Direct attach cables: when copper beats optics
A direct attach cable is a length of twinaxial copper with a transceiver housing moulded onto each end. The housing has the EEPROM that identifies the cable to the switch, but there is no laser, no optical subassembly and, in a passive DAC, no powered electronics at all. It is the cheapest and most reliable way to join two ports that are close together.
Why it wins inside the rack
Compare a 3m link built three ways. Two 10G SR transceivers plus a fibre patch cord is three parts, three things to order and two optical interfaces to keep clean. A 3m 10G DAC is one part at a fraction of the cost.
The power difference matters at scale. A passive DAC draws essentially nothing, while an SFP+ optic is typically around a watt. Multiply that by two ends and by a few hundred links and it shows up in both the power budget and the heat you have to move.
Reliability follows the same logic. There is no laser to degrade and no endface to contaminate, and the two commonest causes of optical link trouble — a dirty connector and a bend past the minimum radius — simply do not apply.
Where it stops working
Distance. Passive twinax attenuates fast, and the faster the signal the faster it gives out. As a practical guide, our catalogue runs to 5m at 10G, 25G, 40G and 100G, and to 3m at 400G. That is not a supply limitation, it is physics: a 400G QSFP-DD signal will not survive 5m of copper cleanly.
Bulk. Twinax is thick and stiff, and its bend radius is unforgiving. A rack full of 5m DACs is harder to manage than the same rack full of fibre, and in a dense top-of-rack layout that congestion can cost you airflow.
Fixed length. A DAC is one part with both ends attached. If you need 2.5m you order 2.5m; you cannot re-terminate it and you cannot reuse the ends independently.
Passive, active and why we stopped selling active copper
Passive DACs are plain copper. Active copper cables (sometimes ACU) add signal conditioning in the housing to push the same copper further, at the cost of power draw, price and a component that can fail.
We no longer carry active copper. The reason is straightforward: for the reaches where passive twinax runs out, an active optical cable is now both cheaper and better — thinner, lighter, longer and lower power. Active copper sat in a narrowing gap and the gap closed.
Breakout cables
A breakout DAC splits one high-speed port into several slower ones: one 400G QSFP-DD into four 100G, or one 200G into four 50G. This is how you connect 100G servers to a 400G top-of-rack switch without spending a 400G port on each.
The catch is that the switch has to support the split. Breakout is a port configuration on the switch, not a property of the cable, and a breakout cable in a port that has not been configured for it will not come up. Check the platform first.
The short version
Inside the rack and under 3m, a DAC is almost always the right answer. Between racks, or anywhere past what passive copper will carry at your speed, move to an AOC or to transceivers and structured fibre.