What do You Know about 400G DAC and AOC?

Data centers, enterprises and high-performance computing environments require resilient and defined 50G, 100G, 200G and 400G cables for interconnecting within and between racks. With the development of 400G technology, 400G direct connect cables for short-haul data center interconnects are now in mass production and on the market, including 400G DAC and 400G AOC.

Currently, 400G DAC and 400G AOC are available in two main package types: QSFP-DD and OSFP, both of which use the 8*50Gb/s PAM4 electrical modulation format. In addition to the 400G direct cable, there are also 400G Direct Attach Cable high-speed breakout cables and 400G AOC active breakout cables, which use one 400G connector at one end of the breakout cable and several connectors of the same lower rate (total rate of 400G) at the other end. 400G DAC/AOC types are listed below:

Product Category	Product name	Description	Maximum transmission distance	Application
400G QSFP-DD DAC Direct Connect Cable	QSFP-DD to QSFP-DD DAC	One 400G QSFP-DD connector on each end, with eight 50G PAM4 electrical channels per connector	3 meters	Direct connection between 400G devices
400G QSFP-DD DAC high-speed breakout cable	QSFP-DD to 2*200G QSFP56 DAC	400G QSFP-DD connector on one end, 2 200G QSFP56 connectors on the other end, each 200G QSFP56 connector has 4 50G PAM4 electrical channels	3 meters	400G-200G Breakout Channels
400G QSFP-DD DAC high-speed breakout cable	QSFP-DD to 4*100G QSFPs DAC	400G QSFP-DD connector on one end, 4 100G QSFP connectors on the other end, each 100G QSFP connector has 2 50G PAM4 electrical channels	3 meters	400G-100G Breakout Channels
400G QSFP-DD DAC high-speed breakout cable	QSFP-DD to 8*50G SFP56 DAC	400G QSFP-DD connector on one end, 8 50G SFP56 connectors on the other end, each 50G SFP56 connector has 1 50G PAM4 electrical channel	3 meters	400G-50G Breakout Channels
400G QSFP-DD AOC Direct Connect Cable	QSFP-DD to QSFP-DD AOC	One 400G QSFP-DD connector on each end, with eight 50G PAM4 electrical channels per connector	70m (OM3); 100m (OM4)	Direct connection between 400G devices
400G QSFP-DD AOC active breakout optical cable	QSFP-DD to 2*200G QSFP56 AOC	One end uses 400G QSFP-DD connectors, the other end uses 2 x 200G QSFP56 connectors, each 200G QSFP56 connector has 4 x 50G PAM4 electrical channels	70m (OM3); 100m (OM4)	400G-200G Breakout Channels
400G QSFP-DD AOC Active Breakout Fiber Optic Cable	QSFP-DD to 8*50G SFP56 AOC	400G QSFP-DD connector on one end, 8 50G SFP56 connectors on the other end, each 50G SFP56 connector has 1 50G PAM4 electrical channel	70m (OM3); 100m (OM4)	400G-50G Breakout Channels
400G OSFP DAC Direct Connect Cable	OSFP to OSFP DAC	One 400G OSFP connector on each end with eight 50G PAM4 electrical channels per connector	3 meters	Direct connection between 400G devices
400G OSFP DAC high-speed breakout cable	OSFP to 2*200G QSFP56 DAC	One end uses a 400G OSFP connector and the other end uses two 200G QSFP56 connectors, each 200G QSFP56 connector has four 50G PAM4 electrical channels	3 meters	400G-200G Breakout Channels
400G OSFP DAC high-speed breakout cable	OSFP to 4*100G QSFPs DAC	400G OSFP connector on one end, 4 100G QSFPs connectors on the other end, each 100G QSFP connector has 2 50G PAM4 electrical channels	3 meters	400G-100G Breakout Channels
400G OSFP DAC high-speed breakout cable	OSFP to 8*50G SFP56 DAC	400G OSFP connector on one end, 8 50G SFP56 connectors on the other end, each 50G SFP56 connector has 1 50G PAM4 electrical channel	3 meters	400G-50G Breakout Channels
400G OSFP AOC Direct Connect Cable	OSFP to OSFP AOC	One 400G OSFP connector on each end with 8 50G PAM4 electrical channels per connector	70m (0OM3); 100m (OM4)	Direct connection between 400G devices

As can be seen from the table above, the biggest difference between 400G DAC and 400G AOC is the transmission distance, 400G DAC can only support up to 3 meters, while 400G Active Optical Cable can support up to 70m (OM3) or 100m (OM4). This is because the transmission medium used by the two is different, 400G Direct Attach Cable is using copper cable as the transmission medium, while 400G AOC is using optical fiber as the transmission medium. In addition to this, the following differences exist between 400G DAC and 400G AOC:

Available types – As far as the current market situation is concerned, there is a wide variety of 400G Direct Attach Cable, while 400G Active Optical Cable active branch optical cables and QSFP-DD to 4*100G QSFP AOC are not in the market for the time being. Therefore, relatively speaking 400G DAC cable can meet more connection needs.

Volume and weight – Because optical fiber is smaller and lighter than copper cores, the volume and weight of 400G Active Optical Cable are small compared to 400G Direct Attach Cable. 400G AOC is half the size of 400G Direct Attach Cable and only a quarter of the weight of 400G Direct Attach Cable. In addition, the bend radius of the 400G Active Optical Cable is smaller than that of the 400G QSFP-DD DAC.

Anti-interference capability – Since the core of 400G AOC is fiber optic, it is an insulating material and does not conduct electricity. Therefore, it is not susceptible to interference from electromagnetic, lightning, or radio signals during data transmission. While the 400G DAC’s cable is a copper core, which conducts electricity and is susceptible to interference such as lightning.

Price – Since fiber is generally more expensive than copper, the price of a 400G Active Optical Cable will usually be higher than the price of a 400G DAC (prerequisite: same level or package). If the transmission distance of 400G DAC can meet your network needs, you can give preference to 400G Direct Attach Cable, which can effectively save costs.

Looking to the Future

Both 400G DAC and 400G AOC are the most cost-effective solutions for interconnecting within a rack or between adjacent racks, but when the transmission distance is greater than 100 meters, using 400G optical modules and matching optical fibers (such as MTP-16 fiber patch cables) with them is the best solution. With the commercialization and scaling of 400G Ethernet, 400G QSFP-DD optical modules and 400G OSFP optical modules have been entering the market and have started mass production.

Q&A About 400G DAC and AOC

Q: Why does 400G DAC/AOC use PAM4 modulation?

A: Because PAM4 is a more effective modulation technology compared to the NRZ modulation method, which can effectively improve bandwidth utilization. At the same baud rate, PAM4 spits twice as much as NRZ, which means that PAM4 can increase network bandwidth without adding additional fiber, effectively improving bandwidth utilization.

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

A: The key technologies for 400G DAC/AOC are PAM4 and DSP, as PAM4 is more sensitive to noise than NRZ, especially in 400G Active Optical Cable. Therefore, DSP is introduced to make up for this shortcoming of PAM4. DSP, as a high-speed digital processing chip, can not only provide a digital clock recovery function (provided by CDR), but also dispersion compensation operation to remove noise, non-linearity, and other interference factors.