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What is the difference between QSFP28 and QSFP56 DD?
Knowledge Base + 2024.01.09

What is the SFP28 Transceiver?

SFP28 stands for Small Form-Factor Pluggable 28. It is a third-generation SFP connection system built for 25G throughput per IEEE 802.3 by specification (25GBASE-CR). SFP28 is an improved version of SFP+, designed for 25G signal transmission. SFP28 has the same type of form factor as SFP+, but it provides an electrical connection of 25Gbps per lane. Compared with SFP+, SFP28 provides higher bandwidth capabilities.

According to the application, SFP28 transceivers can be divided into six categories: dual fiber SFP28, Bidi SFP28, CWDM SFP28, DWDM SFP28, SFP28 DAC, and SFP28 AOC.


The QSFP56 module has four transmit and receive channels, each capable of 53.125 Gb/s operation which together yield an aggregate data rate of 200Gb/s. It utilizes 850nm, 1310nm, CWDM or LWDM wavelengths to transmit data. QSFP56 uses optical MPO or LC connectors for the optical interface and a 38-pin connector for the electrical interface. Unlike previous QSFP form factors, it leverages PAM4 digital modulation scheme.

The QSFP56 was engineered to support 200G Ethernet.


QSFP-DD, QSFP+, QSFP28 and QSFP56 belong to the QSFP form factor, but what are the differences among them? The differences are explained in the following descriptions.


In terms of the appearance, the width, length and thickness of the QSFP-DD are the same as QSFP+, QSFP28 and QSFP56. But the QSFP-DD module is equipped with an 8-lane electrical interface rather than a 4-lane like other QSFP modules and the ASIC ports of QSFP-DD are doubled to support existing interfaces such as CAUI-4. Therefore, the mechanical interface of QSFP-DD on the host board is slightly deeper than that of the other QSFP system transceivers to accommodate the extra row of contacts.

Bandwidth & Application

The QSFP-DD modules can support 400Gbps while QSFP+/QSFP28/QSFP56 can only reach 40Gbps/100Gbps/200Gbps respectively. Therefore, QSFP-DD connectors are used in 400G transceivers, DACs and AOCs, and applied for the 400G data center interconnections. And QSFP+/QSFP28/QSFP56 modules and DAC/AOC are used for 40G/100G/200G networks. interconnection.

Backward Compatibility

QSFP28 (Quad Small Form-Factor Pluggable 28) and QSFP56-DD (Quad Small Form-Factor Pluggable Double Density) are both types of optical transceiver form factors used for high-speed data transmission in data centers and telecommunications networks. The primary differences between QSFP28 and QSFP56-DD lie in their data rates and the number of electrical lanes they support:

Released Year2016201820162010
Number of Electrical Interface Lanes8444
Single Channel Rate25Gbps/50Gbps50Gbps25Gbps10Gbps
Modulation TechnologyNRZ/PAM4PAM4NRZNRZ
Backward CompatibilityQSFP+/QSFP28/QSFP56QSFP+/QSFP28QSFP+/

In summary, QSFP56-DD offers higher data rates and increased bandwidth compared to QSFP28, making it suitable for applications requiring even greater network speeds. The increased number of electrical lanes in QSFP56-DD allows for a scalable solution to meet the evolving demands of high-performance data centers and networks.