The past of the 400G QSFP-DD: the fading of 100G era

Before introducing the 400G optical module, it is necessary to know the 100G optical module. There are three criteria for a successful optical module: small size, low power consumption, and flexibility. As we all know, the SFP/SFP+/QSFP+/QSFP28 form factor is used to implement 1G/10G/40G/100G network transmission respectively. In fact, for the 100G network, there are four different types of modules: CFP, CFP2, CFP4, and QSFP28. As you can see in the figure below, they are getting smaller and smaller in size and power consumption.

In 2018, the 100G data center Ethernet market is in full swing and is the main battleground for major optical module vendors to compete. With the growing maturity of 100G series products of major manufacturers, the shipment volume is rising. The technical threshold of 100G has been crossed and has become the mainstream and first choice for the strong demand of data centers. It is no longer synonymous with high-end optical modules. With the rapid development of technology, in order to meet the growing bandwidth demand of large-scale data centers, we have put our hope on the 400G optical transceivers. 400G optical transceivers are widely considered by the industry as a solution that can effectively reduce bandwidth costs.

The present of the 400G QSFP-DD: the rising star

400G is the next step in the development of 100G, so 400G transceivers are becoming increasingly important. Based on market conditions, 400G is preferred for internal connectivity in large data centers and is currently less used in the transport sector of telecom networks. Since 400G bit rates require PAM4 modulation, the coverage becomes increasingly challenging and limited to a few kilometers, longer distances will require the use of coherent detection techniques, amplification techniques, dispersion compensation techniques, etc. Cisco 400G optics is the first to appear on the market with the form-factor of 400G QSFP-DD.

400G transceivers

What types (models) of 400G QSFP-DD transceivers are available in the market today?

  • 400G SR8 QSFP-DD: the transmission distance of 400G SR8 module through multimode fiber can be up to 70m on OM3 fiber or 100m on OM4 fiber; central wavelength is 850nm; suitable for short-distance interconnection or multi-channel data communication.
  • 400G DR4 QSFP-DD: It supports a max transmission distance of 500m on single-mode fiber. In addition, 400G DR4 is also the base form of 400G silicon optical modules. Another type similar to DR4 is called 400G XDR4 QSFP-DD which with a transmission distance of up to 2km over parallel SMF.
  • 400G FR4 QSFP-DD: It supports link lengths of up to 2km SMF with a duplex LC connector. This module uses CWDM (wavelength division multiplexing) technology, using 8 channels of 53Gbps PAM4 signals on the electrical side, and 4 channels of 106Gbps PAM4 signals on the optical side, which is twice the rate of the electrical side.
  • 400G LR4 QSFP-DD: 400GBase LR4 fiber optic modules are designed with a built-in Gearbox chip that multiplex the two channels’ electrical input data into single-channel outputs signal and then modulates it to the optical receiver end. It defines a 400 Gb/s PHY for operation on the four CWDM center wavelengths of 1271nm, 1291nm, 1311nm, and 1331nm.
  • 400G LR8 QSFP-DD: It supports link lengths of up to 10km over a standard pair of G.652 single-mode fiber (SMF) with duplex LC connectors.
  • 400G ER8 QSFP-DD: It supports link lengths of up to 40km over a standard pair of G.652 single-mode fiber (SMF) with duplex LC connectors.
  • 400G ER4 QSFP-DD: Similar to 400G ER8, the 400G ER4 module supports link lengths of up to 40km over a standard pair of G.652 single-mode fiber (SMF) with duplex LC connectors. But, the 400G ER4 QSFP-DD has only four wavelengths which are The central wavelengths of the 4 LAN WDM channels are 1295.56nm, 1300.05nm, 1304.58nm, and 1309.14nm.

Why QSFP-DD over other 400G capable form factors (OSFP, CFP8, & COBO)?

When compared to the octal small form pluggable (OSFP), the QSFP-DD is quite similar. Both utilize eight lanes to achieve 400G data rates (8x50G), however, the OSFP is planned to later support 800G signals. The QSFP-DD is a better option for those needing the interim step to 200G not only for cost but also due to reduced power consumption. The 400G QSFP-DD has a key technology which is parallelization which enables high-speed interfaces to be implemented using parallel lanes (fibers or wavelengths). Increasing the speed of each lane reduces the number of lanes needed resulting in a lower cost in the long run.

multiple options

OSFP tends to be a better choice for operators immediately moving to 400G with a desire for later scalability, however, OSFP is not backward compatible with QSFP+ and its other forms. This means a move to 400G will come at a higher immediate cost with OSFP with savings later realized by deferring further cost at the 800G upgrade.

CFP8 is another popular form factor for 400G applications due to its support for both 16x25G and 8x50G, however, it has a much larger physical footprint as well as elevated power consumption levels. Unfortunately, it is neither backward compatible nor scalable to 800G.

Our final current option is the unique consortium for onboard optics (COBO). Though newer, this type is installed in the line card. This installation method puts many features of the QSFP-DD out of reach for a COBO, such as lacking the flexibility provided by a hot-pluggable optic. Through an 8 lane 1x400G or a 16 lane 2x400G connector interface, however, it can support both 400G and 800G.

The future of the 400G QSFP-DD: will soon be mainstream

Before the emergence of QSFP-DD, the most popular interfaces in the networking industry consisted of single (SFP/SFP+) or quad lanes (QSFP+/QSFP28). However, to accommodate the expected demand for data bandwidth or channel capacity, eight-lane interfaces are being defined in venues such as Ethernet. The currently available form factors that support eight-lane interfaces do not have all the desired features or density necessary to support the next-generation systems that plan to implement these higher rate interfaces. Thus, the QSFP-DD MSA group extended and defined QSFP-DD based on QSFP (QSFP+/QSFP28).

400G technology is still in its early stages, and suppliers are in a race to find the optimal specifications to meet current market needs by supporting mixed protocols, speeds, and distances. According to market researchers, data consumption will increase by more than 50% every year, and with COVID-19 the expected increase is even higher. In some hyper-scale data centers, data is doubling every year.

With 100G/200G speeds, carriers and hyper-scale data centers have become accustomed to interoperability issues among different vendors using different DSP-based coherent modules and forward error correction (FECs). The upcoming ecosystem of 400G DSP and optical modules will mitigate these issues, changing the landscape from customized to standards-based solutions.

To summary, by analyzing the features of QSFP-DD and comparing it to other 400G optical module form-factors, we found that QSFP-DD has unparalleled advantages in 400G applications such as data center interconnects. It is expected that when the world’s leading hyper-scale data centers start to deploy 400G, QSFP-DD will become the mainstream form-factor of 400G optical modules.