Active optical cables: reach without a transceiver budget
An active optical cable is a fibre assembly with the transceiver built permanently onto each end. To the switch it looks like an optic; to you it is one part number and one thing to install. Nothing to clean, nothing to mate, no patch cord to source separately.
The gap it fills
Copper runs out first. Passive twinax is good to about 5m at 10G through 100G and 3m at 400G, and past that the signal will not survive. Transceivers plus structured fibre will go as far as you want, but you are then buying two modules, a patch cord, and the cassettes and trunks behind them.
An AOC sits between the two. Ours run 1m to 10m at 10G, 25G and 40G, and 1m to 30m at 400G. That covers the awkward middle distances — rack to adjacent rack, across an aisle, up to a middle-of-row switch — where copper has given up and a structured fibre run is more infrastructure than the link deserves.
What you gain
The cable is thin, light and genuinely flexible, so a bundle of AOCs routes and bends in ways a bundle of twinax will not, and it blocks far less airflow.
Because both ends are sealed at the factory, the endfaces are never exposed. Contamination is the single most common cause of optical link faults in the field, and an AOC removes the opportunity entirely.
It is also matched by construction. Both ends come from the same assembly, so there is no question of mismatched wavelengths, mismatched reach classes, or a 300m optic on one end and a 100m optic on the other.
What you give up
It is one unit. If either end fails, the whole cable is replaced — though for us that is a warranty swap rather than a cost, since every cable carries the same lifetime warranty as our transceivers.
The length is fixed at purchase, so a 7m link needs a 7m cable. And unlike a transceiver, an AOC cannot be repurposed onto a different fibre run later.
Coding still applies
Both housings carry EEPROMs, and both are coded for the platform they plug into. A 25G AOC coded for Cisco is a different part from the same cable coded for Arista, even though the fibre and optics between them are identical. If the two ends land in switches from different vendors, you need a cable coded accordingly — tell us and we will build it that way.
Choosing between DAC and AOC
Under 3m and inside one rack, take the DAC — it is cheaper and there is no reason to pay for optics you do not need. Past that, or anywhere cable bulk and airflow are a real constraint, take the AOC. Beyond about 30m, or when the link should outlive the equipment at either end, put in structured fibre and use transceivers.