Learn Why Will 800G Optical Modules Continue To Use QSFP-DD Packaging?

As the smallest package size for 400G optical devices, QSFP-DD can provide higher port density and is backward compatible with QSFP+/QSFP28 packages, making it popular among suppliers. Today, many suppliers have launched 400G QSFP-DD products, such as 400G QSFP-DD optical modules, 400G QSFP-DD DAC/AOC, etc. How much do you know about QSFP-DD packages? This article will help you understand the QSFP-DD package again and understand its differences from QSFP+/QSFP28/QSFP56 and OSFP/CFP8/COBO packages.

QSFP-DD package overview

QSFP-DD (also known as QSFP56-DD) is a dual-density four-channel small form-factor pluggable package, which is a new type of high-speed pluggable module package that complies with IEEE802.3bs and QSFP-DD MSA standards. The electrical interface of this package has 8 channels. Through NRZ modulation technology, the data rate of each channel can reach 25Gb/s, realizing 200G network transmission; through PAM4 modulation technology, the data rate of each channel can reach up to 50Gb/s, realizing 400G network transmission, which is suitable for high-performance computing data centers and cloud networks.

The advantages of QSFP-DD are as follows:

It is backward compatible and compatible with QSFP packages such as QSFP+/QSFP28/QSFP56;

It uses 2×1 stacked integrated cages and connectors to support single-height and double-height cage connector systems;

It uses SMT connectors and 1xN cages, and the cage design and optical module housing optimization can achieve a thermal capacity of at least 12 watts per module. Higher thermal capacity can reduce the requirements for heat dissipation of optical modules, thereby reducing some unnecessary costs.

When designing QSFP-DD, the MSA working group fully considered the flexibility of user use, adopted ASIC design, supported multiple interface rates, and was backward compatible (compatible with QSFP+/QSFP28), thereby reducing port costs and equipment deployment costs.

In short, QSFP-DD is a high-speed, small, pluggable and low-power package, which will become one of the mainstream packages for 400G. So what is the difference between QSFP-DD and the previous QSFP+/QSFP28/QSFP56? What is the difference between it and other 400G packages such as OSFP/CFP8/COBO?

The difference between QSFP-DD and QSFP+/QSFP28/QSFP56

Although QSFP-DD and QSFP+/QSFP28/QSFP56 belong to QSFP package and have the same size, there are still some differences between them. So what is the difference between them?

Structure

In terms of structure, there are some differences between QSFP-DD and QSFP+/QSFP28/QSFP56 in the depth of the motherboard mechanical interface, the number of channels of the electrical interface, and the number of integrated circuits. The details are as follows:

Motherboard mechanical interface depth - When the MSA team designed QSFP-DD, in order to make QSFP-DD and QSFP+/QSFP28/QSFP56 have the same port density and accommodate an additional row of contacts, the mechanical interface on the motherboard is slightly deeper than that of QSFP+/QSFP28/QSFP56.

Number of channels of the electrical interface - As can be seen from the above, the QSFP-DD package is equipped with an 8-channel electrical interface, which doubles the number of channels of the electrical interface compared to the QSFP+/QSFP28/QSFP56 package with a 4-channel electrical interface, which is also due to the QSFP-DD package having an additional row of contacts.

Number of integrated circuits - Although QSFP-DD is the same size as QSFP+/QSFP28/QSFP56, since QSFP-DD has an 8-channel electrical interface, its number of integrated circuits (i.e., ASICs) and density are doubled compared to QSFP+/QSFP28/QSFP56 with a 4-channel electrical interface.

Bandwidth & Application

As can be seen from the above, the maximum bandwidth of QSFP-DD can reach 400Gbps, while the maximum bandwidth of QSFP+/QSFP28/QSFP56 can only reach 40Gbps/100Gbps/200Gbps respectively. QSFP-DD is much higher than QSFP+/QSFP28/QSFP56. For this reason, QSFP-DD is generally used as the packaging of 400G optical modules and 400G high-speed cables (i.e. DAC and AOC) for 400G data center interconnection to solve the problem of high-speed migration of massive data between data centers; while QSFP+/QSFP28/QSFP56 are used for the packaging of 40G/100G/200G optical modules and high-speed cables respectively, and are used for 40G/100G/200G network interconnection.

Compatibility

QSFP-DD is backward compatible and supports the insertion of existing QSFP+/QSFP28/QSFP56 encapsulated modules or connectors into QSFP-DD ports; however, QSFP+/QSFP28/QSFP56 ports cannot support modules or connectors using QSFP-DD encapsulation.

In short, QSFP-DD packaging has been upgraded on the basis of continuing the advantages of previous packaging, increasing bandwidth and better meeting the needs of large-bandwidth network applications. At the same time, backward compatibility can effectively reduce equipment replacement and save network upgrade costs.

Differences between QSFP-DD and OSFP/CFP8/COBO

Although QSFP-DD (QSFP56-DD) and OSFP/CFP8/COBO are all 400G packages, these four 400G packages have certain advantages and disadvantages, as follows:

QSFP-DD - As can be seen from the above, this package is compatible with the existing QSFP+/QSFP28, has a small size, and is easy to maintain.

OSFP - This package also uses an 8-channel electrical interface, with a data rate of 50Gbps per channel, and comes with a heat sink, which can greatly improve the heat dissipation performance; but the new interface standard is incompatible with the existing optoelectronic interface, is slightly larger than the size of QSFP-DD, requires a larger area of printed circuit board (PCB), and has high power consumption.

CFP8 - It is equivalent to a high-speed evolution version of CFP4. The channels of its electrical interface can be 8 channels or 16 channels (mostly 16 channels), which can be quickly put into the market and is suitable for telecommunications backhaul; but the cost is high, and the size and power consumption are large.

COBO - Unlike the above packages, it places the optical module in the PCB board, is not limited by the interface density of the front panel, and can use the motherboard for heat dissipation, has good heat dissipation and small size; but does not support hot plugging, and is difficult to maintain later.

For this reason, QSFP-DD and OSFP have become the preferred packaging technologies for most suppliers. However, QSFP-DD is more suitable for data center applications, while OSFP packaging is more suitable for telecommunications applications.

Will QSFP-DD still be the mainstream package for 800G?

From the above, we can see that compared with OSFP, QSFP-DD (QSFP56-DD) is more suitable for data center applications. With the concentration of east-west traffic in data centers, the internal bandwidth pressure of data centers continues to grow, prompting the application time gap of high-speed optical modules in the telecommunications market and the data communication market to gradually shorten. 400G will be the first to be widely used in the data center market. In other words, QSFP-DD will benefit from this and usher in a good development prospect. Today, 400G has begun to usher in a golden period of development, and major manufacturers have launched QSFP-DD optical modules/DAC/AOC to seize the market.

With the large-scale commercial use of 400G, it is bound to promote the maturity of single-wavelength 100G technology, which also lays a certain foundation for the arrival of 800G. Not long ago, the QSFP-DD800 Multi-Source Agreement (MSA) organization released the first version of the QSFP-DD800 transceiver hardware specification, which is committed to continuing the current QSFP-DD package to support the new generation of QSFP-DD800 with 8 channels and a single-channel rate of 100Gbps. This also means that 800G will still use QSFP-DD, bringing greater cost advantages and commercial value to network operators.