Introduction to Two Data Center Cabling Solutions: DAC Cable vs. AOC Cable

DAC and AOC cables are widely used in high-performance computing networks' cabling systems due to their low latency, low power consumption, and relatively controllable cost advantages in the high-speed interconnection scenario of data centers. Compared with the traditional combination of optical modules and patch cords, these two types of cables provide a simpler and efficient solution.

Whether it is 10G, 25G, 40G, or 100G data transmission needs, DAC and AOC cables provide corresponding interface options, such as SFP+, SFP28, QSFP+, QSFP28, and other specifications, to adapt to the port types of different network equipment. In addition, they also support branch structure configuration, such as splitting from one 40G into four 10G or from 100G into four 25G, to meet the needs of diverse network architecture design.

DACs and AOCs are reliable options for both performance and cost in today's data centers, whether they are used for short-distance direct connectivity or for flexible deployment within a certain range.

DAC And AOC Wiki: Basic Knowledge and Types

DAC, or Direct Attach Cable, is essentially a twisted pair of copper wires with common modular interfaces like SFP+, SFP28, QSFP+, QSFP56, or QSFP28 at both ends, used to create a direct electrical connection between network devices. Because of their simple structure and ease of deployment, DACs are a very practical category of cables, especially for use in scenarios where the distance between devices is short.

These cables are subdivided into two types: passive and active. Although they both transmit electrical signals through copper wires, there are some differences in the way they work. Passive DACs are simpler, relying on a physical connection to complete the signal transfer without additional processing, while active DACs incorporate circuits inside the module to amplify or regulate the signal and improve the stability of the transmission, especially in environments where the transmission distance is a little farther away or where there is a lot of interference.

In practice, DACs are often used to interconnect switches, servers, and storage devices, especially within the same cabinet or between neighboring devices that need to be connected quickly, which can provide an efficient and cost-effective solution.

Passive DAC vs Active DAC vs AOC

An AOC, or Active Optical Cable, is different from a DAC in that it has a multimode fiber at its core. The two ends of the cable are equipped with a module similar to the SFP interface, which is used to realize the conversion between electrical and optical signals. When working, it is necessary to rely on an external power supply to drive the module's internal electronic components, the first electrical signal into an optical signal transmission, and then converted back to electrical signals at the other end.

Due to the longer transmission distance and stronger anti-interference ability of optical fiber, AOC is more suitable for use between racks, such as inside a data center; when establishing connections between switches, servers, or storage devices in different areas, AOC is a very common choice. AOC not only ensures stable high-speed transmission but also avoids the signal attenuation problems that may occur when copper cables are stretched over longer distances.

The Difference Between DAC Cables vs AOC Cables

DAC cables are better suited for interconnecting devices within the same rack, such as between switches, servers, or storage. AOC, on the other hand, is often used to connect across racks, especially in data centers where devices in different areas need to communicate over long distances. However, in addition to different usage scenarios, there are actually quite a few differences between these two types of cables in other areas as well.

Power Consumption Differences

AOC is an active fiber optic cable, which requires electrical-optical-electrical signal conversion during operation and, therefore, consumes more energy during operation, with power consumption roughly between 1 and 2 watts. On the other hand, the power consumption of DAC is relatively low. Active DACs are generally controlled below 1 watt, and as for passive DACs, since they do not rely on electronics at all and rely only on copper wires to conduct signals, their power consumption can be as low as less than 0.15 watts. From the point of view of operation cost, if it is short-distance wiring, the use of DAC is undoubtedly more energy-saving and also saves electricity.

Difference in Transmission Distance

AOC is based on fiber optic technology, so it has a clear advantage in transmission distance and can support up to 100 meters. The DAC is, after all, a copper cable, and the signal runs within a limited distance. Generally speaking, a passive DAC transmission distance of about 7 meters and an active DAC can support up to 10 meters. In high-bandwidth application scenarios, this limitation may be more obvious, such as 100G DAC cable, the longest effective distance is only about 5 meters. Therefore, in the choice of cabling scheme, short distance with DAC, long distance, or rely on AOC.

Cost Comparison

From the manufacturing structure, DAC is much simpler than AOC, and the cost of copper cable itself is lower than that of fiber. In larger data center deployments, if connection distance is not an issue, using DACs in large numbers can significantly reduce overall cabling expenses. Therefore, in short-distance scenarios where cost is a priority, DAC is more cost-effective. However, if long-distance communication is involved, whether to choose AOC or not must be combined with the specific budget and demand for comprehensive judgment.

Difference In Anti-interference Ability

Electromagnetic interference (EMI) in the data center is a problem that can not be ignored. Fiber itself is dielectric and non-conductive, so the AOC cable in the anti-interference category has a natural advantage; it is basically not affected by external electromagnetic fields. The DAC uses copper wires, which are inherently susceptible to electromagnetic wave interference, especially in dense interference sources or cable-intensive cabinet environments; if the wiring and protection are not appropriate, it is easy to cause signal quality degradation and even system instability. Therefore, in environments with a high risk of interference, special attention should be paid to this point when selecting a model.

The Common Application of DAC/AOC

Due to the differences in structure, power consumption, and transmission distance, DACs and AOCs are often used in different types of connectivity scenarios, and each has its own suitable usage environment.

Typical application scenarios of DAC

Taking 10G SFP+ DACs as an example, their most common use is to connect servers to switches located in the same or neighboring racks. Specifically, this type of direct-attach cable is ideally suited for deployment in a ToR (top-of-rack) configuration for interconnecting servers to upper-level switches or as a link connection when stacking 10G switches.

Considering that it typically supports link lengths of up to approximately 7 meters and is very advantageous in terms of power consumption, latency, and cost, 10G DAC is an ideal choice for short-haul, high-density cabling, especially for dense server deployments.

DAC and AOC Application

AOC Advantages in Cable Length and Deployment Flexibility

In contrast, 10G SFP+ AOCs are not limited by the physical characteristics of copper cables and, therefore, have the advantage of being able to support distances up to 100 meters. This makes it a popular option for cross-rack cabling in data centers and is particularly suited for deployment in ToR (Top of Rack), MoR (Middle of Row), and EoR (End of Row) architectures.

In these layouts, where each server is typically connected to an Ethernet switch via one or two cables, AOC cables are well suited to meet these connectivity needs while being distance-independent and flexible in deployment. In addition, AOCs are also widely used in the core interconnection areas of data centers, such as spine and leaf switching architectures, for connecting equipment in the backbone, aggregation, or core layers, especially in scenarios with high bandwidth and distance requirements.

Conclusion

DAC and AOC cables play an irreplaceable role in both high-density server cabinets and data center core networks spanning multiple racks. The former is the preferred choice for short-distance interconnections due to its low latency, low power consumption, and cost-effective features, while the latter is more flexible and adaptable in complex network structures due to its longer transmission distance and interference immunity.

In actual deployment, the choice of which type of cable is used often depends on a variety of factors such as connection distance, power budget, interference environment, and cabling costs. There are no absolute advantages or disadvantages, only a more suitable match. Understanding the core characteristics and typical applications of these two types of cables will not only help optimize network architecture but also find a better balance between budget control and performance.