What Is a Direct Attach Copper (DAC) Cable? (2021)

A DAC (Direct Attach Copper) cable is a fixed-length copper cable with modules at each end that enables direct communication between network devices over copper wire, primarily used within data racks.

DAC cables typically feature modules at either end, such as QSFP+ or QSFP28, and are designed for short-distance, high-speed connections, usually within a rack or between adjacent racks. They are distinguished from optical transceivers by their fixed length and copper-based signaling, which makes them more cost-effective for certain applications.

There are two main types: passive DACs, which use less power and are simpler, and active DACs, which require more power but can extend the reach slightly. Generally, DACs are limited to about 5 meters at higher speeds like 100GbE, with longer distances requiring optical solutions. As speeds increase, cables become thicker and less flexible, complicating routing in dense environments. DACs are favored for their reliability and lower cost compared to optical transceivers, but their short reach limits their use to intra-rack or rack-to-rack connections.

Why It Matters

Understanding DAC cables is essential for data center infrastructure planning, as they offer a cost-effective, reliable solution for high-speed, short-range connections. However, as network speeds and distances grow, reliance on DACs may decrease in favor of optical solutions, making knowledge of their capabilities and limitations critical for network engineers and IT decision-makers.

Background

DAC cables have been a staple in data center networking, especially for 40GbE and 100GbE connections, where their simplicity and low cost are advantageous. The trend toward higher speeds and longer distances is pushing the industry toward optical solutions, which can support longer reach but at higher costs and complexity. The development of breakout DACs, which split high-density modules into multiple lower-speed connections, reflects ongoing efforts to optimize port density and flexibility.

“DACs are usually a fixed length and limited in maximum length by signal integrity, typically around 5 meters for high speeds.”

— STH (Server Technology Hub)

“As network speeds move above 100GbE, DACs will be limited to shorter runs, and optical solutions will become more prevalent.”

— Industry expert

What Remains Unclear

While the technical limitations of DACs are well understood, the exact timeline for their phase-out in favor of optical solutions varies depending on technological advancements, market demand, and specific deployment scenarios. It is also unclear how quickly newer, more flexible copper or hybrid solutions will emerge to extend DAC reach.

What’s Next

Next steps include ongoing development of higher-speed DACs with improved shielding and flexibility, as well as increased adoption of optical transceivers for longer-distance connections. Industry standards and compatibility will also influence how rapidly DACs are phased out in data centers.

Key Questions

What are DAC cables used for?

DAC cables are used for short-distance high-speed connections within data racks, typically connecting switches, servers, and storage devices.

How long can DAC cables reliably operate at high speeds?

Generally, DAC cables are limited to approximately 5 meters at speeds of 100GbE or higher, with shorter lengths preferred for optimal signal integrity.

What are the main differences between passive and active DACs?

Passive DACs consume less power and are simpler but offer shorter reach, while active DACs require more power but can extend the distance slightly, providing better signal quality at higher speeds.

Why are optical transceivers replacing DACs in some applications?

Optical transceivers support longer distances, are more flexible in routing, and are better suited for high-speed, wide-area networks, though they come with higher costs and complexity.

Source: Hacker News