AOC vs DAC vs Fiber: Which to Use for 400G and 800G Links
Use direct attach copper (DAC) inside the rack, where links are about 3 m or less. Use active optical cables (AOC) or active electrical cables for fixed point-to-point links to the next racks. Use pluggable transceivers with structured fiber (MPO trunks and patch cords) for anything that crosses a row, goes through a patch panel, or must survive the next speed upgrade. DAC draws almost no power but is short and thick; AOC is light but fixed in length and cannot be patched; transceivers with fiber cost more per port but reach the whole hall and keep the cabling when the optics change.
Scope of this guide: choosing the media for a link. For which transceiver uses which fiber and connector, see transceiver cabling map: SR4, DR4, SR8, DR8.
The Three Options
| DAC (passive copper) | AOC (active optical cable) | Transceivers + fiber | |
|---|---|---|---|
| What it is | Twinax copper with module plugs fixed at each end | Fiber with optical modules fixed at each end | Two pluggable modules plus separate fiber cabling |
| Typical reach at 400G/800G | About 2–3 m (passive) | Up to tens of metres | 100 m (multimode) to 2 km and beyond (single-mode) |
| Power | Almost none (passive) | Similar to two optical modules | Two optical modules |
| Patch panels | No | No | Yes |
| Change the length | Buy a new cable | Buy a new cable | Swap a patch cord |
| Speed upgrade | Replace cable | Replace cable | Keep the trunks; change modules and maybe cords |
Reach depends on speed and lane rate: at 100G per lane (800G), passive copper reach is shorter than at 50G per lane (400G). Active electrical cables (AEC), which add retimers to copper, sit between DAC and AOC in reach. Always check the switch vendor's supported cable list.
When to Use Each
- DAC: server or GPU node to top-of-rack switch in the same rack. Lowest power and cost per link, but the bulk of many thick cables blocks airflow at high port counts.
- AOC: switch to switch in the next rack or two, where the run is fixed and no patching is needed. Light and thin, but if one end fails the whole cable is replaced.
- Transceivers + fiber: spine and leaf links across a row or hall, any link through a patch panel, and any run longer than tens of metres. MPO trunks installed once can carry 400G, 800G and 1.6T. See AI cluster fiber cabling.
The Hidden Costs
| Factor | DAC | AOC | Transceivers + fiber |
|---|---|---|---|
| Spares | One per length | One per length | Modules + a few cord lengths |
| Moves and changes | Re-cable | Re-cable | Re-patch at the panel |
| Cleaning and inspection | None | Sealed ends | MPO end faces need cleaning |
| Airflow at high density | Poor (thick bundles) | Good | Good |
Fiber adds one task: connectors must be clean. See how to clean MPO connectors.
Breakouts
DAC, AOC and fiber can all break one 400G or 800G port into lower-speed ports. With fiber, the breakout is a harness or cassette that can sit at a panel; with DAC and AOC it is a fixed split cable. See 400G and 800G breakout cabling.
Quick Decision Rule
| Distance and path | Use |
|---|---|
| Same rack, ≤3 m | DAC |
| Next racks, fixed, no panel | AOC or active electrical cable |
| Through a patch panel | Transceivers + fiber |
| Across a row or hall | Transceivers + MPO trunks |
| Between buildings | Transceivers + single-mode fiber |
Frequently Asked Questions
What is the difference between DAC and AOC?
DAC is copper twinax with module plugs at each end and reaches a few metres with almost no power. AOC is fiber with optical modules built into each end and reaches tens of metres.
Is AOC better than transceivers with fiber?
Not for structured cabling. AOC is fixed in length and cannot pass through patch panels. Transceivers with fiber can be re-patched and the trunks reused at the next speed.
How far can DAC reach at 800G?
Passive DAC reach at 100G per lane is about 2 m in most designs. Check the switch vendor's supported cable list.
Why not use DAC everywhere to save power?
Reach is too short and thick copper bundles block airflow in dense racks. Beyond the rack, fiber is the practical choice.
Can I mix DAC and fiber in one network?
Yes. A common design uses DAC inside the rack and transceivers with MPO fiber trunks between racks and rows.
Huawellux Fiber for the Links Beyond the Rack
Huawellux makes the fiber side of the transceiver option: MPO and MTP® patch cables, trunks, breakout harnesses and LC patch cords in OS2, OM4 and OM5, with polarity, gender and length confirmed before production.
Huawellux's specification for MPO and MTP® assemblies (identical for both) is insertion loss ≤0.35 dB for single-mode and multimode, with return loss >60 dB single-mode (OS2, APC) and >30 dB multimode (OM3/OM4/OM5). LC and SC patch cords are listed at insertion loss ≤0.35 dB and return loss ≥50 dB (single-mode UPC), ≥60 dB (single-mode APC) and >30 dB (multimode). Every Huawellux product receives end-face visual inspection under a 400x video microscope and is tested for insertion loss (IL) and return loss (RL) before it ships. Interferometric end-face geometry (fiber height, coplanarity, end-face angle and radius) is checked by sample-based testing in standard production, and 100% interferometer testing of a batch is available on request. Dust caps are fitted on every connector, and a formal test report is available on request.
Huawellux replies to inquiries within 24 hours. Custom orders typically take 1–5 business days to complete production, with final timing confirmed case by case in the quotation. For special/custom requirements, MOQ is 2 km of fiber length. Standard listed patch cords have no formal MOQ. The sample fee is credited against the bulk order.
- Browse MPO patch cables, MTP/MPO trunk cables and breakout harness cables
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