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

Updated on Jun 23, 2022

For interconnection of 400G data center, there are several options including 400G transceiver, 400G DAC Cable (Direct Attach Copper), and 400G AOC Cable (Active Optical Cable). Among these options, each of them has its own merits. Today, this article would tell their differences and teach you how to choose.

400G Transceiver

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 module uses 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 SR8 and DR4, the FR4 optical modules use a duplex LC optical connector. And the FR8 modules (also called 2FR4 modules) use a dual CS connector to connect to 2 fiber pairs.

400G AOC Connectivity

400G DAC Cable

400G Direct Attached Cable (DAC) is suitable for very short-distance data center interconnection and it is cost-efficient. Besides, it uses copper cable as the transmission media. Typically, there are two types of DACs: 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, 400G Active Optical Cable (AOC) uses fiber optical fiber as the transmission media. It is equivalent to using transceivers and separate cables. Besides, 400G AOCs support longer distance transmission than DACs, which can be up to 100m. They are also lighter and smaller than DACs, but more costly. Unlike DACs, AOCs are not affected by Electromagnetic Interference (EMI). 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 is the perfect solution for linked switches and servers inside racks. As shown in the figure below, 400G QSFP-DD DAC is used to connect to 400G ToR switch and server in a 42U server cabinet. Due to the low cost of 400G DAC and height limitation of 42U server cabinet, the 400G DAC becomes an ideal solution for short-distance transmission inside the cabinet.

400G DAC Connectivity

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

The 400G AOC usually connects switches and servers between racks in a data center. As the picture blelow shows, the 400G EoR switch is connected to the server rack and aggregation rack through 400G QSFP-DD AOC . Since the 400G AOC uses optical fiber as the transmission media, it can achieve longer distance transmission and higher bandwidth than the 400G DAC. Besides, it is also used to connect separate switches to create a larger switch architecture.

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.

400G AOC Connectivity

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.

400G AOC Connectivity

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.

400G AOC Connectivity

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 transceiver is a better choice. It can support data transmission distances of up to 10km over OS2 SMF.

  • Up to 40km: 400G QSFP-DD ER8 enables link lengths of up to 40km over OS2 SMF with duplex LC connectors. Besides, it also features low-power, high-density and high-speed, which is a high-efficient option for 400G data center interconnection. However, it uses 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, only 400G-ZR CSFP Coherent optical modules can meet the demands, supporting OS2 SMF with transmission distances up to 120km.



For short, 400G transceivers and 400G DAC/AOC cables are all effective choices for the 400G data center connectivity. 400G AOC and DAC are more suitable for short-distance transmission. However, the 400G AOC supports higher data transfer speed, while the 400G DAC is more cost-efficient. As for 400G transceivers, the application scenarios are more abundant. Data center operators should make appropriate choices based on their actual needs.

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