Four cable types connect servers and switches at 400G and 800G: passive copper (DAC), active copper (ACC), active electrical (AEC) and active optical (AOC). The general rule is to use the shortest, simplest cable that reaches. Passive DAC covers the shortest runs, ACC and AEC extend copper further, and AOC takes over when the link leaves the rack or row.
In short:
- DAC: passive copper, no active parts, lowest cost and power, shortest reach.
- ACC: copper with active signal conditioning to extend reach, about 5 m at 800G according to one supplier.
- AEC: copper with retiming, about 7 to 10 m at 400G and 800G according to the same supplier.
- AOC: optical transceivers built into the cable ends with fiber between them, 100 m and beyond.
These reach figures come from a cable vendor, so confirm against the datasheet of the cable you plan to buy.
How Each Type Works
- DAC (direct attach copper): a fixed copper assembly with transceiver-style ends and no active electronics.
- ACC (active copper cable): copper with signal conditioning, usually equalization in the connectors, to reach further than passive copper.
- AEC (active electrical cable): uses regeneration, retiming or active equalization to improve signal integrity. The retiming step makes the link less sensitive to host and cable routing differences.
- AOC (active optical cable): optical transceiver electronics built into both ends, with fiber between them. It is fixed length, so a fault means replacing the whole cable.
Comparison at 800G (One Supplier's Figures)
- Reach: DAC shortest in-rack runs, ACC about 5 m, AEC about 7 to 10 m, AOC 100 m and beyond.
- Power: DAC negligible, ACC low, AEC low to moderate, AOC moderate.
- Latency: DAC lowest (propagation delay only), ACC very low, AEC low with a retiming step, AOC adds some for conversion at each end.
- Reliability: DAC has no active parts to fail. AOC is fixed length. AEC retiming reduces sensitivity to routing variation.
The source gives no measured latency in nanoseconds and no power in watts, and its reliability points are qualitative. It also sells these products, so the comparison favors its own range.
Cost, Weight and Handling
Passive DAC has the lowest unit cost for short links. ACC and AEC cost more than DAC because of their active electronics. AOC usually costs more than DAC, though another supplier notes it can lower total cost by reducing routing, service time and troubleshooting. Copper gets thicker, heavier and stiffer as speed and length increase, so AOC is lighter and easier to route in dense racks.
Where Each Type Fits
- DAC: inside or near the rack on direct, short paths.
- ACC: short to medium copper paths, including in-rack and adjacent-rack links.
- AEC: dense adjacent-rack links and 800G electrical links.
- AOC: links that cross racks or rows, and switch-to-switch links.
Routed length in a populated rack is often about double the floor-plan distance, so measure the real cable path, including slack, before choosing a type.
Selection Checklist
- Measure the path: use the routed length, not the straight-line distance.
- Start with DAC: use passive DAC wherever the reach allows.
- Step up in order: ACC, then AEC, then AOC or pluggable optics as the distance grows.
- Connector cage: match the cable's connector to the port (OSFP or QSFP-DD). See OSFP vs QSFP-DD.
- Host support: confirm the switch or NIC supports the cable type and the speed.
- Power and cooling: active cables draw power at the port, which adds heat.
- Replacement: fixed-length AOC and AEC mean swapping the whole assembly.
Frequently Asked Questions
What is the difference between ACC and AEC? ACC conditions the signal with equalization. AEC adds retiming, which restores the signal timing and extends reach further.
Which is cheapest? Passive DAC for short links.
When should I use AOC? When the link is longer than copper can reach or crosses racks or rows.
Do these apply to 1.6T? The sources name 1.6T among the relevant speeds but give no reach figures for it. Ask each supplier for tested reach at that speed.
Where to Go Next
- Copper vs optical in high-speed data links
- High-speed Ethernet connectors: OSFP and QSFP
- OCP and ORv3 connectors hub
Sources
- ATOP: best in-rack short-reach interconnect: reach, power, latency and reliability comparison at 800G.
- Axiom DAC, ACC, AEC and AOC guide: how each type works, cost and placement.