What a DAC is

A direct attach cable is a copper cable with transceiver modules fixed to both ends, forming a single assembly that plugs directly into the equipment's ports, without separate modules and fibers. It carries signals over copper, over short distances, typically within a rack or between nearby racks, and it suits links where the distance is short, the rate is moderate, and the cost matters more than the reach, because copper is cheaper than fiber for short links, and because a DAC's fixed ends eliminate the need for separate modules, separate fibers, and separate patching.

A DAC cannot be extended, cannot be re-terminated, and cannot be adapted to a different length, because its ends are fixed, and a DAC ordered at the wrong length cannot serve a link of a different length, however right its rate appears. Measure the link's actual route, not its straight-line distance, and order the length that fits the route, with a little slack for routing, because a DAC that is too short cannot reach, and a DAC that is too long cannot be coiled without degrading its signal, however neatly it is managed.

What an AOC is

An active optical cable is a fiber cable with transceiver modules fixed to both ends, forming a single assembly that plugs directly into the equipment's ports, without separate modules and fibers. It carries signals over fiber, over longer distances than a DAC, with lower power, lower weight, and less electromagnetic interference, and it suits links where the distance exceeds copper's reach, where the environment is electrically noisy, or where the weight and bulk of copper would complicate the installation.

The 100G AOC 10m cable on this site illustrates the form: one QSFP28 end, four SFP28 ends, 10m of fiber, for a short fan-out link within or between racks. An AOC, like a DAC, cannot be extended, re-terminated, or adapted to a different length, because its ends are fixed, and an AOC ordered at the wrong length or with the wrong ends cannot serve, however right its rate appears. Confirm the length and the end types, for the exact equipment at both ends, before ordering, because a cable that mismatches at either end cannot be adapted without replacing it.

Distance: the deciding factor

Distance decides between DAC and AOC, because copper's reach is short and fiber's reach is long, and a link that exceeds copper's reach must use fiber, while a link within copper's reach may use either, with copper cheaper and fiber lighter. A link within a rack, or between adjacent racks, suits a DAC, because the distance is short, the cost is low, and the copper's weight and bulk are manageable, while a link between distant racks, between rows, or between rooms suits an AOC, because the distance exceeds copper's reach, and because fiber's lower weight and interference immunity matter over longer runs.

Measure the link's route, not its straight line, because fiber and copper routes through trays, ducts, and cabinets, and a route that looks short on a map may measure long in the building, and a DAC ordered for a straight-line distance that underestimates the route will be too short, while an AOC ordered for a route that overestimates it will be too long, with slack that must be managed. Order the length that fits the measured route, with a little slack, and confirm the length in the inquiry, naming the equipment at both ends, so the supplier can confirm the cable for that link, rather than for a general case.

Interference and weight: the secondary factors

Electromagnetic interference favours AOC, because fiber is immune to it, while copper picks it up, and a link that runs near power cables, motors, or radio equipment will suffer interference on copper but not on fiber, with errors, retransmissions, and degraded performance on the copper link, however good its rate appears. A buyer whose link runs through an electrically noisy environment should choose AOC, even where the distance suits DAC, because interference immunity matters more than cost where the environment is noisy, and because a link that suffers interference is a link that fails intermittently, in failures that are hard to diagnose.

Weight and bulk favour AOC, because fiber is lighter and thinner than copper, and a link with many cables — a data centre row, a server rack, a storage array — will weigh less, occupy less space, and obstruct less airflow with fiber than with copper, improving cooling, simplifying management, and reducing the load on cable trays. A buyer whose installation has many cables, tight trays, or airflow concerns should choose AOC, even where the distance suits DAC, because the installation's manageability matters as much as the link's rate, and because a congested, overheated rack will fail for thermal reasons, not for link reasons.

Rate and ends: match both

The rate must match the equipment at both ends, because a cable cannot make a port faster than it is, and a faster cable in a slower port wastes the cable's capacity, while a slower cable in a faster port limits the link to the slower rate. The end types must match the equipment's ports at both ends, because a cable with the wrong ends cannot be plugged in without adapters, and adapters add loss, add failure points, and add cost, while reducing the link's reliability, and because a fan-out cable — one end to four ends — must match the fan-out the equipment expects, not just the rate it carries.

A buyer who orders a cable should name the rate, the length, and the end types at both ends, in the inquiry, so the supplier can confirm the cable for that link, rather than for a general case, and so the confirmation received is the confirmation that can be relied on. A cable ordered on rate alone, without length and ends, is a cable that may arrive but not serve, because length and ends are as decisive as rate, and because a cable that mismatches on either cannot be adapted without replacing it.

Cost: count the total, not the cable

A DAC costs less than an AOC, but a link's total cost includes the cable, the ports, the power, the installation, and the management, not just the cable, and a link where a DAC fails from distance or interference will cost more, in replacement, in delay, and in diagnosis, than an AOC ordered once. A buyer who compares cable prices alone will often choose DAC where AOC is cheaper in total, and a buyer who counts the total — including the cost of a failed link, a replacement cable, and a delayed installation — will often find that AOC, for a link where it fits, is the cheaper choice, despite the higher cable price.

Count the installation cost as well: a DAC's weight and bulk may require stronger trays, more space, and more labour, while an AOC's lightness and thinness may simplify installation, reduce tray load, and improve airflow, saving labour and cooling costs that exceed the cable price difference. A buyer who counts installation, power, cooling, and management, not just the cable, will choose the cheaper total, not the cheaper cable, and the cheaper total is the choice that matters.

When neither fits

Some links suit neither DAC nor AOC, because the distance exceeds both, because the ends do not match any fixed cable, or because the link must be reconfigured, extended, or re-terminated in the future, and a fixed cable cannot be changed without replacing it. Such links need separate modules and fibers, where the module can be changed, the fiber can be extended, and the link can be reconfigured, without replacing the whole assembly, and a buyer whose link may change should choose modules and fibers, not a fixed cable, because flexibility matters more than simplicity where the link's future is uncertain.

A buyer who is unsure whether a fixed cable fits should ask, in the inquiry, naming the link, its distance, its ends, and its future, so the supplier can advise whether a fixed cable or separate modules suit it, rather than ordering a cable that may not fit, and rather than ordering modules where a cable would suffice. An inquiry that names the link's facts will receive useful advice, and an inquiry that names only a product word will receive a general answer that fits no link.

What to confirm, before ordering

Before payment, confirm the cable type, the rate, the length, the end types at both ends, the minimum order quantity for that cable, the lead time with its meaning, the warranty terms, the payment and shipping terms, and the certificates that apply to your market, by name, with documents attached. Name the equipment at both ends, the link's route and distance, the quantity, and the market, so the supplier can confirm for your link, rather than for a general case, and so the confirmation you receive is the confirmation you can rely on.

A buyer who confirms the type, the rate, the length, and the ends, in writing, before payment, has done the work that prevents the common failures: the too-short cable, the wrong-ended cable, the interference-degraded cable, and the overweight installation. A buyer who skips that work has saved an hour and risked a shipment, and the hour saved is rarely worth the shipment risked.