What Is a Direct Attach Copper (DAC) Cable

While working for a broadcasting/networking integration gig, we designed, assembled and field-terminated thousands of copper and fibre runs from 1/2 ft to football field lengths.

The amount of time required to terminate a copper cable in the field is seconds, and felt a bit like art. Something about the way it reliably reacted was magical and felt "strong."

Terminating or splicing a fibre cable felt like wrestling a snake covered in melted crayons, and the failure rate was significantly higher across the board. And it wasn't just workmanship, but quality of product, terminating environment, available equipment, misuse by future operators etc.

That said, at a certain point, we as a firm learned that most purchasers would rather the low latency/small footprint of optical/fibre versus copper, maintenece/failure be damned. Though, maybe part of our willingness to push fibre came from knowing that most purchasers would in 1-2 years call us back in to replace the rack terminations with copper :)

Many 10GbE DACs can be hacked to run 25GbE, which is something I had to figure out after realizing that it’s not just plug and play as it was with twisted pair cables coming from 1Gb to 2.5Gb…

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

I'm waiting for 3 DACs and a few other bits to arrive today to move closer to 10G networking at home. Moving house soon and the new place will have 2.5Gbps FTTP (both up and down) so I wanted to be prepared for that. Given my existing broadband is only 500/75Mbps FTTP I was fine with a 1GbE internal network and Wifi-6 meshing. I could have planned to move to 2.5GbE but it may have been a bottleneck at some point, so may as well push straight on to 10G.

I have a USW-Aggregation with 8 SFP+ ports arriving today too. Just have to install Intel X520-DA2 cards in two of my servers (Proxmox host and a general Linux server), and the NAS also has a 10G SFP+ port, and then connect it all up.

Most of it second hand from eBay for half the usual retail price.

I’m a huge fan of fiber optic cables instead of DAC. For one thing, the distance you can run is far greater. For another, they don’t get as hot as metal cables, particularly at high speeds. OM3 and OM4 is actually quite affordable and SFP+ is nice.

Yes on an absolute sense, DAC can be cheaper, but for a homelab or something, it’s not that much different. Also, it’s much cooler to run fiber optic to things (imho).

> This is a bit dependent on the type of DAC, and the vendor. Generally speaking, we are moving into an era as we move above 100GbE speeds where DACs will be limited to 5m runs and shorter runs.

This is generally good enough for intra-rack runs, especially if you put your top-of-rack (ToR) switch in the middle: you basically only have to go about 1m/3' in either direction (up/down).

Surprised it doesn't mention heat - one of the big advantages.

There is also a new generation of "invisible" fiber that is interesting for home use. Basically looks like fishing line so you can run it somewhere that you just don't see unless you're really looking. Fragile tho

I picked up one of these just a few days ago for use with some SFP28 ports. They're cheap, fast, and don't use any power.

Dacs are great … they let you avoid having to convert to optical or having to splice your own cable.

Downside the cables are kind of heavy and sometimes folks dont like that.

Dacs are awesome until you encounter a full rack with nothing but 2 and 4 meter dacs. They appeal very much to the lazy engineer, and a lazy engineer can turn them into a rats nest.

Is this connection galvanically isolated?

Too bad they didn't mention that while the cable is direct attach, the ends each have an EEPROM in them saying what the cable is. This EEPROM can be a source of immense frustration, if it's got the wrong data in it, or if both ends do not agree.