400G QSFP-DD FR4 VS FR8: What Is the Difference?

Posted on Jul 11, 2023

The 400G QSFP-DD transceiver has become one of the most popular form factors in the next-generation network with its advantages of low power consumption, small size, and high speed. There are two types of modules, available for the 2km WDM solution, 400G QSFP-DD FR4 and 400G QSFP-DD FR8. How do they differ from each other? And how to choose? Please find the answers in this article.

What Is 400G QSFP-DD FR4?

The 400G QSFP-DD FR4 optical module is a 400Gbps hot-pluggable transceiver in a QSFP-DD form factor and supports SMF links up to 2km reach over duplex LC connectors. It incorporates 4 independent lanes on CWDM4 center wavelengths of 1271/1291/1311/1331nm running at 100Gbps per lane and compliant with 100G Single Lambda MSA specification.

What Is 400G QSFP-DD FR8?

The 400G QSFP-DD FR8 module adopts the same 50G PAM4 modulation technology as the 400G QSFP-DD SR8. It uses eight channels to transmit data and every channel can reach a transmission rate of 53.125Gbps, achieving a total transmission rate of 400Gbps. However, unlike SR8, which only fits in short-distance transmission, FR8 can be applied in long-distance transmission, with a maximum transmission range of 2km.

400G QSFP-DD FR4 VS FR8 Optical Modules: 4 Key Differences

Difference QSFP-DD FR4 QSFP-DD FR8
Transmission Type SMF SMF
Reach 2km 2km
Optical Connector LC CS
Wavelength CWDM4 Wavelength CWDM4 Wavelength
Optical Modulation 100G PAM4 50G PAM4
Protocol 100Glambda MSA IEEE 802.3bs


The baud rates for the 400G QSFP-DD FR4 and FR8 are 53.125GBd and 26.5625GBd, respectively. As the baud rate increases, the MPI cost and CD cost of PAM4 gradually rise. That is to say, the excessive cost will make it hard to maintain long-term reliable and stable operation, thus requiring a more robust FEC. Both optical modules use KP4 FEC; however, 400G QSFP-DD FR4 has more than 80% channel margin and 400G QSFP-DD FR8 has more than 90% channel margin, making the latter QSFP-DD FR8 module more reliable than the former.


On the electrical side, there is no difference between the 400G QSFP-DD FR4 and 400G QSFP-DD FR8, which are both 8-channel 53Gbps PAM4 modulation. However, on the optical side, the FR8 module uses 8-channel 53Gbps PAM4 modulation, and the 400G FR4 module uses four channels of 106Gbps PAM4 modulation.



The 400G QSFP-DD FR4 uses an 8:4 Gearbox internally, resulting in a poor latency performance of about 102ns, while the 400G QSFP-DD FR8 has only about 84ns. As a result, 400G QSFP-DD FR8 performs with lower latency. Therefore, the 400G QSFP-DD FR8 is more suitable for scenarios requiring high-performance computing.

Power Consumption

On the one hand, the 400G QSFP-DD FR8 offers improved link budgets. But on the other hand, the total laser cost per module is higher due to the adoption of more lasers, resulting in lower output and higher production costs. In contrast, the 400G QSFP-DD FR4 has lower power consumption and simpler heat treatment capabilities.

How to Choose 400G QSFP-DD FR4 and 400G QSFP-DD FR8?

Both 400G QSFP-DD FR4 and FR8 optical modules are desirable transceivers for data centers and cloud computing networks, as both of them are equipped with high performance, great bandwidth, high speed and reliability. The only difference lies in their power consumption and costs. 400G QSFP-DD FR4 transceiver is more suitable for business data centers, while QSFP-DD FR8 transceiver is more appropriate for high performance computing.

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