...

by touisteur

3/23/2026 at 4:36:10 PM

Thank you! I had to stop digging at some point, but honestly I can't wait for someone to port Doom to a ONU.

by kohlschuetter

3/23/2026 at 7:41:55 PM

Forgot about ONU ! Now I need to look into what an ONU can do, and what one can run on it...

by touisteur

3/23/2026 at 10:22:24 PM

Same here!

I also enjoyed the writing style, and so went and read another article on the blog.

by jiehong

3/23/2026 at 3:32:23 PM

So... can we upgrade our existing sfp connectors to 25GBPs if our cable quality is good enough?

by psyclobe

3/23/2026 at 4:31:20 PM

I was going from SFP+ to SFP28. Not sure about SFP to SFP28. But with that EEPROM change you could also try going from SFP to SFP+, just use different bandwidth values.

You could also try resoldering new/thicker Twinax wires to old SFP connectors and update the EEPROM. My soldering skills apparently aren't that great. If yours are or you have plenty of old connectors you should give that a try.

by kohlschuetter

3/23/2026 at 7:40:39 AM

Super brilliant! Great work. I could have used this when a vendor sent me cables that had incompatible codes on both ends.

by jeffrallen

3/23/2026 at 4:32:50 PM

Thank you! I hope you all have trusted vendors, and no-one gets bent over by the sudden flood of cheap SFP28 cables :-)

by kohlschuetter