DAC or AOC?

[Hinterwaeldler]

So, this is not really about TrueNAS, but people on this forum look like they know their stuff ...
I'm a software guy, and 10 gps network gear is still confusing me.

So after reading jgreco's 10Gig Network Primer, I went with SFP+, mainly because 10GBASE-T uses more power (and power consumption leads to heat, and heat leads to noise, which I don't want to have near me).

For the cabling I went with Active Optical Cables (3-5m), because I thought "copper uses more power than fiber", and a DAC (direct attach copper) is a copper cable. But a friend of mine pointed out that passive copper DAC uses less power than AOC, and quite some sources on the internet seem to prove his point, i.e.

DAC Cables vs AOC Cables | FS Community

DAC cables and AOC cables are widely applied in data centers but normally in different working scenarios. What are the differences and how to choose? Find answers here.

community.fs.com

"Normally, the power consumption of AOC cables is higher than DAC ones, which is 1-2w."

So whats the deal with cabling? Do you go with DAC or AOC for short distances, and whats your reasoning?

 

[QonoS]

passive DAC

It is cheaper to get in used condidtion in my experience. The difference in power consumption is so small anyway that noise levels won't be diffent.

Side note: When talking about 10G and saying "copper uses more power than fiber" people usually mean 10GBASE-T, which is copper too. 10GBASE-T consumes more power than passive/active DACs do. In that sense not all copper is equal. 10GBASE-T can do more than 15m. With higher distance power consumption rises.

 

[jgreco]

The 10GBase-T vs DAC thing is already explained, so let's just focus on things that are NOT power hungry 10GBase-T for the remainder of this message:


AOC are meant for somewhat longer runs where DAC length limits come into play.

In the case of passive DAC, you have minimal electronics that eschew the electronic->optical->fiber->optical->electronic transformation in favor of electronic->copper->electronic. The copper in this case is VERY power efficient because it is eliminating the optical/electronic conversion twice, but it is only usable over VERY short distances (think: within a rack for passive DAC, maybe next rack over). It is also the lowest latency option.

For active DAC, some electronics are introduced back in, which introduces slight latency and some minor additional power consumption, and can reach "nearby" to maybe a few racks over.

For AOC, you end up with a situation much more similar to plain optics with fiber (think: very similar to just gluing LC fiber permanently into an optic). It may be slightly lower power because the AOC may be optimized for the shorter length.

In all cases, I don't really recommend this stuff for beginners. True LC fiber is very cheap to replace, so if you get SFP+ optics and just link them with fiber, you can always reuse the optics if you suddenly decide you need to shuffle things around. Additionally, many cards require vendor-coded SFP+'s, which is super-frustrating if you are using DAC/AOC and don't have a properly coded set of ends. By way of comparison, you can ALWAYS buy a card with the correct optics, leave the optics in it, and be virtually guaranteed that you can connect any LC fiber to it, regardless of what is on the other end.

If you are absolutely certain of your architecture, you can buy DAC or AOC and that's fine. Just be aware that it can make switching gear out in the future very difficult. Yesterday's Force10 and Intel X520 (both technically vendor-locked to some extent) all connected via DAC/AOC could be a problem if you later want to upgrade to a different switch or different ethernet card, and you've got all this existing DAC or AOC that won't work with it.

Also, for those of us who like to groom our racks neatly, LC fiber can be ordered to centimeter lengths, so that you do not have craptons of leftover DAC/AOC cable to figure out where to bury it.

 

[Hinterwaeldler]

Thanks, I now have a much clearer picture about the cabling.

 

[jgreco]

I'm a software guy, and 10 gps network gear is still confusing me.

So after reading jgreco's 10Gig Network Primer, I went with SFP+, mainly because 10GBASE-T uses more power (and power consumption leads to heat, and heat leads to noise, which I don't want to have near me).

I've reviewed the 10 Gig Networking Primer, and your confusion isn't unwarranted. Because I generally omitted DAC and AOC from consideration because of the direction I was trying to point new users in, I used the word "copper" to mean 10GBASE-T in some contexts where it could have been confusing if you were also aware of copper DAC.

It's a bit of a balancing act, sometimes, to be helpful yet accessible to new users while also being sufficiently precise to convey meaning correctly to those who know a bit more but who don't know quite enough to get implied context.

I hope that this wasn't an inconvenience or source of undue expense to you. Please feel free to look over the Primer again and see if it is clearer now. I always solicit feedback about confusion such as this, since it is my desire to generate highly useful materials to introduce these tougher topics to new users.