400G QSFP-DD Portfolio and Interoperability

Guide line of 400G Optics

The 400G era is well underway and many customers worldwide have deployed Juniper Networks switch and router platforms that support 400G Ethernet. Part of our ongoing work includes delivering 400G optics modules to customers that meet industry standards for performance and interoperability. These standards are developed by the networking industry as a whole. Pluggable optics is a complex world – now we aim to eliminate as much of this complexity as possible.

The New Alphabet of the Optics World

Engineering teams have developed a broad set of 400G pluggable optics that support an extensive range of use cases for customers, including 500m and 2km single-mode fiber intra-data center interconnects. The 400G optics are based on PAM4 modulation technology that has been standardized in the IEEE 802.3 with some additional optical specifications provided by the 100G Lambda MSA. This was a major transition within the optics technology industry roadmap where past generations of 100G, 40G, 10G and lower-rate optics were based exclusively on NRZ on-off keying (modulation). The transition to PAM4 required not only new laser modulators, digital signal encoding/decoding processors (DSPs) and receiver device designs for optical transceivers, but it also required a completely new set of performance parameters, test methodologies and new lab equipment for test and measurement.

We’re now seeing the fruits of this labor stemming from the IEEE Std. 802.3bs-2017, with the industry offering a wide array of 400G optics. Table 1 below provides a list of standards-based 400G optics known to the author at this time at various stages of maturity.

Table 1: Standards-based 400G capacity QSFP-DD pluggable optics according to IEEE Std 802.3-2018, IEEE Std 802.3cd-2018, IEEE Std 802.3cm-2020, IEEE Std 802.3cn-2019, IEEE P802.3cu D3.2, IEEE P802.3cw D0.99, IEEE P802.3db, OIF-400ZR-01.0, 100G Lambda MSA 400G-LR4-10 Spec. Rev. 1.0, and SNIA SFF TA TWG SFF-8024 Rev. 4.8.

Some of the 400G pluggable optics support more than one rate of Ethernet. Of course, there is support for 400G Ethernet but also, in particular, 4 times 100G Ethernet. In some cases, even 2 times 200G Ethernet can be supported in a common module. These breakout ports of lower speeds are fully independent and can run on separate time domains. This required a new generation of single laser wavelength 100G optics based on the PAM4 technology. These “single lambda” optics have resulted in a significantly lower cost for 100G interconnects, as the number of lasers and modulators are reduced by 4x compared with the legacy 100G optics that use 4x25Gb/s NRZ architecture. These PAM4 optics, QSFP-100G-DR/FR1/LR1, are now being developed and readied for qualification testing.

The IEEE P802.3db task force is developing a standard for 100GBASE-SR1 over multimode fiber that will fill in the shortest reach with a 100G lambda optic. Table 2 below provides a summary of the 100G lambda optics for 100G Ethernet. The DSP deployed in these modules implements the conversion between single wavelength 100Gbps PAM4 optical modulation to 4x25Gb/s NRZ electrical modulation across the module-to-host interface.

The QDD-400G-FR4, -LR4-6, -LR4-10 and -LR8 optics listed in Table 1 support only 400G Ethernet where the 4x 100G-λ (FR4, LR4) or 8x50G-λ (LR8) wavelengths are optically multiplexed/de-multiplexed onto duplex single-mode fiber.

The QDD-400G-DR4 supports both 400GBASE-DR4 onto parallel SMF with 8 fibers (4 Tx plus 4 Rx) via the MPO-12 receptacle connector and breakout to 4 x 100GBASE-DR, which can interconnect using duplex fiber links to QSFP-100G-DR.

The QDD-4x100G-FR1 module, also known as 400G-4xFR1 or 400G-DR4+, supports 4 times 100G Ethernet breakout, which can interconnect to QSFP-100G-FR1 and in 400G Ethernet mode can interoperate with QDD-400G-DR4 at 500 meters.
Similarly, the QDD-4x100G-LR1 module, also known as 400G-4xLR1 or 400G-DR4++, supports 4 times 100G Ethernet breakout, which can interconnect to QSFP-100G-LR1 and, in 400G Ethernet mode, can also interoperate with QDD-400G-DR4 at 500 meters.

The breakout ports also interoperate at the shortest mutual reach, e.g., the 4x100G-DR breakout ports interoperate with QSFP-100G-FR1 or QSFP-100G-LR1 up to 500m. However, the 100G Ethernet breakout ports are not backward compatible with the legacy 100G Ethernet using 4x25G NRZ optics, e.g., QSFP-100G-LR does not interoperate with QSFP-100G-LR4.

What type of optical connectors do the 400G-FR4 / LR4, 400G-DR4 / XDR4 / PLR4, 400G-SR8 and 400G2FR4 transceivers use?

The 400G-FR4/LR4 modules use duplex LC fiber connectors – the same connectors that are used on existing 100G and 40G QSFPs that use duplex SMF (e.g. 100G-CWDM4, 40G-LR4 etc).

The 400G-DR4 / XDR4 / PLR4 modules use a single row, angled (APC) MPO12 connector for use with parallel single mode fiber. Although a MPO12 cable can have up to 12 SMF fibers, only 8 out of the 12 fibers are used (4 for Tx and 4 for Rx). This is the same connector that is used on existing 100G and 40G QSFPs that use parallel SMF (e.g. 100G-PSM4, 40GPLR4 etc). An image, and drawing of an MPO12 connector are shown below:

The 400G-SR8 optics module uses a single row, angled (APC) MPO16 connector for use with parallel multi-mode
fiber. All 16 fibers are used (8 Tx and 8 Rx). An image and drawing of an MPO16 connector are shown below:

The 400G-2FR4 optics module uses 2 x CS connectors, for use with single mode fiber. An image and drawing of
2 x CS connectors is shown below: