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What is the best in rack short reach interconnect

DAC, ACC, AEC and AOC compared for in-rack data center links, with reach limits, power characteristics, latency behaviour and a selection sequence by distance.

August 7, 2026

For in-rack links, passive DAC is the best choice wherever reach allows, because it adds essentially no power and no retiming latency. Beyond passive DAC reach, ACC extends to around 5 m at 800G using equalisation, and AEC reaches around 7 to 10 m using a DSP or retimer in each connector. AOC covers longer runs up to 100 m and beyond. Distance decides, and the shortest viable option is usually the right one.

Four technologies, one decision

Short-reach links inside a rack are carried by one of four things, and the choice between them is more consequential than it looks. Multiplied across a fabric, it affects power, cooling, latency consistency and cost together.

The four are passive direct attach copper, active copper cable, active electrical cable and active optical cable. They differ in what electronics sit inside the connector heads, and that difference determines reach, power and behaviour.

Passive DAC: the default where reach allows

A passive DAC is twinax copper and two connector assemblies with no active electronics. Signal integrity depends entirely on the cable construction and the host SerDes.

That simplicity is the advantage. There is no conversion, no retiming and no meaningful power draw. Latency is the propagation time down the copper and nothing else. There is also no active component to fail, which matters when the cable is buried in a cable management arm behind a production server.

The limit is reach. As lane rates move past 100G, passive copper runs out of margin quickly, and passive DAC at 800G covers only the shortest intra-rack runs.

ACC: equalisation for a few extra metres

An active copper cable adds equalisation and amplification circuits in the connector heads to compensate for cable loss. This extends reach to around 5 m at 800G while keeping power draw low.

ACC fits the case where a link is slightly too long for passive copper but nowhere near long enough to justify optics. Tall cabinets, shared cabinets and rack layouts where the switch is not adjacent to the servers it feeds are typical.

 

AEC: a DSP in each end

An active electrical cable integrates a DSP or retimer in each connector, which reconstructs and cleans the electrical signal rather than simply boosting it. That enables error-free transmission up to around 7 to 10 m at 400G and 800G.

The retiming has a second benefit. Because each end presents a clean electrical eye to the host, the link is less sensitive to host SerDes variation and to cable routing. In dense AI racks where cable bend and congestion are unavoidable, that stability is worth the small power cost.

AOC: fibre in a fixed assembly

An active optical cable puts a miniature optical engine inside each connector shell, with laser drivers and VCSELs at one end and receivers at the other. Reach extends to 100 m and beyond, the cable is light and thin, and it is immune to electromagnetic interference.

The trade is flexibility. An AOC is a fixed-length assembly, so a failure means replacing the whole cable rather than a module, and reconfiguration means new cable. It suits point-to-point links that are unlikely to change.

Comparison at a glance

Type Electronics in connector Typical reach at 800G Power Latency
Passive DAC None Shortest intra-rack runs Negligible Lowest, propagation only
ACC Equalisation and amplification Around 5 m Low Very low
AEC DSP or retimer Around 7 to 10 m Low to moderate Low, with retiming step
AOC Optical engine both ends 100 m and beyond Moderate

A selection sequence by distance

  • Measure the routed length, including slack and cable management path, not the straight-line distance
  • Under passive DAC reach, use passive DAC unless the host SerDes budget is already marginal
  • Between passive reach and around 5 m, use ACC
  • Between around 5 m and 10 m, use AEC
  • Beyond 10 m, move to AOC or to pluggable optics with structured fibre

One caveat on measurement. Routed length in a populated rack is regularly double the floor-plan distance once cable management is accounted for. Specifying from a drawing rather than a measurement is the most common reason a cable arrives too short.

Where fixed assemblies stop making sense

DAC, ACC, AEC and AOC are all fixed-length assemblies. That is efficient for a stable layout and awkward for one that changes. Where a rack is expected to be reconfigured, or where the link crosses into structured cabling, pluggable optics on patched fibre give more flexibility at the cost of more power and more connectors.

ATOP builds proprietary DAC, ACC, AEC and AOC designs alongside its optical transceiver range, in 400G and 800G and across earlier speeds, so the choice can be made on link characteristics rather than on what is available.

What is the difference between DAC, ACC and AEC?

A passive DAC is copper with no electronics, giving the lowest power and latency over the shortest reach. An ACC adds equalisation and amplification in the connector heads, extending reach to around 5 m at 800G. An AEC adds a DSP or retimer in each connector, which reconstructs the signal and extends error-free reach to around 7 to 10 m at 400G and 800G.

How long can a DAC cable be at 800G?

Passive DAC at 800G is limited to the shortest intra-rack runs, because passive copper loses margin quickly as lane rates rise past 100G. For longer runs, active copper cables reach around 5 m and active electrical cables reach around 7 to 10 m. Exact passive reach depends on cable gauge and the host SerDes capability.

Is DAC lower latency than fibre?

Yes, marginally. A passive DAC adds only propagation delay with no conversion or retiming step, so it has the lowest latency of any option. Active copper and AEC add a small amount for equalisation and retiming. Optical links add conversion at each end. The differences are small in absolute terms but consistent.

When should I use AOC instead of DAC?

Use AOC when the link is longer than copper can carry, generally beyond around 10 m at 400G and 800G, or where cable weight and bulk are a problem in dense cable management. AOC also avoids electromagnetic interference. The trade-off is that an AOC is a fixed-length assembly and any failure means replacing the whole cable.

Which short reach interconnect uses the least power?

Passive DAC, by a clear margin, because it contains no active electronics at all. ACC adds a small amount for equalisation, AEC slightly more for retiming, and AOC the most because it converts between electrical and optical at both ends. Using the shortest viable technology for each link is the simplest way to reduce fabric power.
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