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Data Center Connectivity Blog Part 2: DACs vs. AOCs

Accessing a high-speed network from your edge device (cell phone, tablet, laptop, etc.) seems simple. But there is a lot of technology going on behind the scenes. The decisions network installers make when choosing network components directly influences your end-user experience.

In Part 1 of this blog, you learned about several factors to consider when choosing between Direct Attach Copper Cables (DACs), Active Optical Cables (AOCs), and optical transceivers for high-speed networking interconnectivity.

In Part 2 of this blog, we dive deeper into the DAC and AOC technology and compare and contrast these two options. Both DACs and AOCs form an all-in-one cable and transceiver circuitry unit (see the image below). This is different than a typical cable-and-transceiver combination that consists of separate cable and transceiver components with more possible points of failure than the compact DACs and AOCs.

Figure 1. Active Optical Cable application
AOC-Application_Diagram

As you can see in Figure 1, an AOC consists of a cable hardwired to a transceiver that connects to a source PC on one side. A second hardwired cable/transceiver end connects to a display on the other side. Likewise, DACs are constructed in the same way.

Unlike optical transceivers, both DACs and AOCs are easy to maintain, because they do not contain optical connectors that require you to clean them by hand. This is much more convenient and saves you the cleaning costs associated with fiber optic connectors. And DACs and AOCs are less likely to be damaged than delicate optical connectors when you handle them.

If you are looking to save on costs and your data only needs to travel a short distance, choose DACs. They cost less than AOCs. As you learned in Part 1 of this blog, DACs can span up to five to seven meters (about 16 to 23 feet). If you need to drive data farther, AOCs may be your best choice since some can reach distances beyond 100 meters (328 feet).

What’s Inside a DAC or AOC

Here’s how it works: A DAC has four wires, divided into two wire pairs. Each pair derives its electrical signal value from the power difference between the two wires in the pair. The electrical signal travels two ways (back and forth) over each pair. To achieve this, a DAC’s two copper wire pairs are soldered onto a PCB with an EPROM chip that specifies cable length, data rate, and more; have a protective foil covering; and use SFP (small-form-factor-pluggable) or QSFP (quad-SFP) connectors on the ends.

If your data needs to go farther, consider AOCs. They will cost you a bit more, but can drive data signals farther, typically up to 100 meters (328 feet). Both DACs and AOCs are all-in-one solutions, but while DACs use copper cable, AOCs use optical fiber cable, which is more reliable, thinner, and flexible than copper cable. The optical transceivers that are built-in to an AOC change data signals into light that is sent over fiber. Each AOC has eight fibers (four pairs) cut to exact lengths to reduce latency between the four channels in the cable end, so the four signals (over two fibers per signal pair) get to the transceiver at the same time.

Now that you know the basics, let’s investigate each option’s advantages.

Less is More: Limited Distance Equals Cost Savings

If you need to run your connection less than seven meters (about 23 feet), DACs may be your most practical option. Use them for short-distance connections in your rack, for example, to connect top-of-rack switches to servers and storage below. DACs will cost you less than AOCs or fiber cable with modular transceivers attached.

When You Need to Go Farther

For longer distances (up to 100 meters, or 328 feet), AOCs may be your best option. Plus, ultra-high-bandwidth data signals transmit seamlessly over fiber.

Save on Your Energy Bill

DACs consume almost zero power: they use up to one watt less per connection compared to AOCs. For example, suppose you have 250 connections in your rack. Choosing DACs instead of AOCs will decrease the overall power consumption by about 250 Watts, so your installation runs more efficiently and costs less, which makes a big difference in your bottom line.

Don’t Sacrifice Speed

Perfect for sending large amounts of data fast, DACs operate with almost zero latency. AOCs clock in at less than a microsecond of difference than DACs use to transfer data. But DACs may not perform as well as AOCs at higher bandwidths: 100G signals may only travel up to three meters (a little less than 10 feet) over DACs, compared with up to 100 meters (about 328 feet) over AOCs.

Consider Flexibility

DACs are constructed of thick copper cable, and the thickness increases as the bandwidth increases. 100G copper cable is thicker than 10G copper cable. In contrast, no matter the bandwidth, the thickness of the fiber optic cable used in AOCs remains constant. In general, a fiber optic cable (measuring 3-mm in diameter) is about one-third thinner than a typical copper cable (approximately 9-mm). You will find that AOCs are easier than DACs to install in tight spaces.

Signal Clarity

Electromagnetic Interference (EMI), such as ground loops and electrical noise, won’t distort fiber optic cable signals. Because AOCs use fiber, you get much better signal clarity than with the copper wires used in DACs. While DACs require shielding to counteract EMI, AOCs are fully isolated from power issues. Plus, the extra shielding that DACs need to counteract EMI increases their cable thickness, so they take up more space in a network installation.

Plug-and Play

Both DACs and AOCs are easy to install. However, if an DAC or AOC fails, an installer needs to remove it from the network and replace it, which may involve cable re-routing and extra time and effort. This translates to extra labor and materials costs for you.

Interoperability

Active Optical Cables don’t need to meet IBTA, IEEE or SFF standards for transceiver interoperability because they are designed as a closed system with a predefined length. This means that installers can select the best-performing and lowest-cost devices for their systems.

Next Steps

As you have learned in this blog, DACs and AOCs are both viable choices for your high-performance network connectivity. DACs and AOCs both are compact, all-in-one solutions for network connectivity.

DACs are ideal for short-distance applications and are commonly used in high-performance computing systems, large-scale commercial operations, and storage applications. DACs consume almost no power, are extremely cost-effective, and provide outstanding performance. They are great for connecting network servers and storage within a rack to top-of-rack switches.

Active Optical Cables are perfect for long-distance applications because they can run up to 100 meters (328 feet), feature ultra-high-speed bandwidths, are thin and flexible, resist electrical interference, and are easy to install.

So, the next time you pick up your cell phone, tablet, laptop, or other edge networking device, you just might think twice about how your data reaches you instantly!

To learn more about DACs and AOCs, as well as a third option for high-performance network connectivity (optical transceivers), download our free white paper: Data Center Connectivity.

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