The “3-Meter Problem” in Your Top-of-Rack
In data center design, especially in Top-of-Rack (ToR) architectures, every connection matters. Network architects face a constant challenge: connecting servers and switches at high speeds (like 100G, 400G, and beyond) with high reliability, low latency, and minimal cost.
For the short-distance connections that dominate the rack, two solutions have emerged as clear front-runners: Direct Attach Copper (DAC) and Active Optical Cable (AOC).
Choosing the wrong one can lead to unnecessary costs, higher power consumption, or even link failures. This guide will break down the differences so you can make the right engineering and financial decision every time.
What is a DAC (Direct Attach Copper) Cable?
A Direct Attach Copper (DAC) cable is a high-speed, twinax copper cable with transceiver modules (e.g., SFP+, QSFP28) permanently attached at both ends. They are a fixed-length, “plug-and-play” solution.
- Passive DAC: This is the most common type. It contains no active electronic components. It simply acts as a high-fidelity copper pipe, relying on the host switch’s electronics to transmit the signal.
- Pros: Extremely low cost, near-zero power consumption, and the lowest possible latency.
- Cons: Limited distance (typically max 5-7 meters at 100G).
- Active DAC (ADC): This type includes active electronic components within the transceiver heads to boost and condition the signal.
- Pros: Can reach longer distances than passive DACs (up to 15m).
- Cons: Consumes some power (though less than AOCs) and costs more than passive DACs.
What is an AOC (Active Optical Cable)?
An Active Optical Cable (AOC) is functionally similar to a DAC—it’s a fixed-length cable with transceivers on both ends. The critical difference is that it uses multimode fiber optic cable instead of copper.
The AOC’s transceiver heads perform the necessary electrical-to-optical conversion, allowing data to travel as light.
- Pros:
- Distance: Can reach much longer distances (up to 100 meters or more).
- Weight & Size: Significantly lighter and thinner than bulky copper DACs.
- EMI Immunity: Because it uses light, it is completely immune to electromagnetic interference (EMI).
DAC vs. AOC: Head-to-Head Comparison
For many engineers, the choice comes down to the numbers. Here is a direct comparison for a typical 100G QSFP28 link.
| Feature | Passive DAC (Copper) | Active AOC (Fiber) |
| Max Distance | ~7 meters | Up to 100+ meters |
| Cost | Lowest | Medium |
| Power Consumption | ~0W (Negligible) | Low (~0.5 – 1.5W per end) |
| Latency | Lowest (Sub-nanosecond) | Low (but higher than DAC) |
| Weight & Bulk | Heavy & Bulky | Light & Thin |
| Bend Radius | Poor | Excellent |
| EMI Immunity | Susceptible | Immune |
The Decision Guide: Which One Should I Use?
Here are the practical scenarios to end the debate.
Scenario 1: Choose DAC for In-Rack Connections (< 3 Meters)
This is our “Golden Rule”: If the link is 3 meters or less, use a Passive DAC.
For connections inside the same rack—such as connecting servers to the ToR switch—Passive DAC is the undisputed champion.
- Why?
- At this short distance, its limitations (distance, EMI) are irrelevant. You get the benefit of its near-zero cost, zero power consumption, and ultra-low latency. In a data center with thousands of links, saving 1-2 watts per cable adds up to massive operational savings.
Scenario 2: Choose AOC for Inter-Rack Connections (3m to 30m)
Use an AOC for connecting racks.
This is the sweet spot for AOCs. When you need to connect your ToR switch to an End-of-Row (EoR) or spine switch in an adjacent rack, a 7-meter DAC won’t be long enough.
- Why?
- An AOC easily bridges these 5, 10, or 20-meter gaps. Furthermore, in dense, high-count cabling trays, the benefits of AOCs become critical. Their small diameter and light weight improve airflow, reduce cable tray load, and make cable management significantly easier.
Beyond 100G: Considerations for 400G and Up
As speeds increase to 400G and 800G, the signal integrity challenges for copper become exponentially harder. This means passive DAC distances get even shorter (e.g., 2-3 meters max for 400G).
This trend further solidifies our rule: DACs are for “inside the box” (the rack), while AOCs are for “outside the box” (inter-rack).
FAQ: DAC vs. AOC
Q: Do DACs and AOCs use the same port on a switch?
A: Yes. Both DACs and AOCs use the same standard SFP+, SFP28, QSFP+, or QSFP-DD ports on your switch. The switch port doesn’t care if you plug in a copper DAC, a fiber AOC, or a regular optical transceiver.
Q: What is the real maximum length of a passive DAC?
A: It depends on the data rate and the quality of the host switch’s electronics. Our “3-meter rule” is a safe design choice to guarantee performance and reliability.
Q: Does the “DAC vs. AOC” rule also apply to 40G (QSFP+)?
A: Absolutely. The physics are the same. A 40G DAC is the most cost-effective solution for in-rack connections, while a 40G AOC is ideal for connecting racks that are 10m-30m apart.
Q: Can I get “breakout” DACs or AOCs?
A: Yes. Both DACs and AOCs are commonly available in breakout (or “hydra”) configurations, such as a 40G QSFP+ breaking out to 4x 10G SFP+, or 100G to 4x 25G. This is very common for connecting a high-speed switch port to multiple server NICs.
Conclusion: PHILISUN’s Final Recommendation
Choosing between DAC and AOC is simple if you follow this expert rule of thumb:
- For links 3 meters or less (In-Rack): Use a Passive DAC.
- For links 3 meters or more (Inter-Rack): Use an AOC.
By following this, you will build a network that is both high-performance and perfectly cost-optimized.
At PHILISUN, we provide high-reliability, fully-tested DAC (Direct Attach Cables) and AOC (Active Optical Cables) for all your data center needs.
Unsure about your specific data center design? Contact our engineering team for a free consultation. We’ll help you build a reliable, cost-effective network.
