QSFP-DD (Quad Small Form Factor Pluggable-Double Density) is a new modular connector system that utilizes a dual-density, four-channel, small, hot-swappable optical module package. The electrical interface of this package has 8 channels, each channel rate up to 25Gb/s (NRZ modulation) or 50Gb/s (PAM4 modulation), aggregation to provide up to 200Gb/s or 400Gb/s solution, suitable for high-performance computing data centers, cloud networks.

QSFP-DD package advantages

1, using 8-channel electrical interface, with backward compatibility, compatible with QSFP+/QSFP28/QSFP56 and other QSFP packages, reducing port costs and equipment deployment costs.

2. Adopts 2×1 stacked integrated cages and connectors. Due to industry demand, most pluggable packages will eventually develop a dual-height stacked cage connector system in addition to a single-height cage connector system. the QSFP-DD MSA team chose to develop both single-height and dual-height cage connector systems.

3. Using SMT (Surface-Mount Technology) connectors and 1xN cages, cage design optimization and module shell optimization can achieve a heat capacity of at least 12 watts per module. The higher heat capacity reduces the module's requirement for heat dissipation features, thus avoiding unnecessary costs.

QSFP-DD Package vs. QSFP28/QSFP56

QSFP-DD modules support double the number of high-speed electrical interfaces compared to QSFP28/QSFP56 modules. QSFP-DD supports 50Gb/s PAM4 electrical modulation format, which increases the port rate by a factor of 4 compared to QSFP28 modules.

QSFP-DD and QSFP28/QSFP56 systems have the same port density. However, because each QSFP-DD port can accommodate eight channels instead of four, QSFP-DD doubles the number of ASIC ports it supports for existing interfaces such as CAUI-4.

The mechanical interface of the QSFP-DD on the motherboard is slightly deeper than that of the QSFP28/QSFP56 to accommodate the extra row of contacts. the QSFP-DD increases bandwidth over the QSFP28/QSFP56 by just a small increase in length while maintaining the same port density, and has backward compatibility, which reduces the cost of equipment by a significant amount when deploying the network.

Figure1 400G QSFP-DD

Common 400G QSFP-DD Optical Module Types

There are usually eight common types of 400G QSFP-DD optical modules available in the market. The following section provides a basic introduction to QSFP-DD SR8, QSFP-DD DR4, QSFP-DD XDR4, QSFP-DD FR4, QSFP-DD FR8, QSFP-DD LR4, QSFP-DD LR8 and QSFP-DD ER8 optical modules.

400G QSFP-DD SR8

The 400G QSFP-DD SR8 is a 4-channel, hot-swappable optical module for short-haul interconnections, supporting up to 70m (OM3) or 100m (OM4) multimode fiber links via MPO-16 connectors. It is compliant with the QSFP-DD MSA specification and the IEEE 802.3bs protocol, and includes eight parallel transmission channels, each capable of 400 Gbps data transmission at 53.125 Gbps (PAM 4).

400G QSFP-DD DR4

The 400G QSFP-DD DR4 optical module achieves a maximum transmission distance of 500m over single-mode fiber (SMF) using MPO-12 connectors in the 1310nm center wavelength. DR4 stands for "4-channel Dense Wavelength Division Multiplexing (DWDM)", indicating that the 400G DR4 optical module uses four channels to transmit data at 106.25Gbps (PAM4) per channel, for a total transmission rate of up to 425Gbps.

400G QSFP-DD XDR4

The 400G QSFP-DD XDR4 module has a longer transmission distance than the 400G QSFP-DD DR4 module and supports single-mode fiber with a maximum link length of up to 10km using MTP/MPO-12 connectors, which is compliant with the IEEE 802.3bs protocol and the QSFP-DD MSA standard. It includes 4 parallel transmission channels and achieves a total transmission rate of 400G.

400G QSFP-DD FR4

The 400G QSFP-DD FR4 (full-duplex 4-channel, 4-stage timing) is a 400Gbps hot-swappable optical module capable of supporting multimode fiber link lengths of up to 2km with LC connectors.The 400G FR4 optical module consists of four independent channels, each with a rate of 100Gbps at the center wavelength of the CWDM4, and is compliant with the 100G single wavelength MSA specification for use in data centers. MSA specification and can be used in data center high-speed data transmission scenarios. The single-wavelength transmission rate based on PAM4 modulation can reach 106.25Gbps, realizing a total transmission rate of 400G.

