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400G Transceiver, DAC, or AOC: How to Choose?

Updated on Jun 23, 2022 by
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For 400G data center connectivity, several options include 400G transceivers, 400G DAC Cables (Direct Attach Copper Cables), and 400G AOC Cables (Active Optical Cables). This article will explore the differences and benefits of these options and provide insights into selecting the ideal 400G data center networking solution.

400G Transceiver

400G transceivers are high-speed optical modules based on optical fiber transmission technology that can achieve data transmission speeds of up to 400Gbps. The high bandwidth, extended transmission range, and low latency of 400G transceivers position them as primary high-speed connectivity solutions within data centers and enterprise networks.

According to different 400G transceiver form factors, there are CFP8, QSFP-DD, OSFP, COBO, etc., of which the most common type is 400G QSFP-DD. These transceivers are different from each other in terms of transmission distance, connector, media, and so on.

Usually, the 400G SR8 modules use an MPO-16 connector to connect to 8 fiber pairs, realizing 400G transmission. The 400G DR4/XDR4/PLR4 modules use an MPO-12 connector to connect to 4 fiber pairs. Unlike the 400G SR8 and 400G DR4, the 400G FR4 optical modules use a duplex LC optical connector. 

400G AOC Connectivity

400G DAC Cable

400G Direct Attached Cables (DAC Cables) are suitable for very short-distance data center network connectivity. Besides, they use copper cable as the transmission media, resulting in lower cost and latency. Typically, there are two types of DAC cables: passive copper cables for distances from 0 to 5m and active copper cables for distances from 5 to 15m.

400G Direct Attached Cable

400G AOC Cable

Different from 400G DAC cables, 400G Active Optical Cables (AOC) use fiber optical fiber as the transmission media, which is equivalent to using transceivers and separate cables and is not affected by Electromagnetic Interference (EMI). In terms of transmission distance, 400G AOC cables support longer distance transmission than DAC cables, up to 100m, and are usually used for connections between different cabinets in data centers. AOC cables are also lighter and smaller than DAC cables, but more costly. For more detailed information about 400G DAC and AOC cables, please visit: Basics about 400G DAC and 400G AOC.

400G Active Optical Cable

400G Transceiver/DAC/AOC Application Scenarios

Below are several common data center application scenarios, you can also click here FS| 400G Data Center Solutions to learn more.

Connectivity from ToR Switch to Server (Up to 2.5m)

400G DAC cables are the optimal solution for linked switches and servers inside racks. As shown in the figure below, 400G QSFP-DD DAC cables are used to connect to 400G ToR switch and server in a 42U server cabinet. The direct-connect design of 400G DAC cables not only makes deployment easier, but also saves space inside the data center. Due to the low cost of 400G DAC cables and height limitation of 42U server cabinet, the 400G DAC cables become the ideal solution for short-distance transmission inside the cabinet.

400G DAC Connectivity

Connectivity from EoR Switch to Server (Up to 30m)

The 400G AOC cables usually connect switches and servers between racks in a data center. As the picture below shows, the 400G EoR switch is connected to the server rack and aggregation rack through 400G QSFP-DD AOC cables. Since the 400G AOC cables use optical fiber as the transmission media, they can achieve longer distance transmission and higher bandwidth than the 400G DAC cables, and they can be used to connect separate switches to create a larger switch architecture. However, 400G AOC cables are usually more expensive.

400G AOC Connectivity

Connectivity from ToR Switch to Server (Up to 100m)

400GBASE-SR8 QSFP-DD transceivers can be used for leaf-spine switches interconnection and the transmission distance can reach 100m over OM4 MMF with MTP/MPO-16 connector.

Connectivity from ToR Switch to Server (Up to 150m)

Different from 400GBASE-SR8 QSFP-DD transceivers, the 400GBASE-SR4.2 QSFP-DD transceivers support 150m distance transmission over MPO/MTP-12 OM5 MMF.

Connectivity from ToR Switch to Server (Up to 500m)

400GBASE-DR4 QSFP-DD transceivers support up to 500m transmission distance over OS2 SMF with MPO/MTP-12 connector.

Data Center Interconnection (Up to 2-120km)Data Center Interconnection

  • Up to 2km: Usually, the data center interconnection needs transceivers that support longer distance and higher bandwidth transmission. 400G QSFP-DD FR4 transceivers support link lengths of up to 2km over OS2 SMF with duplex LC connector.

  • Up to 10km: For data center interconnection over 2km, 400G QSFP-DD LR8 transceivers are the better choice. They can support data transmission distances of up to 10km over OS2 SMF.

  • Up to 40km: 400G QSFP-DD ER8 transceivers enable link lengths of up to 40km over OS2 SMF with duplex LC connectors. Besides, they also feature low-power, high-density, and high-speed, which is a highly efficient option for 400G data center interconnection. However, they use EML laser, APD detector, and also Mux combiner and Demux splitter, which leads to high cost.

  • Up to 120km: As for hyperscale data center interconnection, 400G QSFP-DD-ZR transceivers can meet the demands, supporting OS2 SMF with transmission distances up to 120km.

Conclusion

Selecting a 400G data center network connectivity solution requires considering your specific needs and budget. 400G AOC and DAC cables are more suitable for short-distance transmission. However, the 400G AOC cables support longer data transmission distances, while the 400G DAC cables are more cost-efficient. As for 400G transceivers, the application scenarios are more abundant. Choose the most suitable solution based on your needs to ensure efficient, stable, and reliable data center connectivity.

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