DAC and AOC Cables

Datacenter networking is a different beast. Every component has to keep up with the most demanding technical specifications. It makes things expensive. If you want to find ways to keep costs under control, you need to know how to optimize every cable in the system. One step towards that optimization is understanding the differences between DAC and AOC cables.

Direct Attach Cables

Direct attach cables (DAC) are made of Twinax copper cables with connectors on both ends. Despite using copper, they can still achieve data-center transmission rates, getting up to 100G as needed.

DAC cables come in two forms: passive and active, and both can transmit over copper. The passive form does not require signal conditioning. This allows it to minimize power consumption and production cost. The active cables have electric components in the transceivers that boost the signal.

DAC cables shine in short-range applications. They are most frequently used to connect switches, servers and storage within a rack (usually at a data center). They are usually pre-terminated in lengths of 10 meters or less. This makes them cheap and easy to use, relative to many other cables in a data center. The shorter length also tends to lower their cost per connection.

Active Optical Cables

Active optical cables (AOC) are multimode fiber optics cables with SFP connectors on both ends. This design requires external power to push the signal, which is why ‘active’ is in the name. The signal does not passively run through the fiber optics lines.

The most common use case for AOC cables is in data centers. They can efficiently link switches, servers and storage that are on different racks in the same center. They typically run lengths of 100 meters or less, and they can achieve speeds up to 400G along those distances.

AOC cables tend to shine when you compare the ratio of their cost and capacity, within the 100-meter ranges they typically serve.

Choosing Between DAC & AOC

Now that we have covered the basic differences between these types of cables, we can discuss use cases. Primarily, there are four categories to consider when choosing between DAC and AOC cables.

Power

Clearly, passive DAC cables use less power than active DAC cables, and it turns out that AOC uses still more power. Passive DAC lines will consume around 0.15W. Active DAC needs around 0.5W, and AOC will run between 1 and 2W. There is more than a 10-fold increase in power consumption between passive DAC and AOC.

Distance

When it comes to performance over distance, AOC is the clear winner. Passive DAC cables are only effective across distances under 7 meters. Active DAC can push that distance up to 15 meters. Meanwhile, AOC lines are effective up to 100 meters, although performance will vary at the extreme ends of this distance.

For any 10G or faster connection beyond 15 meters, AOC is going to be the only viable choice among these three.

Cost

In general, DAC cables cost less to manufacture, and that makes them cheaper. As you would expect, passive DAC is cheaper than active DAC, and AOC is substantially more expensive than active DAC.

Cost savings compound when you optimize your cables. As the cheapest cables are also the shortest, they save money on two fronts. Typically, a network is optimized by minimizing cable length throughout, but in this case, the shortest cables also cost less per meter. When you are able to minimize cable lengths, the savings compound quickly.

Naturally, there are limits to what network design can do for costs. Inevitably, racks will need to communicate, and AOC will prove an affordable option compared to other cables that can run such distances.

EMI

Electromagnetic interference (EMI) is the last major point of comparison. EMI can be a problem in many networks. Since DAC lines use copper, they offer virtually no protection from EMI. Meanwhile, AOC cables are completely shielded and essentially immune to EMI.

When EMI is a point of concern for network design, AOC is the only choice among these three.

High-speed patch cables do a lot of heavy lifting in data centers and high-capacity networks. These three options allow you to fulfill communication needs while optimizing around cost and function. Knowing the advantages of each option can help you with that optimization and lead to the best possible network.

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• Visit the CK Learning Center