The difference between a DAC and an AOC comes down to two things: the medium the signal travels through and how much electronics the cable carries inside. From those two follow its reach, its power draw and its price.
A DAC cable (Direct Attach Copper) is a twinaxial copper pair with two factory-fixed connectors — SFP+, SFP28 or QSFP — one at each end. The signal is never converted to light: that is why a DAC draws almost no power and adds the least latency of any option, and it is also why copper caps its reach at about 7 meters. It is the cable for the server-to-ToR run, inside the rack.
An AOC cable (Active Optical Cable) does perform that conversion: it carries multimode fiber in the middle and sealed optical electronics at each end. With the signal traveling as light, the same link stretches from 7 to 100 meters — between racks or between rows — in exchange for more power draw and more cost.
The third path separates what DAC and AOC integrate: a transceiver in each port and a fiber jumper in between. It costs more on a short run, but each end is chosen separately, the fiber stays installed for several speed generations and reach stops being the ceiling: from 100 meters to tens of kilometers.
This guide compares reach, power draw and relative cost of each technology using standard industry ranges, and gives practical criteria for each run in the data center.
Quick comparison: DAC vs AOC vs transceiver + fiber
| Criterion | DAC (direct attach copper) | AOC (active optical) | Transceiver + fiber |
|---|---|---|---|
| Physical medium | 26–30 AWG twinaxial copper | Integrated multimode fiber | Independent OM3/OM4 or OS2 fiber |
| Typical reach | ≤5–7 m (passive) | 7–100 m | 100 m to 80 km depending on the optic |
| Power draw per end | ≤0.15 W (passive) | 1–2.5 W | 1–4.5 W depending on reach |
| Added latency | Minimal (no optical conversion) | Low | Low |
| Connectors | Fixed, not separable | Fixed, not separable | Removable LC or MPO |
| Link fault | Replace the whole cable | Replace the whole cable | Replace module or jumper separately |
| Structured cabling | Not applicable | Not applicable | Yes (TIA-942) |
| Multivendor scenario | Each end is coded for its platform | Each end is coded for its platform | Each end is chosen and validated separately |
| Cost per short link | The lowest | Intermediate | Higher on short runs, more efficient at scale |
There is no absolute winner: there is a distance range and an operating context where each technology performs best.
What is a DAC cable (Direct Attach Copper)
A DAC is, in essence, a pair of shielded twinaxial copper conductors — usually 26 to 30 AWG — terminated at the factory with two ends that have the same mechanical shape as an SFP+, SFP28, QSFP+ or QSFP28 module. The port on the network platform treats it as a plugged-in module, under the same SFF-8431 (SFP+) and SFF-8436/SFF-8636 (QSFP+/QSFP28) specifications as an optical transceiver.
The great virtue of the DAC is its simplicity: the signal travels over copper with no electro-optical conversion, which translates into the lowest power draw and the lowest added latency of any option. That is why it dominates the server to ToR (top-of-rack) run of 1 to 5 meters, and why it is common in high-performance computing environments where every nanosecond counts.
Passive DAC vs active DAC
| Feature | Passive DAC | Active DAC |
|---|---|---|
| Internal electronics | None (copper only) | Signal conditioning at the ends |
| Typical reach at 10G | Up to 5–7 m | 7–10 m |
| Typical reach at 25G/100G | 3–5 m (IEEE 802.3by / 802.3bj: 5 m) | 5–7 m |
| Power draw per end | ≤0.15 W | 0.5–1 W |
| Relative cost | The lowest on the market | Between passive and AOC |
As the per-lane speed rises, copper loses ground: at 25 Gbps per lane (SFP28 and QSFP28), attenuation and noise hold a passive DAC to about 3-5 meters even with heavy gauges, and the IEEE 802.3bj standard specifies 100GBASE-CR4 up to 5 meters. On top of that, the heavy gauge makes the bundle stiff and bulky compared with the fiber equivalent.
