Pluggable coherent optics are now being developed for 400G applications with two distinct streams emerging. One is high performance CFP2 based for 1000+km reach and multi-service applications and the other is QSFP-DD ZR (and perhaps OSFP) for DCI and metro Ethernet applications. Both streams present new challenges for test and measurement, not just to meet the demanding technical specifications but also to ensure pain-free integration and deployment in an open ecosystem.

Hopefully you were able to join the Light Reading 400G webinar on this subject that I  recently participated in. Whether you were able to or not, I have received several follow up questions in my inbox that are worth examining in greater detail. So, without further ado, here are the top three most asked questions and my responses.

Q. You mentioned in your recent 400G webinar that unframed signals like PRBS/PRBSQ are no longer appropriate for testing 400G class coherent optics. Can you expand on this?

A. The latest 400G DCO modules are highly integrated solutions with a very large & complex DSP IC performing many functions including host SERDES interface, framer, soft decision FEC, mapper and dispersion compensation. Everything past the SERDES, especially the FEC and framer, expect framed traffic. So, a simple unframed PRBS/PRBSQ can only really be used to check connectivity with the SERDES block.

To ‘probe’ inside the module, and to have end-to-end transparency, framed traffic is required. This is also true in production, where the tests are important as part of the optical parametric calibration process.

Q. Why is the module command interface taking on more importance?

A. The DCO module is a fully featured coherent system which includes a framer, FEC mapper and optical parametric tracking. A huge amount of information needs to be constantly ‘pulled’ from the module. Parameters like OSNR and SDFEC BER are critical for real-time tracking of network and link health. What makes this even more challenging is that the module must now react autonomously, predictably, and deterministically to service disruptions such as loss of signal (LOS). These events can have a whole system impact, which must be managed over the module command interface in real time. This interface orchestrates the behavior of the whole module and is a critical aspect of any test and validation process

Q. What about the module to host electrical interface, isn’t that a stable and mature technology?

A. The latest generation of DCO modules, both multi-service CFP2 400G and QSFP-DD ZR use PAM-4 electrical signaling for the high-speed data interface. Typically, 8 lanes of differential PAM-4 at 28 Gbaud carry the 400G of bandwidth to and from the module. Although deployed with 400G client modules, PAM-4 electrical signaling is still extremely demanding from a test and measurement perspective. As well as validation and debugging of all the classic high-speed SERDES issues like signal integrity, we are also now faced with what could be thought of as a ‘protocol’ at the lowest layer, including DSP equalization to manage the link. This new challenge still requires care because it combines classic physical layer issues, along with additional obstacles in and around the firmware driving the DSP equalization.

Without a doubt, pluggable coherent optics will be demanding in terms of test and measurement practices– from chips to systems and from electronics to photons. That is why the VIAVI test and measurement portfolio offers trusted, innovative solutions to help you navigate all the test and validation challenges, from R&D to deployment.

If you were not able to catch my live presentation on 400G Pluggable Coherent Optics – Accelerating Validation and Deployment, I invite you to listen in on the recorded webinar. Register now.

To get the latest news from VIAVI on how to accelerate high-speed network test, visit our High-Speed Network Test Resource Center.

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