[zamadatix]

There are 2 very different main types of "copper" in this context. Each can break down into more specific subcategories but they have a relatively common general behavior with their primary type.

First there is the BASE-T RJ45 stuff, which it sounds like you might have been working with. At 10G or higher speeds this get relatively power hungry and is not really an advantage over fiber unless you are also delivering PoE or are trying to reuse existing cabling.

This type (DAC) is a special type of pre-made cable assembly which eschews much of the advanced signalling/conversion logic. The upside is the power usage is low (often even lower than fiber) and the cost is dirt cheap. The downside is the lengths are much more limited and it's intended to be preterminated SFP-to-SFP (or the like) cable assemblies instead of modular patching/custom built.

There is indeed a latency difference, usually DAC < fiber < BASE-T, but they are all within a few microseconds (not milliseconds) of each other so you really have to be pressed to care about it (to the point you're looking at specialized low latency switches and paying extra to lay things out in a way which minimizes the number of L2 hops rather than the cost).

[myself248]

> it's intended to be preterminated SFP-to-SFP

I wonder why nobody's making field-terminatable DACs for custom lengths. If you've torn one down, they're not exactly complicated inside.

If they existed, would people buy them?

[Aurornis]

DAC cables use twinax. Properly terminating twinax is not easy. It's not as forgiving as twisted pair going into an RJ45.

DAC cables have a limited length range. It's not like 10GBASE-T where you can run the cable hundreds of feet and then put a connector on exactly where you need it. The cables only work at short distances so it's easy to stock the cable sizes close enough to everyone's needs.

[robocat]

Twinax: https://fs.com/blog/what-is-twinax-cable-1928.html

[zamadatix]

I think the only reason RJ45 termination is an (occasional) thing for short patching is you're also in spec to go 100 meters with it so the tools and materials to do so are already commonly in use.

For passive DACs the range of lengths is so low you can just get away with having 2 or 3 different lengths on hand and never need to worry about it. Active DACs start to be too much to bother with again.

Fiber it's possible, but again really only because you can go kilometers with it rather than because people want to make short patch cables themselves.

[jeffrallen]

Nope. For production, you want to reduce risk and variation. DACs are already available in about 5 sizes up to the max 7m length, why would you terminate any other size in the field?

[iso1631]

I did spec a couple of 7m dacs a few months ago to run between two adjacent bays, but normally for more than 2m I'll just drop a SM sfp and run a preterminated fibre cable.

In the field its the armoured fibre on a reel, 100m, 200m, 500m etc, with opticon connectors, or some normal cat5 typically for APs

[bobdvb]

Some network guys I know prefer fibre even within the rack, just because they don't want the weight and the obstructions in the rack. Apparently more than once the weight of the DACs and the bulk of the cable bundle has caused a problem with NICs.

Personally that surprised me, but I can see where they're coming from.

[buildbot]

Yep, I did this for my little DGx spark cluster because 100-200gb copper cables are very thick, heavy, and annoying.

[mwpmaybe]

If you start doing bonded links with DACs or if you have a bunch of servers, the cable management situation gets ugly in a hurry, and the usual solutions like patch panels and keystones aren't applicable. Source: my basement

[bobdvb]

That's my home lab, it's ugly and I am starting to use MTP breakouts instead because those DACs get in the way so much.

[ragall]

> the usual solutions like patch panels and keystones aren't applicable

Why not ?

[wmf]

Support brackets would be 10x cheaper than fiber.

[nucleardog]

Admittedly I'm not buying Enterprise Grade(TM) stuff, but...

For simplicity I just use 10G LR modules everywhere. A pair of fiber transceivers is $25. Pretty sure last batch of 3M pre-terminated fibre cables I was grabbing were like $3 a piece or something. So we'll round up and call it $30.

I can get a 3M DAC for about $20.

So yeah it's cheaper, but the price isn't _that_ different. I was using DACs in quite a few places (and still am), but in general I've found it easier just using fiber. (For one, I've had a few devices that didn't get along with various DACs but worked just fine with the fibre transceivers.)

[iso1631]

The fibre and sfps are a tiny price of the entire solution

DACs will be cheaper than fibre in a bay, but between neighbouring bays about the same.

A 7m passive 40g qsfp dac is £80. A pair of multimode 40g is £66 and 7m multimode cable is £10.

A 2m dac is £28, so fibre is 3 times the price.

[justsomehnguy]

If you do it only once and would never touch it - yes.

But the time spent on digging around and occasionally debugging what and where exactly came off and no longer links (at best) or there is still link up but the are too many errors, add some SLA on top of it... No.

[Filligree]

Doesn't that reduce to RJ45?

[mwpmaybe]

No. It's twin-axial. Think coaxial, but more axial.