Nowadays, data centers to have convenient, stable connections for improved performance and uninterrupted operations. Direct-attached copper cables (DACs) and active optical cables (AOCs) are two of the main components that make this possible. These cables are used to improve data transmission between servers, switches, storage systems, and other devices within the data center. Each type of cable has its own advantages and disadvantages depending on the specific needs or limitations of a particular environment.

What are DAC cables in data centers?

How DAC Cables Work

Direct-connect cables are used to transmit data via differential signaling, where two wires transmit the same signal at opposite voltage levels. This minimizes electromagnetic interference and improves signal quality. Typically, passive DAC cables simply utilize the inherent properties of copper conductors to ensure efficient short-distance data transmission, usually limited to 5 meters. In contrast, active DACs have built-in circuitry that amplifies and conditions the signal, allowing them to support longer distances, sometimes up to 10 meters, while still maintaining high performance and minimal latency. Because they are plug-and-play devices, no additional power supply is required, nor do they require any complex setup to implement.

Types of DAC Cables Passive DAC Cables: Passive connections are cheap and simple; they do not have any signal conditioning circuitry. They are usually used for short-distance applications up to 5 meters. These wires are best suited for cost-effective installations as they consume less power and have a simpler design. Active DAC Cables: The integrated electronics in active cables enhance signal integrity while allowing for longer distances. Latency remains low as they can be extended over 10 meters or even further without compromising latency. Therefore, these types of cables should be used when high performance over longer distances is required. QSFP and SFP Form Factors: DAC cables come in different form factors, the most common of which are QSFP (Quad Small Form Factor Pluggable) and SFP (Small Form Factor Pluggable). For fast 40GbE and 100GbE links, QSFPDAC is used, while SFPDAC supports 1GbE to 10GbE connections. These differences allow people to choose freely depending on the port configuration of the network equipment as well as the performance requirements.

What is an Active Optical Cable?

 Active Optical Cable

An Active Optical Cable (AOC) is a type of cable that uses optical fiber instead of traditional copper wire for high-speed data transmission. Compared to direct-attached copper cables (DACs) that use electrical signals to send information between devices, AOCs use light to transmit data at higher bandwidths over longer distances. Developed specifically to prevent signal degradation over long distances, these cables provide low electromagnetic interference and crosstalk reduction. As a result, they are particularly useful in hyperscale environments or other situations where signal integrity must be maintained over considerable distances.

How AOC Cables Work

Active optical cables (AOCs) convert electrical signals into optical signals, so data can be sent faster and over longer distances than traditional copper wires. The main components of an AOC are the optical transceivers attached to each end of the cable and the fiber optic cable itself. Here's how an AOC cable works:

Transmitter module: This component has a laser diode that converts the incoming electrical signal into an optical signal. It uses the electrical input from the device to encode the optical pulses, which are then sent over the optical fiber.

Fiber optics: Typically made of plastic or glass, this is the main component of any active optical cable. The fiber core guides the transmission of light pulses over long distances between a transmitter and a receiver with virtually no loss of signal power. This is largely due to material properties such as high bandwidth capacity and low attenuation rates.

Receiver module: At one end, there is usually another module called a receiver; it contains, among other things, an optical detector (usually a photodiode) that captures the arriving light pulses and then converts them back into an electrical current or signal for processing elsewhere downstream if needed.

Signal integrity: One of the main characteristics of active optical cables is their ability to maintain signal integrity over long transmission distances. Optical transmission is inherently more resistant to electromagnetic interference (EMI) and crosstalk than copper-based systems, which provide a common ground point for multiple devices along their length. This ensures higher quality data transmission with lower bit error rates (BER).

Power consumption: While the embedded transceivers within each endpoint require some power, this can still result in lower overall power consumption than equivalent copper solutions designed for extended distances (such as within a data center), making active optical cables more energy efficient.

The difference is from the following aspects；

Ⅰ:Transmission distance

• AOC Advantages: Long transmission distance, usually up to 100 meters or more, and some high-end products can even reach several hundred meters. Applicable scenarios: Suitable for long-distance connections between different racks in the data center, or connections across floors and buildings.
• DAC Advantages: Relatively short transmission distance, usually between 3-10 meters. Applicable scenarios: Suitable for short-distance connections within the same rack or between adjacent racks.

Ⅱ. Transmission rate

• AOC Advantages: Supports high-speed transmission, common transmission rates include 10Gbps, 40Gbps, 100Gbps, etc.
• Applicable scenarios: Applicable to applications that require high-speed transmission, such as the backbone network of a data center.

• DAC Advantages: Also supports high-speed transmission, common transmission rates include 10Gbps, 40Gbps, 100Gbps, etc.
• Applicable scenarios: Applicable to applications that require high-speed transmission but have a short distance, such as the interconnection between servers

Ⅲ.Cost

• AOC Advantages: Although the initial cost is high, in the long run, due to the long transmission distance, the use of intermediate equipment (such as switches and routers) can be reduced, thereby reducing the overall cost.
• Disadvantages: The initial investment is high because it includes optical modules and optical fibers.
• DAC Advantages: The initial cost is low because only copper wires and simple connectors are required.
• Disadvantages: The transmission distance is limited and it is suitable for short-distance connections.

Ⅳ. Signal quality 

• AOC Advantages: Optical signal transmission is not affected by electromagnetic interference (EMI), with high signal quality and low transmission loss.
• Applicable scenarios: Suitable for applications that require high signal quality and low latency, such as high-performance computing and financial trading systems.
• DAC Advantages: Signal quality is good over short distances, but as the distance increases, signal quality will be affected by electromagnetic interference.
• Applicable scenarios: Suitable for applications in short-distance, low-interference environments.

Ⅴ. Power consumption

AOC

• Advantages: The power consumption is relatively high because the optical module is required to convert the electrical signal and the optical signal.

         Applicable scenarios: Suitable for applications that have a certain tolerance for power consumption.

DAC

• Advantages: Low power consumption because the electrical signal is transmitted directly without additional conversion process.Applicable scenarios: Suitable for applications that are sensitive to power consumption, such as large-scale data centers.

Ⅵ. Connector type

• AOC Common types: SFP+, QSFP+, QSFP28, CFP, etc.

        Applicable scenarios: Applicable to a variety of interface standards, high flexibility.

• DAC Common types: SFP+, QSFP+, QSFP28, etc.

        Applicable scenarios: Applicable to a variety of interface standards, highly flexible.

Ⅶ. Maintenance and reliability

• AOC Advantages: Fiber optic transmission has high reliability and stability, and is not easily affected by the environment.

Disadvantages: The maintenance cost is relatively high, and professional tools and techniques are required.

• DAC Advantages: Low maintenance cost, simple to use, easy to install and maintain.

Disadvantages: Reliability may be affected in long-distance transmission and high-interference environments

Summary

AOC: Suitable for applications with long distance, high-speed transmission, high signal quality, low latency and high reliability, although the initial cost is higher.

DAC: Suitable for applications with short distance, high-speed transmission, low power consumption and low cost, suitable for connections in the same rack or between adjacent racks.

Conclusion

Active optical cable assembly has become the core solution for high-bandwidth, high-density interconnection in data centers through its lightweight, high-speed, long-distance, strong anti-interference, and low-power consumption features. It is especially suitable for AI and cloud computing; DAC twinax cable remain competitive in short-distance and low-cost scenarios.