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QSFP-DD stands for Quad Small Form Factor Pluggable Double Density. QSFP-DD is though the smallest form factor of 400G transceivers, but provides with the high-density bandwidth in the entire industry, due to which it gained popularity all around.
However, as QSFP-DD, or we can say QSFP56-DD, (the other name of QSFP-DD) came into the market and gained popularity, there was a lot of comparisons made with the other form factors like QSFP56, OSFP, CFP8, and COBO. So, as the comparison of QSFP-DD is always made with the other form factors, in this post, we will thoroughly want to discuss what these all form factors are.
About QSFP-DD
QSFP-DD is the double density QSFP. The QSFP-DD complies with the IEEE802.3bs and QSFP-DD MSA standards. This connector seeing the demand for a higher bandwidth network has been upgraded and is designed to supply eight lane interfaces. Now, this optical transceiver supports 200Gbps and 400Gbps Ethernet applications, which were not supported by any of the previous transceivers. The eight lanes that it operates are supported by either 25 Gb/s NRZ modulation or 50Gb/s PAM4 modulation. Having these transceivers in use will increase the bandwidth and the density of every panel by several times. So, as this provides increased speed and also supports more channels, these are highly compatible to be applied in the high-performance computing data centre and also for the cloud networks.
QSFP-DD: The Advantages Are As Follows:
- The designed system of QSFP-DD is backward compatible. This means the QSFP-DD is compatible with the QSFP modules (such as QSFP+, QSFP28, QSFP56, etc.) The backward compatibility is very critical and advantageous for the end-users and also for the system designers, as it provides flexibility.
- The QSFP-DD is a 2x1 stacked integrated cage. As the industry needs are growing, most of the pluggable form factors seem to have developed a two-high stacked cage-connector system in extension to a one-high cage connector system.
- QSFP-DD has the SMT connector and 1xN cage design. With this design, the thermal support of at least 12W per module can be enabled. With the innovative thermal management systems used in the module and cage designs, the QSFP-DD supports power levels of at least 12W per module. Also, as there is a higher thermal level, the heat dissipation capabilities will get reduced, and also unnecessary costs will be cut down.
- In a single switch slot, QSFP-DD can enable up to 14.4Tb/s aggregate bandwidth. While maintaining the port density, and by quadrupling aggregate switch bandwidth, QSFP-DD can be made to support continuing growth in data centre traffic and network bandwidth demand.
- The ASIC design supports the multiple interface rates, and as it is fully backward compatible with QSFP+ and QSFP28 modules, therefore it reduces the costs of port and equipment deployment.
How Is QSFP-DD Different From QSFP+/QSFP28 & QSFP56
Though QSFP-DD, QSFP+, QSFP28, and QSFP56 are the form factor of QSFP itself, the QSFP-DD is distinct from all three. So, let’s see how QSFP-DD is different from all the three, and how these three are different from each other too, on the basis of the following descriptions:
- Structure & Number Of Electrical Interface Lanes:
The physical structure of the QSFP-DD is more or less similar to the QSFP+, QSFP28, and QSFP56. However, as these are physically the same in size, length, width, and thickness, the QSFP-DD provides an 8-lane electrical interface, instead of providing the 4-lane interface, which the other form factors of QSFP tend to provide. Moreover, the QSFP-DD has ASIC ports that are doubled, which is made to support the existing CAUI-4 interface.
- Bandwidth
The QSFP-DD has been brought into the market, only because there was a need for higher bandwidth. So, compared to the other QSFP modules, the QSFP-DD supports 400Gbps while QSFP+/QSFP28/QSFP56 can respectively reach 40Gbps/100Gbps/200Gbps. Also, if we see the single-channel rate of QSFP-DD, it is 25Gbps NRZ modulation for eight lanes and 50Gbps PAM4 modulation for four lanes, while QSFP56, QSFP28, and QSFP+ have the 50Gbps, 25Gbps, and 10Gbps.
- Application
The application of the QSFP-DD is done at the high-performance computing data centre and the cloud networking centres or at the places that have to set-up their networking and require only the best performance of the network. Also, the other form factors that are QSFP56, QSFP28, and QSFP+ are used at the data centres, or at the places where they suit the best according to their performances.
