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Inside a data center, the last meter matters as much as the first 100 kilometers. For connections between servers, switches, and patch panels, network architects face three choices: Active Optical Cable (AOC), Direct Attach Copper (DAC), or a pluggable optical transceiver with a separate fiber patch cord. Each one wins in a different distance range — and picking the wrong one wastes money, power, or rack space.
DAC (Direct Attach Copper). A fixed-length twinax copper cable with SFP/QSFP connectors permanently attached at both ends. No optics, no laser, no fiber — just copper wire carrying electrical signals. The transceiver ASIC drives the signal directly over the copper medium. Cheapest option per link, lowest power, zero optical components.
AOC (Active Optical Cable). A fixed-length fiber cable with optical transceivers permanently embedded at both ends. The electrical-to-optical conversion happens inside the connector head. To the switch port, it looks like a DAC — but the signal travels over fiber, not copper. Longer reach than DAC, lighter weight, immune to EMI.
Pluggable Optical Transceiver. A removable transceiver (SR8, DR8, FR4, etc.) inserted into the switch cage, with a separate fiber patch cord. The transceiver and the fiber are independent — you can replace one without touching the other. Highest flexibility, widest reach range, supports upgrades without replacing the fiber plant.
| Parameter | DAC | AOC | Pluggable + Fiber |
|---|---|---|---|
| Medium | Copper (twinax) | Fiber (embedded) | Fiber (separate patch cord) |
| Max reach at 400G/800G | 2–3 m | 30–100 m | 100 m–120 km |
| Power per link | ~0.5–1 W | ~2–3 W | ~8–18 W (PAM4) / 16–28 W (coherent) |
| Cost per link | Lowest | 2–4× DAC | 3–10× DAC (varies by reach) |
| Cable weight (per m) | Heavy (~60 g/m for 800G) | Light (~3 g/m) | Light (~3 g/m patch cord) |
| EMI immunity | None (copper) | Full (optical) | Full (optical) |
| Field replaceable | Full cable replacement | Full cable replacement | Transceiver or fiber independently |
| Best for | Intra-rack, adjacent racks | End-of-row, same-row | Cross-row, campus, DCI |
At 800G, a 3-meter DAC cable weighs roughly 180 grams — about the weight of a smartphone. In a fully-populated 32-port switch, that is 5.8 kg of copper hanging off the front panel. Multiply by 32 leaf switches in a rack, and you have 185 kg of cable weight pulling on connectors and stressing the rack mounting. AOCs weigh less than one-twentieth of that for the same reach.
This is not a cosmetic concern. Heavy DAC bundles block airflow through switch front panels, creating hot spots that degrade optics and ASICs over time. In dense 800G deployments, the weight and airflow impact of DACs becomes a thermal engineering constraint — not just a cable preference.
Within the same rack (under 2 m). DAC. Lowest cost, lowest power, simplest. At these distances, the copper signal integrity is adequate and the weight is manageable — one or two cables per server, not hundreds in a bundle.
Adjacent racks and same row (2–30 m). AOC. Copper signal degrades beyond 3 m at 100G PAM4 per lane. AOCs carry the signal optically over these distances, are lightweight enough to route through cable trays without sagging, and cost 60–80% less than pluggable transceivers with separate fiber.
End-of-row and cross-row (30–100 m). AOC for multi-mode distances (up to 100 m on OM4). Pluggable transceiver + fiber patch cord for single-mode distances or if you need the flexibility to replace transceivers independently.
Beyond 100 m. Pluggable transceiver with fiber patch cord. AOC maximum reach is typically 100 m. For anything longer — campus links, building-to-building, metro DCI — pluggable optics paired with structured cabling is the only option.
AOC vs pluggable tradeoff: AOCs cost less per link and eliminate one connector pair (no transceiver-to-patch-cord interface). But if the cable is damaged, both ends must be replaced — you cannot just swap the transceiver. For links where cable damage is unlikely (overhead trays, protected pathways), AOCs win on cost. For links where cables may be disturbed frequently (underfloor, accessible trays), pluggable optics with separate fiber are worth the premium.
APEX Group supplies the full interconnect range — 25G to 800G DACs for intra-rack, AOCs for same-row and end-of-row, and pluggable transceivers (SR8/DR8/FR4/coherent ZR+) with structured cabling for campus and metro links — a single vendor from the server NIC to the DCI fiber.
APEX GROUP — www.apexallinone.com
