Jul 30, 2020

Cabling Your Data Center? Direct Attach Cables (DAC) vs Active Optical Cables (AOC)

Is high density the future of data centers? Absolutely. As more businesses and consumers take advantage of cloud-native, data-intensive services, the principles of high-density design help data center operators do more within a given footprint. From improving space utilization to reducing operating costs, many considerations go into designing and deploying high-density data centers. One of the most fundamental of all, however, involves the selection of cables to connect switches with each other and the numerous servers and networking equipment within the data center.

In this regard, data center operators have a choice – Direct Attach Cables (DAC) or Active Optical Cables (AOC). In both active and passive variants, DACs feature copper wire that connects two fixed transceiver modules on either end. AOCs, on the other hand, use multimode fiber to connect permanently attached transceivers. Both function as alternatives to purchasing individual modules and cables separately.

Is one type better than another? The answer really depends on your needs. Both types of cables have advantages and some aspects that require a bit more consideration.

Cost

When dealing with the high volumes of transceivers and cables needed within a data center, DAC cables provide a cost-effective alternative to AOCs. From the copper used to connect both ends to the lower cost hardware within the transceivers themselves, DACs are an excellent purchase for data center operators buying in bulk to outfit their facilities. By comparison, AOCs, which leverage fiber and other optical technology, are a slightly more expensive choice.

That said, DACs tend to be less compatible with host devices, which include servers, routers and switches. As a result, data center operators must weigh the trade off between more affordable copper equipment and better compatibility. 

Length

In terms of length, DACs are limited to a maximum length of 10 meters. On the other hand, AOCs can run up to a standard maximum length of 50 meters, though optical equipment providers like Precision OT can create custom lengths beyond that if required. AOCs typically leverage OM3 and OM4 multimode cables, with OM4 fiber being backwards compatible with OM3.

The issue with DACs, which limits them in terms of length, is that their electrical signals become quite susceptible to interference and attenuation over longer distances. Of course, with a maximum of 10 meters, DACs are unlikely to be the sole cable of choice for data center providers. While they are less costly than AOCs, they also feature thicker cabling, which makes them harder to run through tight spaces or corners. For data centers that house equipment well over 10 meters apart, AOCs provide the greater reach their operators require.

Durability

Compared to DACs, AOCs are much lighter and thinner which makes them more susceptible to damage if not handled correctly. It is not uncommon for a data center provider to receive their AOCs intact but accidentally break them during unpackaging or while running them through the facility. If you’re looking for some tips on how to handle your AOCs correctly, check out our video on the proper ways to handle your fiber optic equipment. 

Signal Integrity

Data centers are home to many servers, routers and switches – all generating electromagnetic interference. Because DACs use an electric signal to transmit data between both ends, it is possible that the electromagnetic interference will obstruct connectivity. In this circumstance, data center operators are may encounter errors on their data circuits. AOCs, on the other hand, do not encounter this problem as they can run even distances further than 100 meters before engineers could begin to see errors. As such, with the standard length of 50 meters, data center operators leveraging AOCs will not have issues with noise or interference.

A Place for Both

In our experience, data center providers use both DACs and AOCs. Overall, compared to buying individual transceivers and cables, investing in DACs and AOCs saves money and time spent dealing with potentially discrepant cable lengths, panels or splicing fiber. For multi-tenant data centers, it is typical to encounter scenarios where tenants share connectivity in their racks. Some might be on DACs and others on AOCs – it is the responsibility of the provider to meet customer preference. In other cases, one might see a server going to a switch via a DAC and the switch to a router over AOC.

To meet this varying demand, we’re now offering both 25G DACs and 25G AOCs. Our DACs are available with varying cable thickness gauges (AWG) cables in standard length increments of 1m, 3m, 5m and 7m. Our AOCs leverage OM3 and OM4 cables in custom or off-the-shelf lengths up to 50 meters. If required, we can offer AOC lengths beyond 50m. To learn more about our 25G DACs or for more information on our 25G AOCs, contact us at sales@precisionot.com.

To find out more about how we can help you with your data center connectivity requirements overall, contact us!