How much do you know about 400G DAC and 400G AOC?

Data centers, enterprises, and high-performance computing environments require flexible and well-defined 50G, 100G, 200G, and 400G cables for interconnections within and between racks. With the development of 400G technology, 400G direct-connect cables suitable for short-distance data center interconnection have been mass produced and marketed, including 400G DAC and 400G AOC.

Currently on the market, 400G DAC and 400G AOC mainly have two packaging types: QSFP-DD and OSFP, both of which adopt 8×50Gb/s PAM4 electrical modulation format. In addition to the 400G direct-connect cable, there are 400G DAC high-speed branch cables and 400G AOC active branch cables, with a 400G connector at one end and several connectors of the same low rate at the other end (for a total rate of 400G).

The biggest difference between 400G DAC and 400G AOC is in the difference of transmission distance. The transmission distance of 400G DAC can only support 3m, while the transmission distance of 400G AOC can support 70m (OM3) or 100m (OM4). This is because they use different transmission media. 400G DAC USES copper cable as the transmission medium, while 400G AOC USES optical fiber as the transmission medium. In addition, 400G DAC and 400G AOC have the following differences:

• Available types -- According to the current market situation, there are various types of 400G DAC, while 400G AOC active branch cable and QSFP-DD conversion 4*100G QSFP AOC has not entered the market for the time being. Therefore, 400G DAC can meet more connection demands relatively.

• Volume and weight -- Optical fiber is smaller and lighter than copper core, so 400G AOC is smaller and lighter than 400G DAC. A 400G AOC is half the size of a 400G DAC, and weighs only a quarter as much. In addition, the bending radius of 400G AOC is less than 400G DAC.

• Anti-interference capability -- as the core of 400G AOC is an optical fiber, which is an insulating material and non-conductive, it is not vulnerable to electromagnetic, lightning or radio signals during data transmission; But the 400G DAC cable is copper core, will conduct electricity, and therefore is vulnerable to lightning and other interference.

• Price -- Since the price of optical fiber is generally higher than that of copper cable, the price of 400G AOC is usually higher than that of 400G DAC (prerequisite: same level or package). If the transmission distance of 400G DAC can meet your network needs, it is suggested that you can choose 400G DAC first, which can effectively save costs.

Look to the future

Both 400G DAC and 400G AOC are the most cost-effective solutions for interconnecting within or between racks, but for transmission distances greater than 100 meters, it is best to use a 400G optical module with a matching fiber (such as MTP-16 fiber jumper). With the commercialization and scale of 400G Ethernet, 400G QSFP-DD optical module and 400G OSFP optical module have successively entered the market and started mass production.

Question about 400G DAC/AOC

Q: Why do 400G DAC/AOC adopt PAM4 modulation method?

A: Compared with NRZ modulation, PAM4 is a more effective modulation technique, which can effectively improve the bandwidth utilization. At the same baud rate, PAM4 has twice the spit and absorb capacity of NRZ. In other words, PAM4 can increase the network bandwidth without adding additional optical fibers, thus effectively improving the bandwidth utilization.

Q: What are the key technologies of 400G DAC/AOC?

A: The key technologies of 400G DAC/AOC are PAM4 and DSP. Since PAM4 is more sensitive to noise than NRZ, especially in 400G AOC, DSP is introduced to compensate for this shortcoming of PAM4. As a high-speed digital processing chip, DSP can not only provide digital clock recovery function (PROVIDED by CDR), but also perform dispersion compensation operation to remove noise, nonlinear and other interference factors.