In modern data centers, 400G DACs are commonly used for ultra-short-distance, high-speed interconnects. With the development of cloud computing, AI, and other services, the demand for high-bandwidth connections continues to rise. Compared to optical modules, they are lower-cost, plug-and-play, and consume less power, making them suitable for high-density deployments. This article will briefly introduce the relevant aspects of 400G DACs.

What Is a 400G DAC

400G DAC cables are a high-speed copper interconnect solution primarily used for ultra-short-distance transmission of 400Gbps Ethernet or InfiniBand signals. Unlike optical module solutions, DACs do not require separate optical modules and fibers; instead, the copper cable and connector are integrated into a single component.

Each end of the cable typically features a QSFP-DD or OSFP connector, which plugs directly into the ports of switches, network interface cards, or accelerators. Inside the cable, multiple high-speed copper twinax pairs carry electrical signals between devices.

Because DACs transmit signals electrically rather than optically, they are optimized for short-reach applications, usually supporting distances of 1 to 3 meters for passive cables and slightly longer distances for active copper designs.

This integrated design makes 400G DACs a convenient plug-and-play solution for data center deployments where devices are located close to each other, such as inside the same rack.

How a 400G DAC Works

From a technical perspective, a 400G DAC transmits high-speed electrical signals directly through copper conductors. Modern 400G DAC cables typically use eight electrical lanes, each carrying 50Gbps PAM4 signaling, to achieve a total throughput of 400Gbps.

When a switch or server sends data, the host device generates high-speed electrical signals that travel through the copper pairs inside the cable. These signals move directly to the receiving device, where they are processed by the host interface.

Because the connection is entirely electrical, there is no electrical-to-optical conversion involved. This greatly reduces latency and power consumption compared with optical solutions.

However, electrical signals over copper are more susceptible to attenuation and signal degradation over distance. For this reason, DAC cables are designed specifically for short connections where signal integrity can be maintained without complex amplification or optical conversion.

Key Advantages of 400G DAC

One of the core advantages of 400G DAC cables is their excellent cost-effectiveness. Since they do not require optical components such as lasers and photodetectors, they cost significantly less than optical modules or active fiber-optic cables, making them ideal for large-scale deployments that require numerous short-distance links.

Low power consumption is another major highlight. Compared to 400G AOC, passive DAC cables generate almost no additional power, operating directly through the host device’s electrical interface, effectively reducing the overall energy consumption of high-density racks.

The ease of deployment also makes DAC cables highly sought after. As an integrated component, installation is extremely simple; engineers only need to connect the cable directly to the ports at both ends, without the need for additional optical modules or complex fiber management.

Furthermore, DAC cables have a robust structure, facilitating maintenance and management in short-distance applications. Their excellent durability also allows them to withstand frequent plugging and unplugging operations during equipment upgrades and routine maintenance.

Applications of 400G DAC

In modern data centers, 400G DAC cables are primarily used for very short connections where optical links would be unnecessary or inefficient.

One of the most common scenarios is server-to-switch connectivity inside a rack. High-performance servers equipped with 400G network interface cards can connect directly to top-of-rack switches using DAC cables, providing high bandwidth with minimal cost.

DACs are also widely used for switch-to-switch connections within the same rack, particularly in high-density switching environments where ports are located only a few meters apart.

Another important application is in AI and GPU clusters, where multiple accelerator servers are deployed within the same rack. DAC cables provide low-latency, high-bandwidth links that support rapid data exchange during distributed training workloads.

In these scenarios, DACs offer a highly efficient solution because the required distances are extremely short and optical technology would add unnecessary cost and complexity.

Conclusion

As data centers continue to adopt 400G networking technologies, choosing the right interconnect solution becomes increasingly important. While optical modules and active optical cables play critical roles in longer-distance links, 400G DAC cables remain the most practical option for ultra-short connections. For connections within the same rack or between nearby devices, they provide a straightforward solution that combines performance with simplicity.