400G QSFP-DD FR8

The 400G QSFP-DD FR8 utilizes the same 50G PAM4 modulation technology as the 400G QSFP-DD FR4 optical module. It uses 8 channels to transmit data at a rate of 53.125Gbps per channel, with a total rate of 400Gbps. unlike the 400G QSFP-DD SR8 optical module, which is only suitable for short-distance transmission, the 400G QSFP-DD FR8 optical module can be used for medium-distance transmission, with a maximum transmission distance of 2km.

400G QSFP-DD LR4

The 400G QSFP-DD LR4 is a high rate, high bandwidth and long distance data transmission optical module for transmission applications such as 400G Ethernet and data center interconnect. It supports single-mode fiber link lengths of up to 10km via LC connectors and is compliant with the IEEE 802.3bs protocol and QSFP-DD MSA standard.

400G QSFP-DD LR8

The 400G QSFP-DD LR8 optical module uses the same modulation technology as the 400G QSFP-DD SR8 and 400G QSFP-DD FR8 optical modules (50G PAM4 modulation technology) and has 8 parallel transmission channels. However, compared to the 400G QSFP-DD SR8 and 400G QSFP-DD FR8 optical modules, each channel of the 400G QSFP-DD LR8 optical module can achieve up to 10km of single-mode fiber link transmission at a maximum rate of 106.25Gbps, with a total transmission rate of 425G.

400G QSFP-DD ER8

The 400G QSFP-DD ER8 has the same transmission rate, modulation technology and connectors as the 400G QSFP-DD LR8 optical module. It complies with the IEEE 802.3cn protocol and the QSFP-DD MSA standard. The only difference is the transmission distance, 400G QSFP-DD ER8 optical module can realize the maximum transmission distance of 40km, so it is more suitable for ultra-long distance data transmission.

Q&A

Q: Can I use OSFP optical modules at one end of a 400G link and QSFP-DD optical modules at the other end?

A: Yes. OSFP and QSFP-DD describe the physical size of the optical modules. OSFP and QSFP-DD optical modules are compatible with each other as long as the Ethernet media type is the same (i.e., 400G-DR4 or 400G-FR4, etc. at both ends of the link).

Q: Can I plug a QSFP-DD optical module into an OSFP port?

A: No. The physical dimensions of QSFP-DD and OSFP modules are different. You can only use the corresponding size optical module in the corresponding system. If you are using OSFP package, you must use the corresponding 400G OSFP optical module.If you are using QSFP-DD package, you must use the corresponding 400G QSFP-DD optical module.

Q: What is the difference between 40/100G optical modules and 400G optical modules?

A: Compared to 40G/100G optical modules, 400G optical modules are shifting from traditional low-end optical modules with single-carrier modulation and coherent detection to polarization-multiplexed multicarrier applications.In addition 400G optical modules have more package sizes, while their low power consumption, high bandwidth and high speeds come with increased network deployment costs.

Q: What is the difference between using NRZ or PAM-4 for electrical and optical signaling channels?

A: NRZ stands for "non-return-to-zero" modulation, which describes the electrical or optical signal channel, where there are only two amplitude levels, one representing the number "1" and the other the number "0".", which is the main modulation scheme for transmitting up to 25Gb/s data.And PAM-4 stands for Pulse Amplitude Modulation with Four Levels, where the "4" indicates that the electrical or optical signal carrying the digital data has four different signal levels. In this case each signal level represents two digits, which allows the PAM-4 waveform to transmit two times more bits than the NRZ waveform at the same symbol cycle rate, meaning that PAM4 can achieve higher bit rates at half the baud rate.

Q: What are the advantages and disadvantages of using OSFP or QSFP-DD optical modules?

A: QSFP-DD is based on the QSFP size with an extra row of electrical pins. This makes it very backward compatible with 40G and 100G QSFP optical modules and is also compatible with QSFP28 optical modules. However, the OSFP size is not compatible with some of the optical interfaces, and adapters are required for interoperability.While QSFP-DD modules rely on the switch's external heatsink to achieve greater transmission power, OSFP modules have heatsinks that are integrated into the module enclosure and have a larger thermal surface area than QSFP-DD modules, resulting in optimal thermal contact between the heatsink and the heatsinks, and improved heat dissipation performance.