The Decision Is Rarely About Speed
On paper, QSFP28 DAC and QSFP28 AOC look very similar. Both support 100G Ethernet, both are widely deployed, and both are commonly used for short-reach connections inside data centers. If you only look at bandwidth numbers, there is almost no difference.
But in real deployments, speed is rarely the deciding factor. The real question usually comes later, during installation, troubleshooting, expansion, and daily operation. That is where the differences between DAC and AOC start to matter.
Understanding What You Are Actually Deploying
QSFP28 DAC is essentially a passive copper cable with QSFP28 connectors on both ends. There are no active optical components involved. Once plugged in, the switch handles most of the signal processing.
AOC, on the other hand, integrates optical transceivers and multimode fiber into a single assembly. The optics are sealed inside the cable, which means the signal is converted to optical and back again, even though the cable looks just as simple from the outside.
This fundamental difference shapes how each solution behaves over its lifetime.
Installation Speed and Human Error
In large-scale deployments, installation speed is not just about convenience. It directly affects rollout schedules and labor costs.
DAC cables are straightforward. You plug them in, check link status, and move on. There is no concern about fiber polarity, no cleaning procedures, and no sensitivity to bend radius within short distances.
AOC cables are also easy to deploy, but they introduce subtle handling considerations. Even though the fiber is pre-terminated, excessive bending or rough handling can still affect long-term reliability. These issues do not always appear immediately, which makes them harder to diagnose later.
Cable Management and Physical Behavior
Copper and fiber behave very differently inside racks.
DAC cables are thicker and less flexible. In dense racks, this can make cable routing more challenging, especially when many short links are packed closely together. However, their stiffness can also be an advantage, as cables tend to stay where they are routed and are less likely to shift accidentally.
AOC cables are thinner and more flexible. They are easier to route cleanly, particularly in high-density spine or leaf layers. That said, their flexibility can sometimes work against them, as cables may move or bend more easily during maintenance activities.
Power Consumption Over Time
Power consumption often becomes visible only when multiplied across hundreds or thousands of ports.
QSFP28 DAC consumes minimal power because it does not perform optical-electrical conversion. This makes it attractive for environments where power efficiency is tightly controlled and incremental savings add up.
AOC cables consume more power due to the embedded optical components. While the difference per link is small, at scale it can influence rack-level power budgets and cooling requirements.
Distance Requirements and Hidden Trade-Offs
Distance is one of the most commonly cited differences. DAC is typically used for very short connections, while AOC supports longer reach, often up to 70 meters or more.
What is less discussed is how often that extra reach is actually needed. In many data centers, the majority of connections fall well within DAC’s supported range. Choosing AOC in those cases provides flexibility that may never be used.
However, when layouts are less predictable or when racks may be rearranged in the future, AOC’s extra reach can prevent the need for recabling later.
Failure Modes and Troubleshooting
When something goes wrong, simplicity matters.
With DAC, troubleshooting is usually straightforward. If a link fails, the cable is often the first and only suspect. Replacing it is quick and inexpensive.
With AOC, failures can be harder to isolate. Because the optics are embedded, it is not always immediately clear whether the issue lies in the cable, the transceiver electronics, or the host port. This can increase mean time to repair, especially in busy environments.
Inventory and Logistics Considerations
From a logistics standpoint, DAC cables are easier to stock. They are generally cheaper, have longer shelf life, and are less sensitive to storage conditions.
AOC cables require more careful handling and typically cost more. For organizations managing large inventories across multiple sites, this difference can affect procurement and spare strategy.
Lifecycle Cost Versus Initial Cost
Initial cost comparisons often favor DAC, but lifecycle cost is where the decision becomes clearer.
DAC’s lower power consumption, simpler troubleshooting, and lower replacement cost often result in lower total cost of ownership for short-reach links.
AOC may justify its higher cost in scenarios where distance flexibility reduces future rework or where cable density and airflow benefits outweigh power considerations.
Use Case Alignment Matters More Than Technology
Neither DAC nor AOC is universally better. Each excels in specific scenarios.
DAC aligns well with fixed, dense, short-reach architectures where predictability and efficiency are priorities. AOC fits better in environments where layouts change frequently or where longer internal distances are unavoidable.
The key is matching the technology to the operational reality, not just the technical specification.
Conclusion
QSFP28 DAC vs AOC is not a debate about performance. It is a conversation about operational philosophy. DAC favors simplicity, efficiency, and predictability. AOC offers flexibility and cleaner cabling at the cost of higher power use and complexity. Understanding how your data center actually operates—today and in the future—is the only reliable way to make the right choice.
