GoKawiilAt STH, we use the term “DAC” to reference Direct Attach Copper cabling dozens of times each year. Almost every time we use the term, someone asks the question, “What is a DAC?” Since at STH we believe it is important to help impart knowledge, even if many readers already know the answer, we felt like it was time for a quick guide.
What is a DAC (or Direct Attach Copper) Cable?
In simple terms, a DAC has modules at either end of a ~26-28 AWG twinax copper cable that allows direct communication between devices over copper wire. Both ends have specific connectors and the cable length is fixed. Electromagnetic shielding around the copper cable increases as the speeds increase to keep communication reliable.
In this example, we have two QSFP+ connectors on either end. There is then a fixed cable that goes between the two ends allowing devices to communicate. This cable, unlike optical transceivers, is usually a fixed length and limited in maximum length by signal integrity.
As part of our fiber optic guide series, we are mostly focusing on optics. Optical communication is essential for the long-range transmission of data. As networks get faster, and we push into the 400GbE era and beyond, the distance that copper communication can reliably and practically travel at those speeds is limited. For the next few years, we are still likely to see copper DACs between devices in a single rack, but going forward, most rack-to-rack and beyond connectivity will happen via optical communication.
What is a Breakout DAC Cable?
We are going to note that you may see one other important type of DAC cable, the breakout DAC. With modules such as QSFP+ for 40GbE networking and QSFP28 for 100GbE networking the “Q” stands for Quad. As a result, one way to conceptualize the QSFP+ connector above is that it is carrying four (quad) SFP+ channels. SFP+ is 10Gbps, QSFP+ is 40Gbps, four (quad) 10Gbps links give us 40Gbps of bandwidth. The same conceptual model holds for SFP28 and QSFP28. As a result, a somewhat common practice is to use the higher-density QSFP+/ QSFP28 form factors and split them to connect to 2-4 lower-speed devices. Here is an example with four SFP+ ends on one side and a single QSFP+ side on the other:
We are going to quickly note that while conceptually this works, not all switches, routers, NICs, servers, storage, and other components support breakout. These days, most do, but there are still quite a few exceptions where they do not. There are even NICs like the HPE 620QSFP28 4x 25GbE Single QSFP28 Port Ethernet Adapter, that are intended to have a QSFP28/ QSFP+ port used with DACs or as four separate connections.
Although you can see one physical port above, you can see the NIC as four separate 25GbE devices not just a 100GbE device:
The important thing here is that DACs and this breakout DAC cable model is so prevalent that we even see specific cards using breakout DACs just to increase port density.
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