ANTONY SAVVAS PAM4 TESTING
require a pretty sophisticated definition for PAM4. For higher baud rates (56 GBaud and higher) usable T&M equipment is still limited. For example, for true BER testing, SHF is the only company offering measurement capability up to 58 GBaud.” WILL IT BECOME EASIER? Optical Connections also asked the industry whether the test and measurement challenge will become any easier. Ixia’s Seifert said, “Over time, probably several years, the testing will become a little easier as the fundamental PAM4-based technology matures. But almost without regard to the maturity of any new technology, the testing cycle actually lengthens. There is a continued need to reconcile the hundreds, if not thousands, of different networking products, optics and cable components and their many different configurations and form factors that all have to work together successfully. This is a gigantic task that can take five to seven years after an initial new technology - like PAM4 - hits the market and settles down.” He adds however, that with new chips that have 112 Gb/s per lane signalling under rapid development, and several new IEEE draft standards in the works to support those efforts, by the time the industry gets a grip on today’s 56Gb/s per lane signalling with PAM4 and FEC, there will be another round of testing required for products based on these new chips. SHF’s De Lippe confirms, “The electrical and optical components and instruments will become faster and better tailored to the specific PAM requirements. However, PAM4 will always remain more challenging than NRZ.” NRZ FUTURE? So, what about NRZ, which PAM4 seeks to replace, is it on the way out? And if not, where is it still relevant for use? VIAVI’s Brooks says, “PAM4 is an emerging technology and is still a premium solution, while NRZ-based signalling offers a lower-cost solution as it is a more mature technology and has significantly higher volumes. Large amounts of legacy elements and ICs will have NRZ-based signalling for many years. It will continue to offer certain advantages for many generations.” Seifert adds, “NRZ-based products have very large installed bases that are and/or would be extremely expensive to replace. This is why the majority of PAM4-based equipment has the ability to be reconfigured as a PAM4 port back into the NRZ encoding mode as desired by the user. The idea is to be able to support the large installed base of legacy NRZ products.” The market has clearly found an efficient interconnect solution with PAM4 and the test and measurement sector seems to be making progress towards supporting it, with gradual steps being taken rather than giant leaps to make sure all players can safely take advantage.
a large range of testing issues that involve interoperability between different devices from different manufacturers, auto- negotiation and link training testing, and benchmark performance testing when all the previous testing is complete.” Brooks concurs, “PAM4 links have a background error rate so now we need to know what are ‘good’ and ‘bad’ errors and which error profiles can cause the error-correcting FEC to fail. The ability to quickly determine the error ‘fingerprint’ is absolutely key to successful and economic production and deployment. Real traffic with real profiles is needed to validate the complex functionality within the FEC and ICs.” So, is the test and measurement industry up to the job in this new testing area? Brooks says, “Many new challenges exist with these high speed PAM4-based solutions – both electrically and optically. The R&D phase is running well but some innovation is required to ensure that volume production needs can be met against the technological and economic factors like throughput of production. VIAVI has several unique concepts that not only accelerate product development and validation but also address the critical needs for cost-effective volume production and deployment.” On the same subject, Frédéric de Lippe, head of sales at SHF Communication Technologies AG, adds, “For baud rates up to 32 GBaud a selection of measurement equipment is commercially available. For some parameters, however, different vendors tend to use slightly different definitions, if not clearly defined by a standardisation body. Parameters which had no ambiguity at all for NRZ may
Many new challenges exist with high-speed PAM4-based solutions
PAUL BROOKS DIRECTOR OF STRATEGY, LAB AND PRODUCTION, VIAVI
PAM4 TESTING ISSUES l DEMAND ON MEASUREMENT BANDWIDTH The multitude of signal trajectories broaden the un-usable time range (the so called “dead zone”) of a PAM4 eye compared to a conventional NRZ eye. Insufficient measurement bandwidth has the drastic impact of widening this dead zone, resulting in significant eye closure. Thus the measurement systemmust be either very broadband and/or equalisation techniques have to be utilised. For the latter however, this can make measurement parameters difficult to compare if different equalisation methods or even different DSP algorithms have been used. l SENSITIVITY As each of the three individual PAM4 eyes have less than 1/3 of the amplitude of the equivalent NRZ signal, impairments like noise close the eye more rapidly. Thus, the receiving electronics must be very accurate - the sensitivity needs to be improved much more than by a factor of 3 to achieve the same BER as for NRZ. At very high speed this is difficult to achieve. l TEST SIGNAL GENERATION In terms of test signal creation, PAM4 signals are often generated by supposedly more flexible Arbitrary Waveform Generators (AWG) while NRZ signals are generated by Pattern Generators (BPG). AWGs, however, have significant disadvantages like limited memory and very narrow sample windows. These drawbacks can still only be overcome by BPG architectures which work at broadband and generate the patterns logically in “real time’ without invoking user patterns upload. Current state-of-the-art BPGs however are flexible enough to generate all variations of PAM4 signals with and without pre-distortion. A system from SHF, for example, provides the flexibility of an AWG while it maintains the advantages of a BPG for baud rates up to and beyond 100 Gbaud. Source: Thomas Lee, technology advisor at SHF Communication Technologies AG
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ISSUE 17 | Q2 2019
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