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25G Direct Attach Cables (DAC & AOC) Overview

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Posted on March 4, 2020
October 23, 2020
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25G direct attach cables (DAC & AOC) are the ideal and cost-effective solutions for short-distance connectivity within a rack or between adjacent racks in 25G data transmission. There are generally two types of 25G direct attach cables: SFP28 passive direct attach twinax cables (25G DACs) and SFP28 active optical cables (25G AOCs). The 25G SFP28 DAC cables are available in length up to 5m (16.4ft), while the SFP28 AOC cables can reach a much longer length to 30m. Both of them are widely applied in storage area networks, data centers and high-performance computing connectivity. The following will explain the differences between 25G DAC and AOC in all aspects and offer answers to some frequently asked questions.

25G Direct Attach Cables (DAC & AOC) Differences

25G direct attach cables (DAC & AOC) are not only different in the cable distances, but also in the weight, bending radius, heat dissipation, power consumption, and so on. The detailed differences between the 25G direct attach cables (DAC & AOC) are discussed below.


  • Weight and bending radius: with fiber optic cable as transmission media, 25G AOC has much smaller size and lighter weight when compared with 25G DAC. So it can be flexibly deployed in places where space is a premium, and can be transported from one place to another. Also, the bending radius of 25G SFP28 AOC is significant than 25G DAC, which makes 25G AOC cable more suitable for high-density cabling structures.

  • Heat dissipation & power consumption: as 25G DAC is passive and has no power consumption, using it won’t produce heat, thus making it a more versatile with wider temperature ranges. Moreover, its cooling requirement is not strict due to the thermal design, applying 25G DAC can significantly save the data center operating expenses. 25G DAC will keep the transmission stability even in harsh natural conditions and external interference.

  • Electromagnetic interference (EMI): since the cable type of 25G SFP28 AOC is fiber, it is immune to interference from electromagnetic, lightning, or radio signals during data transmission. The 25G DAC might be susceptible to electromagnetic interface, such as adverse response and degradation, etc, but the impact is not significant. If there are special requirements for the EMI in the network, the 25G AOC will be an ideal solution to ensure good confidentiality during the transmission, which can be applied for secure transmission in large data center networks.

  • Price: the price of the 25G AOC is often more expensive than that of 25G DAC. Both 25G SFP28 DAC and AOC can be applied for the transmission within 5m. When the transmission distance exceeds 5m, the SFP28 AOC or 25G modules combined with fiber optic cables are the better solutions.

Further Consideration

Both 25G DAC and AOC are cost-effective and ideal solutions for short-distance transmission between rack and rack in a data center. When it comes to the transmission distance over 100m, 25G fiber optic transceivers combined with matched optical fiber cables are more common and efficient solutions. Different types of optical transceivers can be selected for various transmission distances.

25G Direct Attach Cables: FAQs

The following are some frequently asked questions concerning 25G direct attach cables.

Q: What’re the benefits of 25G direct attach cables?

A: 25G direct attach cables are less costly than using discreet transceivers with field-connected structured cabling because the interconnection is simplified. There aren’t as many connectors, adapters, patch panels, and other infrastructure elements along the path of the communication channel. Additionally, particularly with passive DAC cables, power consumption is less when compared to the use of transceivers because they are self-contained components and not bound by transmission specifications as transceivers. Moreover, DACs and AOCs offer plug and play simplicity - with only one component to manage, rather than multiple items that must be interconnected together.

Q: Can 25G DAC/AOC be backward compatible with 10G or 1G?

A: The 25G AOC cables can support backward compatibility with 10G/1G. But whether 25G DAC cable can be backward compatible is determined by the brands of the networking device.

Q: Can I have different compatibilities for each end of 25G direct attach cables?

A: Of course, different compatibilities for each end of 25G direct attach cables can be provided on the market (For the DACs provided by the original brand providers like Cisco, the different compatibilities for each end of 25G direct attach cable can’t be achieved, but some third party suppliers can provide the different compatibilities for each end of 25G direct attach cables through customization). FS.COM offers customized dual compatibility solutions for 25G direct attach cables so that you can choose compatibilities for each end individually. All you need to do is to select the "Dual Compatibility Solutions" option in Compatible Brands drop-down box and note down your devices’ model numbers on the custom page.

Q: What are the applications of 25G direct attach cables?

A: Direct attach cables can be used in a variety of applications and locations in a data center. In general, this pre-terminated solution is particularly effective for the following applications:


  • Top of Rack/Adjacent Rack: 25G direct attach cables are ideal for shorter ToR or rack-to-rack runs with cost-conscious budgets.

  • Middle of Row - since the 25G DAC cable can run no more than 5m, they may be the perfect solution in the middle of row applications.

  • End of Row - 25G AOCs are most likely the best option for EoR configurations since the applicability of the 25G DACs reaches their limit at around 30m in length.

  • Zone-to-Zone - 25G SFP28 AOC is the clear solution for longer zone-to-zone runs due to the advantages of using fiber optic cables as mentioned previously.

Q: How to choose a reliable supplier for 25G direct attach cables?

A: Technology, quality, compatibilities, technical support, warranty policy, and after-sale services are needed to be considered when choosing a reliable supplier. A trustworthy supplier should provide comprehensive customer services, return and exchange services, professional technical support, product testing, warranty services, etc.

Related Article: What Is 25GBASE-T?