Quick answer: Active optical against copper comes down to distance and environment. A passive copper DAC is cheap, draws no power and reaches about 5 m at 25G. An AOC converts the signal to light inside the connector shell, draws typically under 1 W per end, weighs less and bends tighter, but costs more and cannot be repaired in the field at all.
Two racks sit three meters apart and the design calls for 25G. A copper DAC does the job for less money, and an AOC does it with a thinner, lighter cable that bends where copper refuses to. The choice rarely turns on speed. It turns on distance, airflow, weight on the connector and what happens when a cable is damaged during a maintenance window. This guide compares the two on the properties that decide the install.
How the two cables move a signal
A copper DAC carries the electrical signal the whole way, and the plug shell holds little more than a latch and a circuit board. An AOC holds an electrical to optical converter at each end, so the signal travels as light between the two shells and returns to copper inside each one.
Copper DAC behavior
Passive copper is the simplest option: no active parts, no power budget, nothing to fail except the cable and the contacts. Signal loss rises with length, which is why a 25G copper DAC is limited to about 5 m. Past that length, equalization cannot recover enough of the signal to keep the link reliable.
AOC behavior
An AOC buys distance with power. Each end draws typically under 1 W to drive the laser and the receiver, and the link works well beyond copper reach. The trade is that the ends are sealed: a damaged connector means a new cable, and the module power has to fit the switch power budget.
Weight, bend radius and power draw
In a dense rack these three properties decide how the install goes, far more than the reach on a datasheet.
Weight and bundle size
Optical cable is far lighter than copper of the same lane count, so a bundle running from a top of rack switch to a patch panel puts less load on trays and on the switch connectors. Lighter bundles also hurt less when a technician has to move one to reach a port behind it.
Bend radius
A copper DAC has a stiff minimum bend radius and holds the shape it is bent into, which is a problem in a cabinet with a shallow door. An AOC bends tighter and springs back, so it routes around corners that copper cannot take. Neither one should be forced past its rated radius.
Power
An AOC draws typically under 1 W per end, and that counts against the switch power and heat budget for every port. A passive copper DAC draws nothing at all. Across a full rack the difference is small per port but real at the power supply.
Where each one fails
Failures look different. Copper fails at the contact and the latch, while an AOC fails at the shell and in the fiber inside it.
| Factor | Copper DAC | AOC | Practical effect |
|---|
| 25G reach | 5 m limit | Well beyond 5 m | Distance decides first |
| Weight | Heavy at high lane counts | Light per lane | Tray and connector load |
| Bend radius | Stiff, holds a set | Tighter, springs back | Routing in shallow cabinets |
| Power draw | None | Typically under 1 W per end | Switch power and heat budget |
Practical scenarios
- Top of rack to adjacent rack: Three meters at 25G is copper territory. A passive DAC costs less and adds no power, and the run is short enough that loss never becomes the issue.
- Cross aisle at 25G: Twenty meters breaks the copper limit. An AOC carries the link, saves weight in the tray and keeps its bend radius inside the manager.
- High density spine: Thirty two links in one rack make the power budget visible. Add the AOC draw per end before sizing the supply, or use copper where distance allows.
- Field repair: A crushed copper DAC can often be swapped from stock, while a crushed AOC cannot be repaired at all, so keep spares in the lengths you actually use.
FAQ
Is an AOC faster than copper?
No. Both carry the same lane speeds. The difference is reach: a copper DAC is limited to about 5 m at 25G, while an AOC works over much longer runs because the signal travels as light between the two ends.
Do AOC cables need special ports?
They plug into the same cage as a copper DAC or an optical module. The difference sits inside the shell, where the electrical to optical conversion happens. Power comes from the port, so the switch power budget has to include it.
Which one survives a tight cabinet better?
An AOC, in most cases. It bends tighter and weighs less, so it puts less strain on the connector and the tray. A copper DAC is stiffer and can hold a bend that keeps pressure on the plug.
Next step
Size the link before you order. The AOC selection guide covers length, lane count and power budget, the fiber cable selection guide explains jacket and fiber choices for the same run, and the fiber loss budget guide shows how to add up every interface in the path.
