DAC vs AOC: Choosing the Right Short-Reach Interconnect

Two cables, one job

Both DAC (direct attach copper) and AOC (active optical cable) connect two fixed ends without pluggable modules. Both come pre-terminated, both are factory-tested, and both are usually the cheapest way to link equipment inside a rack or between adjacent racks.

The difference is what carries the signal.

DAC: copper all the way

A DAC is a copper cable with the transceiver circuitry built into the connector ends. There is no optical conversion at all.

• Reach: typically 1–3 m for 100G passive DAC; 5 m and beyond usually requires an active copper version
• Power: essentially zero for passive DAC, since there is no active component
• Cost: lowest per link at short distances
• Weight and bend radius: heavier, and stiffer — this matters when cabling dense racks
• Failure mode: signal attenuation, which grows quickly with distance and cable gauge

AOC: copper to the connector, light through the cable

An AOC converts the electrical signal to light inside the connector housing, sends it through multimode fiber, and converts it back at the other end.

• Reach: commonly 1–100 m, and available in longer runs
• Power: higher than passive DAC — the cable ends have active electronics
• Cost: higher than DAC at short lengths, but increasingly competitive past 3–5 m
• Weight and bend radius: much lighter and more flexible
• Failure mode: power dissipation at the connector ends

Side-by-side

| | Passive DAC | AOC | |---|---|---| | Typical reach | 1–3 m | 1–100 m | | Power | ~0 | Active, higher | | Relative cost at 1 m | Lowest | Higher | | Relative cost at 10 m | Not viable | Competitive | | Cable weight | Heavy | Light | | Bend radius | Poor | Good | | Breakout to 4×25G / 4×100G | Yes | Yes |

How to decide

1. Distance first. Under 3 m inside a rack, passive DAC is almost always the cheapest working answer.
2. Then rack topology. If the run crosses between racks or through cable management that demands tight bends, AOC's flexibility saves installation time and reduces the risk of damaged cables.
3. Then air flow. DAC blocks airflow in dense racks; AOC does not. In high-density GPU racks this alone can decide the choice.
4. Then breakout needs. Both DAC and AOC support breakout. A 40G QSFP+ to 4×SFP+ or a 100G QSFP28 to 4×SFP28 breakout cable is often cheaper than buying extra switch ports.

Common practical note

AOC and DAC are fixed-length assemblies. Order the length you need — unlike a module plus patch cord, you cannot adjust an AOC on site. Measure twice.

Canjin Technology supplies SFP+, QSFP+ and QSFP28 active optical cables including breakout versions, in standard lengths from 1 m upward.