DAC Twinax for Top of Rack Switching


Top of rack switching as the basic data center architecture requires one or two access switch installed on the top of each server rack, which provides easy cable management and maintenance. Additionally, it reduces the cable requirement as all sever connections are terminated to its own rack. Direct attach cables (DAC) Twinax cable can achieve interconnection up to 15 m at 10 Gbps or 40 Gbps with low power. Most server gears come default with DAC Twinax cables in-rack along with a couple of short-throw lasers back to the core. This article will present the cabling solutions of the DAC cables for data center top of rack deployment.

Why Use DAC Twinax Instead of Cat5e or Cat6 Cables for Top of Rack Switching?

What media option would be the best to invest in for the top of rack application? Category copper cable (Cat5e or Cat6) used to be the preferable options. Cat5e or Cat6 cables, standardized on 10GBASE-T, are overall cheap, backward compatible and much easier for cable management, especially for shorter cable runs. But these days, DAC Twinax cables are more suitable for top of rack switching owing to the following reasons.

First, most of the hosts we are buying are not with RJ45 on board and all the switches and firewalls we’ve used for years were SFP+. Second, the extra power consumption for 10GBASE-T is higher than DAC cables. Additionally, DAC cables have lower latency hit. Last but not least, people often prefer DAC Twinax owing to its flexibility. People can easily swap a DAC cable for an SFP+ transceiver and generally connect to anything I need to.

Note that the decision between Cat5e/Cat6 cables and DAC comes down to what your strategy for server hardware is, if you don’t run high-density servers, or you use blade enclosures to reduce your server port count, Cat5e or Cat6 cables might be fine and should be much cheaper.

DAC Twinax Cables

Data-Center-InterconnectionFor interconnection in the top of racks, the direct attach cable assemblies are used to connect the server to switch, storage to switch or switch to switch. Just as the following image shows, DAC Twinax cables, available in direct attach the passive or active copper cable, active optical cable or breakout direct attach cable assemblies, can cope with different interconnection applications.

Copper Twinax cable with various length options are popular on the market. For example, 10G SFP+ DACs are commonly used in interconnect application below 15 m, e.g. server to switch or storage to switch interconnection in the same rack. SFP-H10GB-CU2M is the 10G SFP+ passive direct attach copper cable used for a short-reach direct connection. And for 40GbE, 40G QSFP+ DACs and AOCs are used, the former is usually used for short-interconnection below 15 m while the latter is used for longer interconnect application that over 15 m but less than 100 m. Of course, higher speed and more bandwidth are needed for spine switches. 100G DACs, like QSFP28 DACs are used in this case.

DAC Twinax Used in Top of Rack Switching

FS.COM offers a comprehensive solution for data center ToR deployment including cables, transceiver optics and direct attach cabling products to satisfy various demands on the data rate. Depending on different layers, we offer different solutions:

  • Use Case for Interconnecting Rack Servers and ToR Switch

For interconnection between rack servers and ToR switch within the rack, the Category 6a (Cat 6a) Ethernet patch cables or SFP+ direct attach cables can be used. The cable length depends on the distance from the rack server to the ToR switch that is limited to the vertical height of the cabinet.

  • ToR Switch to Network Switch

When connecting the ToR switch to network switch, it requires increased transmission speeds and distances. For distances below 15 m, the SFP+ direct attach cable is still an ideal option. But if the distances are more than 15 m, we recommend you to use the multimode fiber with the corresponding optics. In addition, for distances, less than 100 m, the SFP+ AOC (active optical cable) is a viable option.

  • Network to Core Switch

At the core of data center, direct attach cables designed with QSFP+ connectors can be used for short distance transmission of up to 15 m to support up to 40Gbps. To support distances above 15 m, the OM3 or OM4 fiber is recommended to use with the QSFP+ optics. In addition, for distances less than 100 m, the QSFP+ AOC is a viable option.


Twinax copper cables including the DAC and AOC cables are the mainly used cable type to support high transfer rates between servers, switches and storage devices intra rack or inter-rack in a data center. This article has concluded the reasons why we should DAC cables and the cabling solution for top of rack switching.

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