- Modulation Technology
If we talk about the modulation scheme of all the form factors, then the QSFP+, QSFP28, and QSFP56, each have one type of modulation that is NRZ, MRZ, and PAM4 respectively. But if we talk about the modulation of QSFP-DD, it’s a combination. If you want to draw four lanes, the modulation for eight lanes will be 25Gbps NRZ, and for four lanes it will be 50Gbps PAM4.
- Backward Compatibility
The backward compatibility is very crucial and important for any newer version of any form factor that is coming into the market. Having the system supporting the Backward Compatibility will eliminate the need to start over and deploy new equipment while upgrading to the newer product. So, from reducing the costs to making the deployment of the latest system easy, backward compatibility does its work, well.
QSFP-DD And OSFP/CFP8/COBO
Now, after talking about the form factors of QSFP, we also need to talk about the form factors of the 400G optics, which are there in the market. So, the form factors that are available in the market of 400G optics are OSFP, CFP8, and COBO.
QSFP-DD and OSFP
The OSFP is another new pluggable form factor of 400G optics. The OSFP and QSFP-DD both form factors that run eight high-speed electrical lanes, support the 400Gb/s, which can reach upto 800Gb/s as well. Though these both form factors appear to be similar but have three key differences too. The differences are:
- OSFP allows more power of 15W than the QSFP-DD, which allows 12W. However, the OSFP allows an early adoption, as it is easier to release a technology designed for 15W than 12W, so the consumption of power is more in the OSFP than that of the QSFP-DD. If more power consumption is allowed, the more unnecessary cost will rise, so, QSFP will cost more than QSFP-DD.
- The QSFP-DD with the QSFP+ (40G), QSFP28 (100G), and QSFP56 (200G) is backward compliant. At the same time, OSFP requires the QSFP to OSFP converter.
- While the OSFP integrates thermal management directly into the form factor, the QSFP-DD does not do the same.
QSFP-DD and CFP8
According to CFP MSA, there are radical differences in the CFP8 and QSFP-DD.
- CFP8 has 16x channels 25G NRZ on the electrical side, instead of 8x 50G PAM4 which QSFP-DD has.
- The power consumption CFP8 uses is 24W, which is much higher than the QSFP-DD, which is 12W, since CFP8 is three times larger than that of the QSFP-DD. The more the power consumption will be there, the more unnecessary cost will be increased.
- The CFP8 can’t be used on the QSFP+ and QSFP28 port, while QSFP-DD can be used on the same.
- Though CFP8 and QSFP-DD both have the maximum bandwidth of 400Gb/s, the CFP8 can only support in the form of 16x25G or 8x50G, while QSFP-DD supports both and also 200 Gb/s that is 8x25G.
QSFP-DD and COBO
COBO that is Consortium for On-Board Optics is a non-profit, member-driven, and mutual-benefit corporation that brings together industry leaders to defeat the limitations that are linked with the moving optics inside networking equipment by producing innovative industry specifications.
So, the COBO is installed in the controlled environment, internally to the line-card. The COBO lacks flexibility and also does not support hot-pluggable, so to maintain COBO, it takes more effort than that to maintain the QSFP-DD. The form factor, COBO, has two electrical interfaces, the eight-lane, and sixteen lanes, to meet the 1x400G and 2x400G transmission needs.
Endnote
However, now, as we have compared all the form factors of QSFP and also of the 400G optics, we can clearly see what the differences between all of them are. However, as the QSFP-DD are more compatible in comparison to other form factors of the QSFP; and QSFP-DD and OSFP are better than the other form factors of 400G optics, these are more popular ones in the market too. Also, as the QSFP-DD is the latest form factor and will be soon be brought into use widely, it will be a forerunner in the good development prospect as well. Alongside, when the total bandwidth of 400Gb/s will be applied extensively, it will also lay down the groundwork of the 800G, as the industry leaders have formed the QSFP-DD800MSA group to extend the bandwidth of the QSFP-DD factor from 400Gb/s to 800Gb/s.
