Industry Insights

Addressing 400G and Beyond System Design Challenges

2019-07-31  |  6 min read 

The wireline internet network must continue to evolve to keep up with advancements in wireless communications such as 5G. 400G standards, including Ethernet, Fibre Channel, and Common Electrical I/O, are only recently available, and some are still under development. A big challenge in the industry is implementing 400G cost-effectively. The high cost of 400G implementation has slowed the process. However, as 5G devices begin to hit the market, 400GE will appear in data centers.

It will likely take multiple iterations to perfect signal generation and decoding of newly developed PAM4 devices supporting 400GE speeds. Building hardware early in the design cycle can be costly and time-consuming. As an alternative to investing in hardware device turns, engineers use simulation, modeling software, and the latest test instruments to characterize 400GE devices.

Modern simulation software uses Algorithmic Modeling Interface (AMI), which allows engineers to select integrated circuit chips from various vendors to use in their simulated design. The AMI models encapsulate the performance of the transmitter and receiver, enabling engineers to test their designs before building hardware.  

400GE Measurement Challenges

Achieving 400GE speeds in network systems requires making new and more challenging measurements. Test instruments such as high-performance oscilloscopes and bit error ratio testers (BERTs) help engineers characterize and improve the quality of their designs. Instruments and software used in the past likely cannot support the higher signal speeds, increased baud rate, and new measurements required to test PAM4 designs. New instruments and software can help engineers ensure that their designs comply with the latest standards.

Here are some of the new measurement challenges and the recommended instruments to characterize PAM4 devices:

Transmitter test

  • Oscilloscopes must be capable of higher bandwidth with an ideal fourth-order Bessel-Thomson response
  • Oscilloscopes with lower noise can apply equalization to open the eye of the signal for analysis
  • PAM4 receiver testing requires an oscilloscope that can emulate the complex, multi-transition levels, 400G built-in clock recovery, and inherent data-dependent jitter
  • These standards require new optical and electrical measurements, such as transmitter dispersion eye closure, signal to noise and distortion ratio, and other output jitter measurements best made by next-generation oscilloscopes with built-in measurements

Receiver test

  • A receiver test requires a PAM4 capable BERT for BER measurements and receiver characterization with additional error analysis capabilities, such as forward error correction (FEC)
  • Generate newly defined stressed signals with an arbitrary waveform generator (AWG) to test a receiver’s inputs and characterize with a BERT to ensure receiver BER and sensitivity (stressed eye signals can be difficult to generate without a capable AWG)
  • An optical modulation analyzer and high-speed digitizer test solution are needed for 400G coherent transmitter tests, such as error vector magnitude, a global signal metric to characterize the quality of the coherent transmitter

Channel test

  • A vector network analyzer and physical layer test software can characterize channel and interconnect designs

Network test

  • A gigabit Ethernet tester characterizes the protocol layer, including forward error correction for PAM4 signals

Looking beyond 400G, development of 800G standards has just begun. Data throughput of 800G / 1 Tb will be an evolutionary increase from 400G — basically faster PAM4. Development for the electrical needs is in progress, with chip-to-chip, chip-to-module, and then backplane projects. The results are initial definitions for 53-58 gigabaud electrical links. Optical definitions will follow the electrical developments but have not begun yet. Selecting optical transceivers that comply with industry standards is critical for data center operators to support speeds of 400G and beyond. Fortunately, the high-performance instruments and integrated software used for 400G device development are already capable for future 800G device development and test.

Keysight experts are actively engaged in the standards organizations. They work to integrate and develop test instruments and software that will provide engineers with the tools they need for new high-speed device design and test. As technology and standards continue to advance, Keysight is at the forefront, providing test equipment capable of measurements at 400G and beyond.

Learn more about PAM4 or 400GE