Network Cables and Connectors: Direct Attached Cables (DACs)

Today we’re reviewing an oft-over looked network connector, the Direct Attached Cable!

The Direct Attached Cable (DAC) has transceivers pre-built into them. They come in both SFP+/SFP28 form factor as well as QSFP+/QSFP28 form factor (check out our previous post on what these transceivers are). These cables are very useful because having a preset length and being a complete solution, they are substantially cheaper than buying an SFP and then using a fiber optic cable with it. Fiber optics (depending on many factors we’ll discuss later) are often four to ten times the price of a DAC.

Additionally, since DACs normally have copper wires in them instead of glass, they are much more flexible and can don’t have as many concerns with ends getting dirty, minimum bend diameter or other things that effect optical cables. They do, however, have distance limitations.

Active vs. Passive DAC

In addition to SFP+ and QSFP, there is another differentiator in DACs: whether they are active or passive. Passive DACs essentially are just wires between the two slots. There are no additional smarts in them to amplify the signal or send it further distances. Normally these top out at five meters or less.

There are then active DACs, which take an active part in making sure the signal makes it to the other side. Active DACs have technologies in the SFP/QSFP ends of the cable that amplify the signal and allow it to travel further distances. They can go up to 300 meters.

Active Optical Cables (AOCs)

After a certain distance, though, the copper in active DACs is normally replaced with fiber. These are called Active Optical Cables (AOC). They function similarly to normal DACs in that they’re self-contained cables with the SFPs built in, but instead of copper they use fiber. These cables are still often cheaper than SFPs and fiber cables, are immune to EMI, eliminate polarity problems, but do have normal fiber bend constraints.

To DAC or Not to DAC: That is The Question

So why would you ever not use DAC? Quite simply, knowledge of the distance that you need to traverse data. Sometimes you need a 1 meter, sometimes a 3 meter, sometimes a 10 meter.  If you buy DACs, that’s what you have, there’s no changing it. If you buy transceivers and fiber cables, for a minimal expense, you can change the length of fiber cable. Also, if you have a fiber patch panel, you have to use an SFP and a fiber cable.

There’s one more problem with DAC (really all transceivers): each switch manufacturer has their own hardware qualified lists (HQL) that says what devices/ports they support. If you have a Cisco switch, it has its own list. Dell has its own list. Surprise, other manufacturer’s cables aren’t on the HQL, which means that one side or the other of the DAC would be unsupported. Going between manufacturers is a case where it’s just easier to pay more for independent transceivers and use fiber cables instead of DACs.

There are also some manufacturers who only support their own DACs on very specific switches. HP is notorious for this — you can have an HP-branded DAC, but it still won’t work in their switch because it’s for a different model. This can create a monstrous amount of frustration just trying to plug in devices. Overall, most server-side transceiver slots don’t really care whose transceiver plugs into them, so DACs are an excellent choice for plugging servers into switches.

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