Both of these are the same idea: a cable with the connectors permanently attached at both ends, so a switch-to-switch or server-to-switch link needs no separate transceivers and no patch cord. A DAC (direct attach copper) assembly does it with twinaxial copper and passive or lightly conditioned connectors. An AOC (active optical cable) does it with fiber and real optical transceivers fused into each end. From the switch's point of view both plug into the same SFP or QSFP cages and negotiate the same speeds.
The difference is what the physics allows. Copper runs out of signal integrity within a few metres, so DAC is a within-rack or adjacent-rack part; beyond that the link simply will not come up reliably. AOC keeps the convenience of a fixed assembly but carries the signal on fiber, so the same product class reaches tens of metres — across a row, between cabinets, up a riser. That reach costs more per link and draws real power at each end, because there are lasers in there. The decision is therefore almost entirely a distance decision, with weight and airflow as the tiebreaker in dense racks.
One more axis matters at scale and is easy to miss: neither is field-serviceable in the way a transceiver pair is. If one end of a DAC or an AOC fails, the whole assembly is replaced, and the assembly is a fixed length. A pluggable transceiver pair on a structured fiber plant costs more up front and lets you swap one module or re-patch a length. That is why large fabrics use all three deliberately rather than picking a favourite.
At a glance
Side by side
| Factor | AOC (active optical cable) | DAC (direct attach copper) |
|---|---|---|
| Practical reach | Tens of metres — commonly available from about 1 m out to 30 m and beyond | Short: roughly 1-3 m at 100/400G, up to about 5-7 m at 10/25G; active DAC extends the top end |
| Medium | Fiber, with transceivers permanently fused into both ends | Twinaxial copper, connectors moulded onto both ends |
| Cost per link | Higher than DAC, lower than two pluggable optics plus a patch cord | Lowest of the three options at the same speed |
| Power per end | Real draw — there is a laser and, at high speeds, signal processing in each connector | Negligible for passive (an ID EEPROM only); active DAC draws a modest amount |
| Weight and bulk | Thin and light; easy to bundle and route, better airflow in dense racks | Thick and stiff with a large bend radius; bulky where many run together |
| EMI behaviour | Immune — fiber carries no electrical signal between the endpoints | Copper; susceptible to interference and crosstalk in dense, noisy environments |
| Serviceability | Fixed length; a failure at either end replaces the whole assembly | Fixed length; a failure at either end replaces the whole assembly |
| Typical role | Row-scale and inter-cabinet links, and long AV runs where HDMI over copper has stopped working | Intra-rack server-to-top-of-rack and switch stacking links |
Choose AOC (active optical cable) when
- The run is longer than a few metres but you still want one fixed assembly instead of two optics and a patch cord
- The link crosses cabinets or a row and there is no structured fiber plant to patch into
- Rack density or airflow is a real constraint and thick copper bundles are already a problem
- The path passes through electrically noisy plant where EMI immunity is worth paying for
- It is a long AV run — fiber HDMI assemblies are the same idea applied to a display cable past the point copper stops working
Choose DAC (direct attach copper) when
- The link stays inside one rack or reaches an adjacent one, within a few metres
- You are cabling many identical short runs and want the lowest cost and lowest power per port
- Server-to-top-of-rack or switch stacking links dominate the design
- Rack power and cooling budgets are tight and every watt per port counts across thousands of links
Bottom line
Treat this as a distance decision first and a density decision second. Inside a rack, DAC is the pragmatic default — cheapest, near-zero power, and simple. Once a span leaves the rack or the copper bundles start fighting the airflow, AOC keeps the single-assembly convenience while buying real reach and thin, light cabling. Past the point where you want to re-patch lengths, swap a single module, or run through a structured plant, both fixed assemblies lose to a pair of pluggable transceivers on fiber. Most well-designed rows use all three on purpose: DAC in the rack, AOC across it, and optics on fiber for anything that leaves the room.
FAQ
Common questions
- Is an AOC just a DAC with fiber in it?
- Functionally that is a fair description — both are fixed-length assemblies with the connectors permanently attached, so neither needs separate transceivers or a patch cord. The difference is that an AOC has real optical transceivers fused into each end, converting the electrical signal to light and back. That is what buys the reach, and it is also why an AOC draws meaningful power at each end while a passive DAC draws almost none. Physically an AOC is thin and flexible fiber; a DAC is thick, stiff twinax.
- How far can an active optical cable run?
- Much further than copper. AOC assemblies are commonly available from about a metre out to 30 m and beyond, with the exact maximum depending on the speed and the specific product. That covers the cases DAC cannot reach: across a row, between cabinets, up a short riser. Past the range a fixed assembly is practical to route and stock, the better answer is a pair of pluggable optics on a structured fiber plant, because that lets you re-patch the length instead of ordering a new cable.
- Can I mix AOC, DAC and pluggable optics in the same switch?
- Yes, and well-designed fabrics do. All three plug into the same SFP or QSFP cages and negotiate the same speeds, so a single switch commonly runs DAC for in-rack links, AOC across the row and transceivers on fiber for anything leaving the room. Selecting by span length rather than by preference is standard practice and is how you optimise cost and power without compromising the runs that genuinely need reach.
- What happens if one end of an AOC fails?
- The whole assembly is replaced, because the transceivers are permanently attached and the length is fixed. That is the trade a fixed assembly makes in exchange for being cheaper and simpler than two optics plus a patch cord. It is also the reason large deployments hold spare assemblies in the lengths they actually use, and the reason a fabric expected to be re-patched or re-lengthed over its life is usually better built on pluggable optics and structured fiber.
- Are fiber HDMI cables the same thing as an AOC?
- They are the same principle applied to a display cable. A fiber HDMI assembly puts the optical conversion inside the connectors so the run can be far longer than passive copper HDMI supports, which is typically the point past roughly 15 m where a passive cable stops holding a high-resolution signal reliably. Like a datacentre AOC it is directional and fixed-length, so the source and display ends are not interchangeable and the length has to be measured before ordering rather than after.