What Is a DAC Cable? Direct Attach Copper Explained

A passive DAC contains no electronics. The transceiver body holds the connector, the latch and an EEPROM that identifies the assembly to the switch; the signal passes through the copper unconditioned. Passive assemblies draw no power beyond what the port supplies for identification, add no processing delay of their own, and are the shortest and cheapest option.

An active DAC adds signal conditioning in the connector body — equalisation and re-driving — so the assembly can reach further than a passive one of the same gauge. It draws more power from the port and is used where the physical run is at the edge of what a passive assembly will carry cleanly.

Cable gauge is the third variable and it is on the label for a reason. A thinner conductor bends into a dense rack more easily and impedes airflow less; a thicker one carries the signal further. On a rack full of high-speed links, the bundle behind the switch is a real thermal consideration, not a cosmetic one.

An active optical cable (AOC) is the same idea in glass: transceivers permanently attached to a fiber assembly, fixed length, no field termination. It reaches much further than copper and weighs less in a bundle, at a higher cost per link, so the boundary between DAC and AOC is essentially where copper stops being reliable.

Pluggable optics plus a fiber patch cord are the flexible option: any length the plant supports, transceivers that can be replaced independently, and cords that can be re-used. They cost more per link and add two more failure points and an optical power budget to manage. For a top-of-rack switch to servers in the same rack, that flexibility buys nothing.

So the ordinary rule is: inside the rack and to the rack next door, DAC. Down the row or between rows, AOC. Between rooms or where the length is not yet decided, optics and structured fiber.

Confirm the form factor at both ends first — SFP+ at 10 Gigabit, SFP28 at 25, QSFP+ at 40 and QSFP28 at 100 — and whether the link is a straight assembly or a breakout that fans one QSFP into four SFP ports. A breakout is a different part number and a different switch configuration.

Then confirm coding. Switches read the EEPROM in the connector body and many will refuse, or log a warning about, an assembly whose identification does not match what the platform expects. Since a DAC has a body at each end, an assembly joining two different platforms has to satisfy both.

Then measure. The length is fixed at purchase, so measure the routed path — up the rack, through the manager, down the other side — rather than the straight-line distance, and leave enough slack for the cable to be dressed without violating its bend radius. A DAC that is 200 mm short is scrap.