1. What form-factors are used for 400G optics and cables? 

There are two form-factors defined for 400G optical modules and cables: 

• The OSFP: The OSFP stands for “Octal Small Form-factor Pluggable”. It is described as an “Octal” module because the electrical interface of an OSFP connector consists of 8 electrical lanes, running at 50Gb/s each, for a total of bandwidth of 400Gb/s. 

• The QSFP-DD: The QSFP-DD stands for “Quad Small Form-factor Pluggable (QSFP) – Double Density (DD)”. The electrical interface of a QSFP-DD connector also has 8 electrical lanes, running at 50Gb/s each, for a total bandwidth of 400Gb/s. The QSFP-DD form factor is similar to the QSFP form factor, except a second row of electrical contacts has been added to increase the number of high-speed electrical lanes from 4 (in a QSFP) to 8 (in a QSFP-DD).

  1. Can I plug an OSFP module into a QSFP-DD port, or a QSFP-DD module into an OSFP port? 

No. The OSFP and the QSFP-DD are two physically distinct form factors. If you have an OSFP system, then OSFP optics and cables must be used. If you have a QSFP-DD system, then QSFP-DD optics and cables must be used. 

  1. QSFP-DD vs OSFP: what’s the difference?

OSFP = Octal Small Form Factor Pluggable

QSFP-DD = Quad Small Form Factor Pluggable Double Density

OSFP has a max power consumption of < 15W where QSFP-DD is designed for < 12W. The thermal management for OSFP is in included in the transceiver where as for QSFP-DD the design is up to the host device. QSFP-DD, as the name already states, is basically a QSFP – therefore the ports are backwards compatible for QSFP56, QSFP28 or QSFP+ which has a lot of advantages for the system vendors. 


The major advantage for OSFP is the bigger size. Usually it’s ‘the smaller, the better’ but in this case OSFP are more robust in terms of heat dissipation, and more spacious in terms of electrical and optical components; this could come in handy for future use cases. Also, the size allows OSFPs to be 800G ready.

Both QSFP-DD and OSFP are designed for intra-DC application including DAC, AOC and optical connection up to 2km. Additional variants will come for other applications (DCI) with longer reach and other technology like DWDM super channel. Whether you want to use QSFP or QSFP-DD will be up to your network setup and scalability, and the choice which devices you’d like to use. QSFP-DD devices can hold up to 36x QSFP-DD in 1U, while OSFP can only hold 32x – on the other hand heat dissipation is a challenge for QSFP-DD devices.

  1. Can I have an OSFP on one end of a 400G link, and a QSFP-DD on the other end? 

Yes. The OSFP and QSFP-DD describe the physical form factor of the module. As long as the Ethernet media types are the same (i.e. both ends of the link are 400G-DR4, or 400G-FR4 etc.), OSFP and 400G QSFP-DD modules will interop with each other.

  1. Can I plug a 100G QSFP module into a QSFP-DD port? 

Yes. A QSFP (40G or 100G) module can also be inserted into a QSFP-DD port (without a mechanical adapter). When using a QSFP module in an QSFP-DD port, the QSFP-DD port must be configured for a data rate of 100G (or 40G), instead of 400G.

  1. What is the maximum supported distance for 400G Transceivers and Cables? 

The maximum distance currently supported by Fibermall 400G transceivers is 100m/500m/2km/10km/40km. For copper DACs, a max length of 3m is supported for OSFP cables, and a max length of 2.5m is supported for QSFP-DD cables. For DWDM links, Fiber Mall will provide support for 400ZR / ZR+ coherent modules capable of closing 120km (for the 400ZR) and 2000km+ (for the 400ZR+) DWDM links. The 400ZR modules integrate coherent DWDM transmission into the switch/router port, eliminating the requirement for expensive optical transport equipment.

  1. What are the types of 400G QSFP-DD?

The most available types in the market today are 400G DR4, 400G-SR8, FR4, LR4, ER4, LR8,and ER8.

But, only several vendors can provide all these modules above, there are still some technology challenges to conquer.types of 400G QSFP-DD transceivers

  1. QSFP-DD vs QSFP56: what’s the difference?

Although the footprint of the physical transceiver is the same, its electrical interface is quite different. Since the QSFP-DD is a double density module, it carries 8 lanes instead of 4. In turn, this increases the amount of ASIC ports to support CAUI-4 and other existing interfaces. When using standard NRZ modulation, each interface can support 25GB/s for a total of 200G throughput (8x25G). If additional lane density is needed, higher level PAM4 modulation technologies can be leveraged to deliver a 400G (8x50G) signal.

While the QSFP56 can achieve 200G data rates, the lower lane count means that even with PAM4 modulation 400G data rates are not possible. For applications involving 400G data center interconnects, leveraging QSFP-DD transceivers, direct attach cables (DACs), and active optical cables (AOCs) creates a high-performance link capable of scaling to next generation data rates.

A key tenant of the scalability of QSFP-DD transceivers is that they are also backwards compatible with the QSFP56 – the QSFP-DD also called QSFP56-DD. Already deployed QSFP+ transceivers can remain in use without compromising performance of the network beyond the QSFP-DD. This keeps upgrade costs in check by compartmentalizing and deferring replacement of QSFP to QSFP-DD modules to a later time.

  1. Is QSFP-DD preferred for 400G deployments?

Yes, but it also depends on the application. If you don’t need the 200G interim step, there may be deferred operational and capital expense savings by moving straight to 400G. However, if budget or downtime is a concern, the interim step may allow for deferred initial expenses and long term operational savings.
The market is moving towards adopting QSFP-DD as a form factor for 800G, with the first Multi-Source Agreement (MSA) specifications for a QSFP-DD800 form factor already released. While the future may hold other technological advancements in the 800G race, it appears the QSFP-DD form factor is here to stay.