As data center demand expands, careful cost management is important on the micro and macro scales. According to a recent McKinsey report, ITÂ hardware and network infrastructure costs account for more than half of the capex needed for AI data center expansion. Even choosing between DAC and AOC server connections can have a significant impact on your return on investment and system performance. How do they compare?
What Are DAC Cables?
DAC stands for Direct Attach Copper cabling. This type of high-speed networking cable uses a twinax copper core to transmit electrical signals.
Active (powered) variants of DAC have integrated signal conditioning. For example, Active Copper Cables include signal amplifiers, and Active Electrical Cables use clock data recovery units to eliminate jitter.
What Is AOC Technology?
AOC stands for Active Optical Cables. Unlike the DAC category, AOC technology consists of fiber-optic cables. Optical transceivers and multimode fiber allow for excellent signal quality and high data transfer rates.
AOC cabling and optical transceivers are available in countless pluggable configurations, but generally on the higher end of the spectrum:
- QSFP+ 40G
- QSFP28 100G
- QSFP-DD 400G
- OSFP 400G
There are also breakouts with form factors for 4x25G or 4x10G switch port connections.
What Are the Main Differences Between DAC and AOC?
DACs are passive or active high-speed copper cables with a short-to-medium range. Passive DACs consume very little power, but active versions include powered circuits for signal enhancement.
AOC uses high-performance fiber-optic technology. Optical transceivers and AOC cabling can transmit high-speed data over long distances with minimal signal degradation.
Common Applications for DAC Cables and AOC in Data Centers
DAC cables are ideal for short-range connections. For example:
- Within server racks
- Between adjacent racks
- Top-of-Rack switch connections
- Switches to routers
- Servers to storage area networks
Active versions of DAC cables provide a greater range, but the maximum distance between connections is about 20 feet (7 meters) in 400G data centers.
AOC allows for much longer connections. This type of cabling is an excellent choice for End-of-Rack and Middle-of-Row switches, as well as other connections across racks. In high-performance computing environments, AOCs also allow for connections between networking areas, such as leaf-spine switches and core architecture.
AOC Vs. DAC Comparison Chart
| AOC | DAC (Passive) | DAC (Active) | |
| Cable type | Fiber optics | Copper | Copper |
| Power consumption | >1 W | <0.15 W | 0.5 to 1 W |
| Cost | $$$ | $ | $$ |
| Maximum length at 400G | 100 meters | 3 meters | 7 meters |
| Installation | Complex | Simple | Simple |
| Latency | Low | Very low | Very low |
| Key benefits | Immune to EMI, excellent signal quality | Energy efficiency | Balance of cost and performance |
| Common applications | EoR, MoR, leaf-spine, and core | ToR and inter-rack | ToR and adjacent racks |
Pros and Cons of Passive DAC Cables
To show the differences between DAC and AOC more clearly, this section focuses on the advantages and disadvantages of passive DAC cables.
Energy Efficiency (Pro)
Passive DAC cabling connections require minimal power to run. Real-world power consumption is typically <0.1 W per cable. DAC connections help lower data center operating costs and power demands. The difference is especially noticeable in large- or hyperscale environments, where DACs can potentially cut per-switch power consumption by half.
Low Cost (Pro)
Copper twinax cables are relatively inexpensive, with most of the cost coming from the SFP or QSFP transceivers. Using DACs wherever possible can significantly reduce infrastructure costs.
Deployment and Maintenance (Pro)
DAC cables are durable and resilient. They don’t have many components, so few things can go wrong. IT techs can install and replace them easily, and accidental bumps or environmental factors are less likely to cause problems.
This indirectly translates to lower opex for data centers. The higher the cost of IT labor, the greater the savings with robust and long-lasting cables.
Thermal Control (Pro and Con)
DAC cables have several advantages for heat control. First, the copper core helps with dissipation, moving heat away from server components. With no or little power consumption, the cables stay cooler.
When server space is an issue in HPC data centers, however, DACs can become a liability for heat control. To achieve 400G, DAC cables need to be larger to accommodate the additional channels. This can block airflow in congested clusters.
Transmission Distance (Con)
One of the biggest factors when comparing AOC vs. DAC is the deployable distance. In 400G data centers, DAC cables have a maximum distance of 10 feet (3 meters). Scaling to 800G reduces the max to about 6 feet (2 meters).
Active DACs (ACC and AEC) can significantly extend this distance, but they also come with power and cost tradeoffs. Think of active DAC as the middle ground between passive DAC and AOC.
Advantages of AOC Technology
Optical fiber offers the cutting-edge performance that HPC data centers need to compete. AOCs offer improvements across the board, but with a higher price tag compared to DACs.
EMI Immunity
External equipment can generate electromagnetic interference that affects DAC cables. Shielding cables reduces the effect, but no DAC (active or passive) is immune to EMI.
Optical technology is different. Light signals are impervious to EMI. This makes AOC ideal for areas near machinery, motors, or HVAC systems.
Distance (Pro)
With a max distance of over 300 feet (100 meters) at 400G, AOC can adapt to any connection requirements. AOC cables and servers prepare AI data centers for 800G and beyond.
Flexibility (Pro)
AOC cabling is thin, lightweight, and flexible. Not only does this reduce the weight on raceways, but it also simplifies connection layouts when space is an issue.
Airflow (Pro)
Even at 400G, AOCs are narrow. They allow plenty of air through racks and avoid congestion.
Bandwidth (Pro)
Optical signals support high bandwidth and data transfer speeds with minimal degradation. For data centers that require maximum processing capabilities, AOC is a must.
High-Performance AOC and DAC Cabling Solutions
At Equal Optics, every DAC and AOC cable is engineered for performance, compatibility, and reliability — backed by lifetime support. Contact our team to optimize your next data center upgrade.
