How to choose 400G DAC & 400G AOC?

NADDOD Jason Data Center Architect Aug 2, 2023

Due to the continuous growth in demand for data center and cloud computing resources, it has driven the development of ultra-large-scale public cloud data centers. Global data traffic continues to rise, prompting data centers to develop towards higher speeds, larger bandwidths, and lower latency, making 400G Ethernet the inevitable trend for data centers. In order to meet the interconnection needs within and between racks in data centers, enterprises, and high-performance computing environments, 400G DAC and AOC cables have become the main choices for short-distance interconnection due to their low power consumption, high stability, fast response, and easy installation.


The following are the main types of 400G DAC and 400G AOC classifications currently available on the market:

Package type:

QSFP-DD: the most mainstream packaging mode in 400G data centers, which is a common packaging type for 400G DAC and 400G AOC, using the 8*50Gb/s PAM4 electrical modulation format.

OSFP: another packaging type used for 400G DAC and 400G AOC, also using the 8*50Gb/s PAM4 electrical modulation format.

Cable type:

DAC: Direct Attach Cable, commonly known as DAC, refers to copper cables used for direct connection without the need for optoelectronic conversion to transmit electrical signals. It is the preferred option for short-distance applications and is widely used for data transmission between servers within or between adjacent racks in data centers. Its biggest advantage is its low cost, ultra-low power consumption, and high reliability.

AOC: Active Optical Cable, commonly known as AOC, refers to active optical cables. There are optical transmitters and receivers on the circuit board of the modules on both ends of the AOC cable, which convert electrical signals into optical signals at the transmitting end, transmit them through the fiber, and convert them back into electrical signals at the receiving end. Due to the use of optical signal transmission, it can transmit over longer distances than DAC. However, due to the complexity of design and processing technology and the need for optoelectronic conversion, its cost and power consumption are higher than that of DAC.

Direct connection and breakout:

Direct connection cable: both ends are fixed QSFP-DD packaged optical modules, connected by optical fibers or copper cables in between.

Breakout cable: one end of the cable uses a 400G connector, and the other end uses several identical low-speed connectors, with a total rate of 400G. It is suitable for scenarios that require distributing a high-speed optical signal to multiple devices.

Naddod 400G AOC DAC

The continuous development and application of these 400G DAC and 400G AOC cables and their branching cables provide more choices for data centers, enterprises, and high-performance computing environments to meet their interconnection needs in different scenarios.

Product Category Product Name Description MAX Distance Application


Two 400G QSFP-DD connectors are located on both ends, with each connector 8 x 50G PAM4 electrical channels.



Direct connection between 

400G devices

400G QSFP-DD DAC Breakout Cable QSFP-DD to 2*200G QSFP56 DAC


One end uses a 400G QSFP-DD connector, while the other end uses two 200G QSFP56 connectors, each with 4 x 50G PAM4 electrical channels.


3m 400G-200G branch link
400G QSFP-DD DAC Breakout Cable QSFP-DD to 8*50G SFP56 DAC

One end uses a 400G QSFP-DD connector, while the other end uses eight 50G SFP56 connectors, each with 1 x 50G PAM4 electrical channel.


3m 400G-50G branch link


Two 400G QSFP-DD connectors are located on both ends, with each connector 8 x 50G PAM4 electrical channels.




Direct connection between 

400G devices

400G QSFP-DD AOC Breakout Cable QSFP-DD to 2*200G QSFP56 AOC


One end uses a 400G QSFP-DD connector, while the other end uses two 200G QSFP56 connectors, each with 4 x 50G PAM4 electrical channels.




400G-200G branch link
400G QSFP-DD AOC Breakout Cable QSFP-DD to 4*100G QSFP56 AOC


One end uses a 400G QSFP-DD connector, while the other end uses four 100G QSFP56 connectors, each with 2 x 50G PAM4 electrical channels.




400G-100G branch link

Based on the table above, it can be seen that the most significant difference between 400G DAC and 400G AOC is the transmission distance.


400G DAC has a shorter transmission distance, supporting a maximum of only 3 meters, while 400G AOC can support longer distances, up to 70 meters (OM3) or 100 meters (OM4), because they use different transmission media. 400G DAC uses copper cables, while 400G AOC uses optical fibers.


In addition to the transmission distance, there are also other differences between 400G DAC and 400G AOC:

  • Maintenance cost: DAC lacks the electro-optical conversion unit required by AOC. The power consumption of passive DAC during operation is much lower than that of AOC, and the heat generated is smaller, resulting in better natural heat dissipation. Therefore, the cooling requirements are lower than those of AOC, making it more energy-efficient, environmentally friendly, and significantly reducing operating expenses in the long run.


  • Price cost: The manufacturing cost of optical fibers is higher, so the price of 400G AOC is usually higher than that of 400G DAC (assuming the same level or packaging). If the transmission distance of 400G DAC can meet your network requirements, choosing 400G DAC first can save costs to some extent.


  • Convenience and flexibility of application: Optical fibers have smaller volume and weight compared to copper cables. The volume of 400G AOC's optical fiber cable is about half that of 400G DAC, and the weight is only one-fourth of that of 400G DAC. Compared with the copper cable of 400G DAC, the optical fiber cable of 400G AOC has a smaller diameter and bending radius, making it more flexible and convenient to use between racks and easier to organize.


