800G Pluggable Optics Drive a New Approach to Test and Validation

By Guest blogger - Dr. Paul Brooks, High-Speed Optical Transport Strategy, VIAVI Solutions

The pluggable optic ecosystem has undergone a revolution in complexity over the past decade. Technology has evolved from 100G modules using analog clock and data recovery (CDR) circuits, Non-Return-to-Zero (NRZ) modulation and simple firmware, to complex digital signal processing (DSP)-based equalization and CDRs, Pulse Amplitude Modulation 4-level (PAM-4) modulation and advanced module management like content management interoperability services (CMIS).


The complex and highly integrated photonics are directly impacted by the DSP and are all intimately linked to the firmware. The module is no longer a loose collection of distinct domains — the electrical, the photonics, the firmware and the module management are now a closely orchestrated system. As a result of this tight coupling, the electrical interface becomes even more challenging to troubleshoot, debug and validate.


The move to 100G electrical signaling for the host electrical interface pushes the boundaries of DSP performance and requires deep insight into the dynamic interaction between host and module for issues such as crosstalk and CDR-based link flaps. Simply put, an integrated approach to module test and validation is a must in the post 400G world.


An orchestrated approach pulls together full visibility and control in each of the three domains – photonic, electrical and control/firmware. This is the only way to fully understand the subtle interactions inside the module DSP, and how this relates to what the host thinks is going on.


Even the simplest event – the interruption and recovery of the module optical signal – requires true simultaneous multi-domain visibility and analysis. From the moment the optical signal is interrupted it is critical to know the order of events:

  • When does the module assert loss of signal (LOS)?  
  • When and how does it mute its electrical transmitter?
  • What does the module CMIS report?
  • Does all this happen quickly, correctly and unambiguously?


The situation of recovery from LOS is even more demanding. It is important to know what happens when the optical signal is re-asserted:

  • Can the module self-recover?
  • How long does it take to de-assert LOS?
  • Does the module-to-host electrical interface de-mute correctly and without errors?


The latest generation of DSP ICs required for 800G are driving a revolutionary increase in technical complexity, set against a backdrop of aggressive time-to-market timetables and cost expectations.  We have moved from the isolated test islands used at 100G and below, to a deeply integrated approach to module development, debugging and validation in order to address the needs of the 800G module ecosystem. If you would like a more in-depth review of what the path forward with 800G pluggable optics may look like, visit: Preparing the Way for 800G: 100G Electrical and 800G Pluggable Optics.

Posted: 1 June 2021 by Guest blogger - Dr. Paul Brooks, High-Speed Optical Transport Strategy, VIAVI Solutions | with 0 comments

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