Tech Briefs Magazine - May 2021 - PIT-16

TECHNOLOGY LEADERS
Test & Measurement Systems

Forward error encoding is a process that starts on data at
the Physical Coding Sublayer (PCS) before data transitions
into Physical Media Attachment (PMA). This PCS/PMA interface manages data error coding, interleaving, scrambling and
alignment contributions. This PCS/PMA encoding system
poses error rate analysis challenges as the process of observing the physical bit error generating root cause is now
obscured under a considerable amount of digital error correction and interleaving circuitry. The desire to examine the
physical errors in an optical transmission that leads to unrecoverable data frames is a complex process, and one that test
instrumentation suppliers are actively advancing today.
Specialized tools such as layer1 BERT and KP4 FEC multiport
analysis systems now play an integral part of receiver tolerance and general FEC aware debug tools (Figure 4).
The PCS/PMA gap that exists between a FEC corrected optical signal and its actual raw physical transmission can be
bridged with Keysight's 400G FEC aware receiver test system,
which analyzes FEC encoded data streams and can direct an
oscilloscope to localize (trigger on) the physical optical interface at locations where errors are occurring and offers system
designers a tool for the first time which connects the post FEC
error analysis with analysis and visualization of the physical
transmission side by side.
Summary
Currently, the highest capacity direct modulated data-communications systems operate at 400Gbps. These systems have
multiple lanes of 100Gbps, either using four transmitters
and four fibers or having four wavelength transmitters and a
single fiber. First generation 800Gbps links will be 2× scaled
up 400Gbps systems through higher density connectors such
as QSFP-DD and OSFP interconnects. In this scenario, with
just more lanes of 100Gbps to aggregate to 800Gbps, the
specifications and test methods will remain similar to what
they are for 400Gbps systems. Native ×4 lane wide 800Gbps
links will depend on advancements in both electrical and
optical specs which are currently underway. This next speed
class at 800Gbps will most likely advance to a native 200Gbps
per lane speed both electrically and optically while adhering
to strong market needs to reduce overall power consumption and cost.
When 200Gbps single lane transmission is achieved, the
test methods and techniques from 100Gbps will likely be
highly leveraged, however, the 200Gbps field will likely
employ advances in modulation methods as a focus on
increased transmission efficiency and managing known bandwidth bottlenecks are key pressure points on this industry.
The measurement partners at Keysight are integral contributors to these cutting-edge standards efforts to ensure that
effective test solutions continue be available as these technologies evolve to 800Gbps and 1.6Tbps for next generation
data center architectures.
This article was written by Greg D. Le Cheminant, Measurement
Applications Specialist, Digital Communications Analysis, Internet
Infrastructure Solutions; and John Calvin, Strategic Planner and
Datacom Technology Lead, IP Wireline Solutions; Keysight
Technologies (Santa Rosa, CA). For more information, visit
http://info.hotims.com/79413-225.

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Tech Briefs Magazine - May 2021

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