  • Stability and anti-interference ability: Since 400G AOC uses optical fibers as the core, which is an insulating material and does not conduct electricity, it is not easily affected by electromagnetic, lightning, or radio signals during data transmission. Additionally, optical cables are less affected by temperature fluctuations compared to copper cables. In contrast, the 400G DAC cable uses a copper core, which is conductive, making it more susceptible to electromagnetic interference such as lightning.

Selection of 400G AOC and DAC for Data Center Interconnects

The products used for interconnecting data centers within cabinets, between cabinets, and between centers and users are essentially three types: optical modules, direct attach copper cables (DAC), and active optical cables (AOC). Industry experts generally prefer to use AOCs for architectures. It is commonly believed that DACs are bulky, difficult to manage and wire, and may be susceptible to electromagnetic interference due to the characteristics of electrical signals. Does this mean that copper cables have no value?


Not necessarily. Copper cables also have their own advantages. With the development of cable technology and manufacturing, modern copper cables can support higher data transmission rates than traditional copper cable interfaces. Direct attach copper cables can also be interchanged and hot-swapped with optical modules and AOCs. Compared with optical cables, copper cables are more economical and passive copper cables have lower power consumption. They can bring great benefits to data center investment and operation, and also have smaller environmental impact.


Recently, a well-known North American internet company has purchased a large number of direct attach copper cables for the new construction and upgrading of data center equipment rooms, which indirectly indicates that first-class international data center users still favor and use these types of products. Direct attach copper cables within 3 meters still have widespread application value.

In terms of transmission scenarios, optical modules and optical fiber have the widest range of applications, from telecom convergence front propagation and medium propagation networks to data center switches. AOC is suitable for high-bandwidth interconnection of large distances up to 100 meters, while DAC is suitable for connecting servers and GPUs to top-of-rack switches.


In terms of cost, optical modules + optical fiber are more expensive (but more flexible in networking, and still the most mainstream solution for overseas cloud vendors from a total cost perspective). AOC is cost-effective due to its integrated design, while DAC has the lowest cost since it does not require optical-electrical conversion. However, its transmission distance is greatly limited at high speeds.


We believe that the "optical in, copper out" trend in telecom access networks will also occur in data center interconnect networks. Previously, due to the high cost of high-speed optical modules, AOC solutions were not significantly advantageous, but with the decrease in the cost of high-speed optical modules, the DAC solution is expected to accelerate upgrading to the AOC solution in the era of AI supercomputers.


When choosing 400G AOC and DAC, the following factors need to be considered:


  1. Transmission distance: If a longer transmission distance is required, such as more than 3 meters, 400G AOC may be a better choice.
  2. Flexibility and interchangeability: If frequent replacement or expansion of layouts is required, 400G AOC provides greater flexibility.
  3. Cost and simplicity: If the budget is limited or simple installation and management are required, 400G DAC may be more suitable.


In summary, the choice between 400G AOC and 400G DAC depends on specific needs and prioritized factors. In practical applications, the choice is usually made based on the layout, budget, and performance requirements of the data center. Choosing a reliable supplier is also crucial when purchasing high-speed cable products. NADDOD is a leading provider of optical network solutions, dedicated to building a smart world of interconnected things with innovative computing and networking solutions.


Since 2017, NADDOD has been continuously providing innovative, efficient, and reliable products, solutions, and services to users, offering the best switches, 400G, 200G, and 100G AOC/DAC/optical module products for application scenarios such as data centers, high-performance computing, edge computing, and artificial intelligence, greatly improving customers' business acceleration capabilities with low cost and outstanding performance.


NADDOD is customer-centric and continuously creates excellent value for customers in various industries. With a professional technical team, rich application scenario implementation and service experience, the products and solutions have won customers' trust and favor with high quality and outstanding performance, and are widely used in industries and key areas such as high-performance computing, data centers, education and research, biomedicine, finance, energy, autonomous driving, the internet, manufacturing, and operators.


If you are looking for high-quality 400G AOC/DAC products, NADDOD will provide you with the best network upgrade solution with rich experience and high-quality products, escorting your business.

Q&A about 400G DAC/AOC

Q: Why do 400G DAC/AOC need to use PAM4 modulation?

A: Because compared to NRZ modulation, PAM4 is a more efficient modulation technology that can effectively improve bandwidth utilization. At the same baud rate, the throughput of PAM4 is twice that of NRZ. Therefore, PAM4 can increase network bandwidth without adding additional fibers, effectively improving bandwidth utilization.


Q: Can I plug a 400G QSFP-DD cable into an OSFP port?

A: No, QSFP-DD and OSFP have different form factors and sizes, and are not compatible with each other.


Q: Can I use 400G AOC instead of 400G DAC?

A: Yes, 400G AOC has a longer transmission distance than DAC, making it the better choice for long-distance transmission.


Q: Can I customize DAC or AOC cables with non-standard lengths?

A: The length of 400G cables can be customized according to customer requirements.