The QSFP-DD is the smallest 400G form factor optical module on the market today. It is also the optical module that offers the highest transmission bandwidth density in 400G applications, with backward compatibility to previous generations of QSFP form factor modules, making it widely popular in many data center and enterprise applications. This article will provide a detailed comparison of the current mainstream 400G optical modules, including QSFP-DD, QSFP56, OSFP, CFP8, COBO, and other modules. By reading this post, you will know the difference between these 400G optical modules.
What is a 400G QSFP-DD Optical Module?
QSFP-DD stands for Quad Small Form Factor Pluggable Double Density. The IEEE802.bs protocol and the MSA standard define QSFP-DD specification. The ''double density'' in this standard means that the number of high-speed electrical interfaces of this module is twice that of the standard QSFP28 module. The QSFP-DD optical module can increase the data rate of each transmission channel up to 25Gb/s through NRZ modulation technology, reaching a maximum transmission rate of 200Gbps. Using PAM4 modulation technology, the data rate of each transmission channel can be increased to 50Gb/s, reaching a maximum transmission rate of 400Gbps. 400G QSFP-DD optical transceivers are generally used for 400G Ethernet applications in high-performance computing data centers and cloud networks.
The following section will list the benefits of the QSFP-DD form factor:
- Backward compatibility: This feature allows QSFP-DD to fully backward compatibility with existing QSFP modules including QSFP+, QSFP28, QSFP56, etc. This gives engineers or terminal users greater flexibility when building new networks or performing network upgrades.
- Special connector design: The QSFP-DD features a 2x1 stacked integrated cage/connector, supporting both one-high cage connector and two-high stacked cage connector systems.
- SMT connectors and a 1xN cage design: This design allows the module to be self-contained with at least 12W of the thermal support. It enables this module to have higher thermal performance and reduces unnecessary costs for thermal design during operation.
- ASIC design: It can support a variety of port rates and is fully backward compatible with previous QSFP form factors including QSFP+ and QSFP28 modules, which reduces the cost of port and device deployment.
QSFP-DD vs QSFP+/QSFP28/QSFP56
The following sections will describe the differences between various QSFP form factors, including QSFP-DD, QSFP+, QSFP28, and QSFP56.
Structure
As different QSFP form factors, QSFP-DD, QSFP+, QSFP28, and QSFP56 have the same width, length, and thickness. This means they are identical in appearance. In terms of internal transmission channel design, QSFP+, QSFP28, and QSFP56 are 4-channel electrical interfaces, while 400G QSFP-DD optical modules are 8-lane electrical interface, and it also provides doubles the number of ASIC ports to support existing interfaces such as CAUI-4. Due to the need to accommodate an extra row of contacts, the mechanical interface of the QSFP-DD on the host board is slightly deeper than other QSFP system transceivers.
Bandwidth & Application
The QSFP-DD module provides maximum bandwidth 400Gbps, whereas the QSFP+/QSFP28/QSFP56 can only achieve 40Gbps/100Gbps/200Gbps respectively so the QSFP-DD connectors are applied to 400G optical modules, DACs and AOCs for 400G data center interconnects and enterprise application. While QSFP+/QSFP28/QSFP56 modules and DAC/AOC are utilized for interconnecting 40G/100G/200G networks.
Backward Compatibility
As we mentioned earlier, the QSFP-DD standard is backward compatible with both transceiver modules of the previous generation of QSFP systems because it also uses the QSFP form factor. QSFP-DD technology increases the bandwidth and maximum transmission rate of the module based on the appearance of the QSFP system. The feature of backward compatibility allows QSFP-DD modules to avoid large-scale replacement of existing equipment during installation, thereby achieving the purpose of reducing the cost of network upgrades.
The following table shows the difference between QSFP-DD/QSFP56/QSFP28/QSFP+:
|
Form Factor |
Released Year |
Electrical Interface Lanes |
Single Channel Rate |
Modulation Mode |
Backward Compatibility |
|
QSFP-DD |
2016 |
8 |
25Gbps/50Gbps |
NRZ/PAM4 |
QSFP+/QSFP28/QSFP56 |
|
QSFP56 |
2018 |
4 |
50Gbps |
PAM4 |
QSFP+/QSFP28 |
|
QSFP28 |
2016 |
4 |
25Gbps |
NRZ |
QSFP+ |
|
QSFP+ |
2010 |
4 |
10Gbps |
NRZ |
/ |
Conclusion: Will QSFP-DD Still Popular In 800G Ethernet In The Future?
Many people wonder whether QSFP-DD can continue to be popular in 800G Ethernet, so that it can be used with confidence to prevent large-scale equipment replacement in the future. Currently, the market for 400G Ethernet is slowly expanding as data centers demand more and more network bandwidth, and QSFP-DD modules are widely used in 400G Ethernet due to their numerous benefits. For the development prospect, the single-wave 100G technology is becoming more and more mature when 400G Ethernet is commercially available on a large scale, which lays the foundation for the 800G transmission rate‘s release.
And the QSFP-DD800 Multi-Source Agreement (MSA) organization has released the first version of the hardware specification for the QSFP-DD800 transceiver. This specification intends to continue using the QSFP-DD form factor to support single-channel transmission rates up to 100Gbps, which means that this form factor will continue to be used in the future, and it will remain popular for 800G Ethernet in the future.