Views: 399 Author: Anna Publish Time: 2026-05-19 Origin: Site
QSFP-DD (also known as QSFP56-DD) is a dual-density quad-channel small form factor pluggable package, a new type of high-speed pluggable module package that complies with the IEEE 802.3bs and QSFP-DD MSA standards. This package has 8 electrical interfaces, achieving a data rate of up to 25Gb/s per channel through NRZ modulation technology, enabling 200G network transmission; and up to 50Gb/s per channel through PAM4 modulation technology, enabling 400G network transmission. It is suitable for high-performance computing data centers and cloud networks.
It is an extension of QSFP (a widely adopted quad-channel electrical interface package, including QSFP+ and QSFP28).
It uses a 2×1 stacked integrated cage and connector. Due to industry demand, most pluggable packages eventually develop a dual-height stacked cage connector system, in addition to single-height cage connector systems. Typically, single-height cage connector systems are included in the initial MSA specification, while double-height cage connector systems are left to independent vendors. To better serve the industry, the QSFP-DD MSA group chose to develop both single-height and double-height cage connector systems simultaneously.
QSFP-DD utilizes SMT (Surface-Mount Technology) connectors and 1xN cages. Cage design optimization and module housing optimization enable at least 12 watts of thermal capacity per module. The QSFP-DD specification defines thermal capacity levels up to 14 watts and levels exceeding 14 watts. Thanks to innovative thermal management technologies in the module and cage design, QSFP-DD modules support at least 12 watts of thermal capacity in typical system designs, benefiting from the extensive knowledge and experience in QSFP series package system design. Higher thermal capacity reduces the module's thermal requirements, thus avoiding unnecessary costs.
It employs an 8-channel electrical interface. The QSFP-DD electrical interface employs eight channels, each with a rate up to 25Gb/s (NRZ modulation) or 50Gb/s (PAM4 modulation), providing aggregation solutions up to 200Gb/s or 400Gb/s. QSFP-DD can achieve up to 14.4Tb/s of aggregated bandwidth in a single switch slot. By quadrupling the aggregation switch bandwidth without increasing port density, QSFP-DD can support the continued growth in network bandwidth demands and data center traffic.
Prior to QSFP-DD, the most popular interfaces were generally single-channel (SFP and SFP+) or four-channel (QSFP+ and QSFP28). To accommodate anticipated demands for data bandwidth or channel capacity, industry organizations defined eight-channel interfaces. However, the available packages supporting eight-channel interfaces at the time did not possess the desired characteristics or density required for next-generation systems supporting higher-speed interfaces. Therefore, the QSFP-DD MSA group extended and defined QSFP-DD based on QSFP (QSFP+ and QSFP28).
QSFP has four electrical channels, each with a rate of 10Gb/s (QSFP+) or 25Gb/s (QSFP28), providing 40Gb/s or 100Gb/s solutions through aggregation. QSFP-DD, with its pluggable package, uses eight electrical channels, each with rates up to 25Gb/s (NRZ modulation) or 50Gb/s (PAM4 modulation), providing up to 200Gb/s or 400Gb/s solutions through aggregation.
Systems designed using QSFP-DD modules are backward compatible, supporting existing QSFP+/QSFP28 modules and providing flexibility for end users and system designers. Backward compatibility is crucial for the industry—systems need to fully leverage this because ASIC designs are intended to support multiple interface rates. End users can utilize newer ASICs and system products, reducing port costs and enabling the insertion of various currently available QSFP+/QSFP28 modules to support their required media and transmission distances without requiring separate system products. This significantly reduces the risk of deploying new devices, while system designers can leverage known technologies and designs to build universal products that support multiple pluggable variants. Module designers do not need to migrate their low-rate designs to new, non-backward compatible packages, thus reducing overall costs. Backward compatibility is extremely important due to the significant cost savings it provides during large-scale device deployment.
However, because each QSFP-DD port can accommodate 8 channels instead of 4, QSFP-DD doubles the number of ASIC ports it supports for existing interfaces such as CAUI-4.
The mechanical interface of QSFP-DD on the motherboard is slightly deeper than that of QSFP+/QSFP28 to accommodate an additional row of contacts. QSFP-DD has the same height and width as QSFP, allowing system designers to achieve the same system port count density for designs based on QSFP+/QSFP28 or QSFP-DD. Any current QSFP+ or QSFP28 module can be plugged into a QSFP-DD port.
Q: What is the difference between QSFP-DD SR4 and SR8?
A: SR4 uses four fibers to achieve 400G, suitable for short distances, while SR8 uses eight fibers, providing greater flexibility for high-density applications.
Q: How does PAM4 modulation improve 400G performance?
A: PAM4 encodes two bits per symbol, doubling the data rate compared to NRZ, thus enabling 400G on fewer channels and improving signal efficiency.
Q: What are typical use cases for 400G transceivers in HPC?
A: Typical use cases include artificial intelligence training, machine learning inference, data center interconnects, and large-scale scientific simulations.
Q: Is the 400G QSFP-DD module backward compatible with older QSFP systems?
A: Yes, QSFP-DD is backward compatible with QSFP28 and QSFP56, allowing integration with existing infrastructure.
Q: What are the main challenges of adopting 400G technology?
A: Challenges include upgrading cabling, ensuring compatibility, and managing costs while transitioning to higher-speed networks.
