DAC vs AOC vs transceivers

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For short links inside a rack or a row there are three choices: a direct-attach copper cable (DAC), an active optical cable (AOC), or two transceivers with a structured fibre patch between them. The industry trend of 2023–2025 is clear: mature networks are moving to transceivers + single-mode fibre even for short 100G/400G runs.

The core argument

When a DAC or AOC fails, you re-pull the entire cable at both ends — often through a populated rack. When a transceiver fails, you swap one module and the cabling stays put.

Against DAC and AOC

100G/400G DACs are thick and stiff, have a poor bend radius, top out at roughly 3 m at 400G, and run hot.
Failure clusters happen: one operator reported 5 failures out of ~30 100G DACs in three months.
AOC (built with chip-on-board optics) combines the worst of both worlds — the same re-pull on failure as a DAC, plus compatibility questions between the two fixed ends.
10GBASE-T SFP+ (RJ45) modules draw 2.5–4+ W, overheat in dense ports and reach only ~30 m. The operator consensus is blunt: avoid them.

For transceivers + fibre

Benefit	Why it matters
Future-proof cabling	The fibre stays when you upgrade 100G → 400G; only modules change
Faster link-up	No electrical debounce on optical links
Failure is local	Replace one module, not a cable through the rack
Serviceability	Modules are stocked and swapped; cables are not

The fair counter-argument

A passive DAC on a very short link (within one rack, 1–2 m) is inexpensive and reliable in practice. The case against DAC is strongest at 100G/400G and across rows.

A note on module selection

When choosing transceivers for short runs, short-reach optics (SR/DR on multi-mode or single-mode as appropriate) cover in-rack and in-row distances; see Optical Ethernet interfaces for the naming.

In CodingBox

Transceivers — unlike sealed cables — carry readable, codeable memory. CodingBox reads their identity and DDM on the Check transceiver screen and lets you recode a module for the switch you are plugging it into, which is one more reason the module-based design is easier to service.

What is physically inside each cable family and how that maps to identity bytes, DDM and failure modes: Cable internals.

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