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The Data Link Layer – How DAC and AOC Cables Can Work For You

Feature - The Data Link Layer - How DAC and AOC Cables Can Work For You - Cable Management Blog

As the need for data storage and speed increases, the need for hyperscale data centers has increased. So has the need for edge data centers as well. While large-scale centers serve companies like Amazon, Microsoft, and Google, other organizations are looking at smaller data centers closer to the end-user. In both cases, the data link layer of the data center is critical. Enter Direct Attach Copper (DACs) cables and Active Optical Cables (AOCs).

The Data Link Layer - How DAC and AOC Cables Can Work For You - Cable Management Blog
The data link layer of the data center is critical to ensuring your resources and used to their full potential

What is that data link layer? It’s the physical layer, the connection between servers that ensures all the computing resources are used to their full potential. The speed and integrity of these connectors can make a huge difference. 

They include Direct Attach Copper (DACs) cables, Active Optical Cables (AOCs), and fiber optic cable assemblies connected into transceivers throughout the data center. How does each one work, and why are they so critical to installation, maintenance, and deployment?

The Need for Speed

There are two aspects to the need for speed: the need for speed in shorter cables between servers, and the need for speed over longer distances. Different kinds of cables work differently in each instance. 

For example, DACs are most often used over short distances, connecting units in the same server rack. They can be active or passive – active connections are part of signal processing circuitry, and passive connections simply carry power. In the case of a DAC, the cable is made of copper rather than fiber. 


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AOCs usually connect devices within the same row, but they cover longer distances than their copper cousins. However, they do not work in End of Row (EOR) or Middle of Row (MOR) configurations where certain types of patch panels are used. They are usually provided in fixed lengths from a few meters long to more than 100 meters. AOCs are active and include transceivers, control chips, and modules.

Both are fast, similar in speed to optic fiber cables, but that speed can be compromised by cable damage or in the case of DACs, electromagnetic interference. Both must be tested with a tool that can accept dual SFP/QSFP transceivers and generate and analyze traffic.

So how do you test them? Well, there are methods that include automation, but there are other factors to consider. 

Automation Matters

 Speed drives us to DACs and AOCs in some cases, but they can become damaged in a variety of ways. This often doesn’t even happen in the installation process, but in the shipping and handling before they even arrive at the data center. Sometimes it happens if they are stored and moved frequently. 

So the first place to test them is before installation. This ensures they are working before they are put into service. It’s easy to see how testing all cables at installation can be costly and time-consuming but not testing early can be costly later on. 

The solution is rapid, automated testing that can be done by running a test pattern where the results can be compared to a Bit Error Rate (BER) threshold. DAC and AOC cables including breakouts usually have a BER rating on their datasheets, especially when they are meant to be used with devices implementing the RS-FEC algorithm.

The tests only take a minute per cable and result in reports including a cable identifier, such as the serial number, identifying clearly any faulty equipment. 

Proper Power Planning

What’s the other advantage of DACs and AOCs? Energy savings. Point to point high-speed cables take less power and can save money, especially at scale. While DACs offer more dramatic numbers per cable, AOCs offer savings as well when multiple transceivers are replaced by cables. 

They’re not ideal for every case in every data center, but where they can be used as a key part of deployment, they can provide significant energy savings.

Living on the Edge Deployment

The other argument for DAC and AOC deployment and testing at installation exists on the edge. More Edge deployments force centers to increase speed, security, and efficiency at the same time as they minimize latency.

Opting to wait and address any connectivity issues during troubleshooting results in costly mistakes and skipping troubleshooting steps in favor of speedy repairs, sometimes those that are not necessary. Not only is this costly – cables can vary from tens of dollars to thousands but it can also lead to confusing labels and the increased probability of unplugging a live cable.

The fact that DACs and AOCs can be tested so quickly and easily at the time of installation is another great argument for their use in the data link layer. But no matter what cable configuration your data center uses, from point to point high-speed cables to other fiber and optical options, the management of that data link layer is critical to smooth data center operations.

Looking for High Speed Cables?

WD 25G SFP28 SFP+ DAC Cable - 25GBASE-CR, SFP28 to SFP28 Passive Direct Attach Copper, Twinax Cable

Ready to start optimizing your data link layer? Have questions about what cables might be right for you and your application? Whether you are deploying a brand new data center or making moves and changes, we’re here to help. Contact AnD Cable Products today for more information. We’re here to help every step of the way. 

About the Author

Louis Chompff - Founder, AnD Cable Products, Rack and Cable ManagementLouis Chompff – Founder & Managing Director, AnD Cable Products
Louis established AnD Cable Products – Intelligently Designed Cable Management in 1989. Prior to this he enjoyed a 20+ year career with a leading global telecommunications company in a variety of senior data management positions. Louis is an enthusiastic inventor who designed, patented and brought to market his innovative Zero U cable management racks and Unitag cable labels, both of which have become industry-leading network cable management products. AnD Cable Products only offer products that are intelligently designed, increase efficiency, are durable and reliable, re-usable, easy to use or reduce equipment costs. He is the principal author of the Cable Management Blog, where you can find network cable management ideas, server rack cabling techniques and rack space saving tips, data center trends, latest innovations and more.
Visit https://andcable.com or shop online at https://andcable.com/shop/