What is an AOC cable (Active Optical Cable)
An AOC solves the reach problem of copper by integrating the optics inside the cable: at each end there is conversion electronics with 850 nm VCSEL lasers and photodetectors, joined by multimode fiber that runs embedded and sealed from the factory. Toward the network platform, the AOC presents itself just like a transceiver: a QSFP28 end, for example, runs the same 4 lanes of 25.78125 Gbps — 103.125 Gbps aggregate — as a conventional 100G module.
Its advantages over the DAC are clear in the 7 to 100 meter range:
- Lower weight and diameter: the embedded fiber weighs a fraction of 26 AWG twinax and allows tighter bend radii.
- Electromagnetic immunity: as an optical medium, it is unaffected by interference from nearby power supplies or electrical bundles.
- Reach: it covers distances between racks, rows and even adjacent rooms without repeaters.
Its structural constraint is the same as the DAC’s: the ends are fixed. The length is decided at purchase, it cannot be terminated or repaired in the field, and a fault at any point — connector, electronics or fiber — forces the replacement of the whole assembly. On critical links, that means keeping spares of every length and every end combination present in the room.
Reach by medium and speed
The following table summarizes typical industry reach by speed and technology. Transceiver values correspond to standard IEEE 802.3ae (10G), 802.3by (25G), 802.3ba (40G) and 802.3bm (100G) optics:
| Link | Passive DAC | Active DAC | AOC | Transceiver + fiber |
|---|---|---|---|---|
| 10G (SFP+) | Up to 7 m | 7–10 m | Up to 100 m | SR: 300/400 m (OM3/OM4) · LR: 10 km (OS2) |
| 25G (SFP28) | 3–5 m | 5–7 m | Up to 100 m | SR: 70/100 m (OM3/OM4) · LR: 10 km |
| 40G (QSFP+) | 3–5 m | 5–7 m | Up to 100 m | SR4: 100/150 m (OM3/OM4) · LR4: 10 km |
| 100G (QSFP28) | 3–5 m | 5–7 m | 70–100 m | SR4: 70/100 m · CWDM4: 2 km · LR4: 10 km (OS2) |
At 10G, AOC and SR optics overlap almost completely: the decision comes down to cost and operations. From 25G per lane onward, however, the transceiver with fiber is the only option that scales beyond 100 meters, and the only one that reaches links between buildings or between sites through LR, ER or CWDM/DWDM optics.
Power draw, latency and thermal management
At row or room scale, power draw becomes a design line item. Typical ranges per end are: less than 0.15 W for a passive DAC, 0.5-1 W for an active DAC, 1-2.5 W for an AOC and 1-4.5 W for optical transceivers depending on reach (a 10G SFP+ SR is around 1 W; a 100G QSFP28 SR4, 2.5-3.5 W).
A ToR network platform with 48 servers connected over passive DAC dissipates several tens of watts less than the same unit populated with active optics — heat the cooling system does not have to remove. On latency, the passive DAC also wins: with no electro-optical conversion, the added latency is reduced to propagation over copper.
The other side is mechanical: heavy twinax takes up more tray cross-section and, at high densities, can obstruct the rear airflow of the rack; fiber keeps the tray light and clear.
When each option makes sense
When to use DAC
- Runs of 1 to 5 m inside the rack: server to ToR network platform and stacking between adjacent chassis.
- Tight budget per port: it is the cheapest way to populate dozens of short links.
- Latency and power draw as priorities: high-performance computing and distributed storage inside the rack.
When to use AOC
- Links of 10 to 100 m without structured cabling: hops between racks or rows where no fiber plant with patch panels exists (or is justified).
- Congested trays: when the weight and diameter of twinax are a real management problem.
- Temporary deployments or labs, where the lack of modularity matters less than installation speed.
When to use transceiver + structured fiber
- Distances above 7-10 m with a long-term view: the OM4 or OS2 fiber plant with patch panels is installed once and outlives several speed generations (10G → 25G → 100G reuse the same trunk).
