400G/800G from electrical validation to Ethernet testing and beyond

400G Ethernet

400GE is today’s highest client rate in the Ethernet ecosystem according to the IEEE 802.3bs standard, offering improved capabilities for key aggregation and high‑bandwidth interconnect applications. These applications cover a wide range: cloud‑scale data centers, internet exchanges, colocation services, wireless infrastructure, service provider and operator networks and video distribution infrastructure.

EXFO’s 400G testing solutions are the most advanced in the industry, featuring a powerful, versatile and scalable test and measurement solution that incorporates rackmounts (LTB‑8/LTB-2) and portable (FTB-4 Pro/FTB-2 Pro) platforms with a wide variety of modules, providing unique flexibility.

The new 400G technology defines new physical layer specifications for 200 Gbit/s and 400 Gbit/s operation and introduces mandatory forward error correction (FEC) for correcting errors, targeting to compensate signal degradation and optimize link quality. Our high-speed testers feature a FEC statistics tool based on the Reed Salomon coding, also known as RS (544, 514), for which the codeword is structured with 544 symbols, where 514 symbols are dedicated for payload and 30 symbols for parity. This type of FEC can correct up to 15 symbols in error per codeword.

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PAM4: The key enabler of 400G technology

Latest 400G rate has been materialized in high-speed interfaces thanks to the implementation of 4-level pulse amplitude modulation (PAM4) that essentially doubles the density of data with respect to NRZ. PAM4 as a new coding technique, allows to drive 8 lanes of 50G or 4 lanes of 100G for different variants of CFP8, OSFP and QSFP-DD, replacing classical NRZ-based interfaces. In this framework, PAM4-based components deliver advantages but also introduce big challenges for testing, since analyzing the volume of a four-level system is more complex than NRZ. Also, PAM4’s amplitude is three times shorter than that of NRZ, the signal to noise ratio is lower and the spacing between encoding levels is tighter, as a result it is more susceptible to noise.

It is then highly complex to analyze and debug 400G PAM4-based interfaces since several parameters of the transceivers (e.g., optical power at Tx and Rx, the electrical power consumption, temperature, skew, etc.) must be tested before, during and after the installation in networks.

We provide a wide array of tools to analyze and debug PAM-4 based transceivers, such as pre-emphasis, equalization and PAM4 histogram (reference tool for representing the 4 electrical levels of the modulation, enabling the real time PAM4 analysis per lane).

Validation of transceivers in manufacturing

The 200G/400G new-generation transceivers are mass produced, at a pace of thousands per day, they must meet strict quality and reliability standards to function accurately in today’s demanding high-speed networks, including 5G. Manufacturers need simplified solutions capable of providing quick and accurate transceiver evaluation and validation at different stages of production to maintain that speed of production.

It is to address this need that we have designed our unique end-to-end solution for qualification of next-gen transceivers, including client-side testers and powered with a suite of analysis tools to quickly and easily validate the 200G/400G optical interfaces.