400G DAC Cable Solutions for Short-Reach Connectivity

As global data traffic escalates, the demand for high-density, low-latency networking solutions has never been higher. The transition to 400G Ethernet represents a critical milestone in data center evolution, requiring interconnects that can handle massive throughput without compromising on power efficiency or cost. In short-reach environments—typically within a single rack or between adjacent cabinets—Direct Attach Copper (DAC) technology has emerged as the gold standard. By providing a robust, purely electrical path for data, 400G DAC cables offer a strategic balance of performance and sustainability, ensuring that the backbone of modern digital infrastructure remains both fast and cost-effective.

400G Direct Attach Copper (DAC) cable is a high-speed, cost-effective interconnect solution designed for short-reach data transmission within high-performance computing environments. These cables consist of a twinax copper assembly terminated with industry-standard transceiver modules, such as QSFP-DD or OSFP, at both ends. Unlike active optical cables (AOC) that rely on electrical-to-optical conversion, DACs operate purely on electrical signals, which inherently minimizes power consumption and eliminates the latency introduced by signal conversion processes. The 400G throughput is typically achieved through eight channels of 50G PAM4 (Pulse Amplitude Modulation) signaling.

The primary application of 400G DAC cables is centered within the "Top-of-Rack" (ToR) switching architecture of hyperscale data centers and enterprise server rooms. In these environments, DACs are the preferred choice for connecting servers to their respective switches within the same rack or in immediately adjacent racks. Because the physical distance between a server's Network Interface Card (NIC) and the rack switch is usually under three meters, the distance limitations of copper are irrelevant, while the benefits of zero power consumption and low cost become paramount. This creates a highly efficient edge layer that supports the massive bandwidth requirements of AI training clusters and high-frequency trading platforms where every microsecond of latency is scrutinized.

Beyond simple point-to-point connections, 400G DACs are increasingly utilized in "breakout" configurations to optimize port utilization and bridge the gap between different generations of hardware. A single 400G port on a spine switch can be split into four 100G or eight 50G connections using a breakout DAC cable, allowing older 100G servers to integrate seamlessly into a 400G fabric without the need for expensive intermediate transceivers. Furthermore, as data centers transition toward liquid cooling and high-density GPU clusters for Large Language Model (LLM) processing, the robustness of copper becomes a mechanical advantage. DAC cables are less sensitive to dust, debris, and tight bend radii compared to fiber, making them ideal for the cramped, high-energy environments found in the heart of modern AI infrastructure.

AICPLIGHT 400G DAC portfolio is engineered to address the specific density and thermal requirements of next-generation networking hardware. These DAC cables provide the foundation for low-latency, energy-efficient interconnects within modern hyperscale environments.

Often referred to as the industry-standard "workhorse" for 400G networks, the QSFP-DD (Quad Small Form-factor Pluggable Double Density) interface maintains critical backward compatibility while effectively doubling port density compared to previous generations. This passive cable is the primary choice for Top-of-Rack (ToR) switching, linking 400G switches to high-density servers within a range of 0.5m to 3m. It achieves near-zero power consumption and ultra-low latency, making it ideal for standard enterprise rack deployments.

Figure 1: A 400G QSFP-DD passive direct attach copper twinax cable provides a high-speed link between two Arista DCS-7060DX4-32-F switches for short-reach data center connectivity.

As the market utilizes both OSFP and QSFP-DD standards, physical interoperability has become a vital requirement for heterogeneous hardware environments. This "hybrid" DAC cable facilitates a seamless, direct electrical connection between a QSFP-DD equipped switch and an OSFP-based server or storage array. By eliminating the need for expensive adapters or transceiver conversions, this solution ensures that infrastructure remains flexible regardless of the vendor mix. This is particularly critical when integrating high-performance AI compute nodes into existing network fabrics, providing a stable 400G link that supports RDMA over Converged Ethernet (RoCE) applications with maximum efficiency.

Figure 2: 400G QSFP-DD to OSFP passive DAC cable enables seamless integration between an Arista switch and an H100 GPU server equipped with a ConnectX-7 NIC.

Designed for high-efficiency traffic distribution, this breakout (splitter) cable is a key component for high-radix switching architectures. The "Finned Top" design on the OSFP connector serves a dual purpose: it acts as an integrated heat sink that utilizes the switch's internal airflow to maintain optimal thermal operating points. This cable splits a single 400G OSFP port into two 200G QSFP56 channels, allowing a single high-radix switch to serve multiple 200G nodes. This effectively doubles the connectivity density of a single rack unit and is widely adopted in AI training clusters where high-bandwidth distribution to multiple accelerators is required.

Figure 3: A 400G OSFP to 2x 200G QSFP56 breakout cable splits high-bandwidth traffic from an Arista DCS-7050PX4-32S switch to multiple Mellanox network interface cards.

Similar to the OSFP breakout, the QSFP-DD version serves as the go-to solution for standard 400G leaf switches connecting to 200G NICs. This cable provides a logical and physical migration path for data centers transitioning from legacy 200G systems into a cohesive 400G core fabric. By enabling the reuse of existing 200G infrastructure while upgrading the switching core, it protects hardware investment and reduces the total cost of ownership (TCO). The passive copper construction ensures that these high-density breakout configurations do not add to the thermal load of the rack, maintaining a "green" and cost-effective interconnect strategy for short-reach applications.

Figure 4: This configuration demonstrates a 400G QSFP-DD to 2x 200G QSFP56 breakout DAC cable connecting an Arista 400G switch to a legacy 200G OEM switch.

In the high-stakes world of hyperscale computing and AI model training, the choice of interconnect is far from a minor detail; it is a fundamental driver of operational efficiency. The 400G DAC cable stands out as a superior solution for short-reach connectivity, offering unmatched benefits in terms of zero power consumption, minimal latency, and mechanical durability.

AICPLIGHT's comprehensive portfolio—ranging from standard QSFP-DD cables to specialized OSFP-to-QSFP-DD hybrids and high-efficiency breakouts—is designed to meet the rigorous demands of today's heterogeneous hardware environments. By integrating these high-performance copper solutions, data center architects can optimize their port density, protect their legacy investments, and build a scalable foundation capable of supporting the most intensive computing workloads of the future.