- Ends that evolve separately: you can change the optic on one side — from SR to LR, for example — without touching the other end or the jumper in between.
- Simple operations and spares: after a fault, you replace the affected module or jumper; a single spare transceiver model covers many links, with no need to inventory fixed lengths.
- Reaches beyond DAC/AOC range: campus, inter-building or metropolitan links with LR, ER, ZR or WDM optics.
To size it with real products: a 10G link over fiber is solved with 10G SFP+ duplex modules; 40G aggregates with QSFP+ 40G multifiber modules over MPO-12 from the product catalog; and the 100G spine layer with 100G QSFP28 transceivers, all with compatibility validated by model, firmware, port and optical part number.
The multivendor factor: one end for each platform
A DAC or an AOC has both ends defined at the factory, which raises the usual question: what happens when the link joins network platforms from different vendors? With an off-the-shelf cable, that single component has to be accepted by two different port policies.
At EON Technology that is precisely one of the scenarios where we add value: we code each end of the DAC or AOC independently, for the platform it faces. One end can be adapted for one vendor and the other end for a different one, on the same cable.
The procedure is the same one we apply to transceivers. Our pre-sales and post-sales engineering team validates both platforms on the link — model, firmware, port and optical part number on each side — and the transceiver recoding service adapts and validates the compatibility of each end before dispatch, for stable operation under the agreed technical conditions. The service includes 90-day operating coverage. The product warranty, from 1 to 5 years, is independent, and our compatibility matrix documents the platforms already validated by brand.
With transceiver + fiber the link breaks down differently: each module is validated against its own network platform and the fiber in between is a passive, neutral medium. It remains the best option when distance, structured cabling or trunk reuse is what matters — but multivendor, on its own, is no longer a reason to rule out a DAC or an AOC.
Common mistakes when choosing between DAC, AOC and transceivers
- Stretching the DAC beyond its range. A 25G passive DAC running at 7 m may link, but with a rising error rate that shows up as intermittent retransmissions, hard to diagnose.
- Comparing unit price only. The DAC wins on the purchase order; the life-cycle cost includes spares per length, re-cabling on every speed migration and hours of tray management. Structured fiber amortizes better over 5-10 years.
- Treating the AOC as repairable. It is not: a fault means full replacement and pulling the cable out of the tray. Without spares of the exact length on site, the link stays down.
- Ignoring twinax bulk. Forty-eight 26 AWG DACs at the back of a rack are a real obstacle to airflow and to the technician’s hands.
- Taking compatibility for granted on multivendor links. Both cables and modules must be verified against the model, firmware and specific port of each platform — and in a DAC or AOC, each end separately — before deployment, not after.
- Cabling runs that will migrate speed with fixed-length media. If the plan is to move access from 10G to 25G, an OM4 fiber plant is kept; a tangle of DAC/AOC is replaced in full.
How to decide: a practical checklist
Before defining the medium for each run, answer four questions:
- Distance? Less than 5-7 m: DAC. Between 7 and 100 m: AOC or transceiver + fiber. More than 100 m: transceiver + fiber.
- Are both ends from the same vendor? If not, do not rule out the DAC or the AOC: each end is coded for the platform it faces. What you need to do is validate both before buying.
- Does structured fiber exist or is it planned? If so, using it with SR/LR modules almost always simplifies operations.
- Will the run migrate speed? An OM4/OS2 plant with panels survives the migration; cables with fixed length and fixed ends do not.
The fine design — optics, fiber and the real distances in your room — is best resolved over concrete floor plans. EON serves Latin America and Spain from a centralized operation in Colombia. Typical dispatch in 48h*. Request a quote for your connectivity project and validate every link before buying, or talk to an engineer about the coding of each end.
*Subject to inventory, destination country, carrier, customs, the customer’s import process and third-party logistics conditions.