Summer 2019 - Optical Connections Magazine


ADAPTING TO A PAM4 WORLD Small steps on a long road

Global growth in cloud computing is driving demand for higher-performance infrastructure. And to keep pace with demand, developers are transitioning to 400G technologies to enable compact, faster and lower cost-per-bit solutions, writes Antony Savvas.

S everal core technologies are enabling 400G, including the use of higher order modulation. A newer modulation scheme provides four-level pulse amplitude modulation, otherwise known as PAM4, which transmits two bits per symbol - doubling the data rate compared to conventional NRZ (non-return-to-zero) modulation. Optical Connections looks at the market progress of PAM4 and considers whether the industry is geared up to take full advantage, despite some challenges to the technology. PAM4 is very prone to noise and presents some particularly complex problems for test and measurement. It effectively doubles data rates at the expense of the reduced signal-to-noise ratio (SNR). In testing, new measurements are required to characterise impairments that were not an issue in previous NRZ designs. With PAM4 signals having a lower signal-to-noise ratio and one third of the amplitude of equivalent NRZ, they require much more advanced tools and features for their successful validation and deployment. The 400 Gigabit Ethernet (GE) standards define PAM4 multi-level signalling as a recommended modulation format to implement serial 400GE data centre interfaces. This is an evolution from the two state NRZ modulation used in 100GE, for instance. MARKET TAKEUP With all this in mind, what is the current take-up of PAM4 and in what sectors of networking in particular? Robert Blum, director for strategic marketing and business development at Intel Corporation, says, “We’re using PAM4 in

our Stratix10 FPGAs, for instance, and our 400G DR4 modules are using PAM4 on both the electrical and optical side.” Paul Brooks, director of strategy, lab and production at test and measurement firm VIAVI Solutions, says, “Higher-order modulation is established in many areas but now is transitioning to high-speed client optics. A healthy ecosystem of ICs and optical modules will enable network elements such as Ethernet switches to be brought to market very soon. PAM4 will offer broad market appeal to areas from high performance router-to-router interconnects through to high-density links within data centres where increased bandwidth density at the faceplate is critical.” Brooks said the first-generation PAM4 silicon is already available for both phy devices (required for optical modules) and more complex switch silicon. Some challenges still remain in taking PAM4- based optical client interfaces to volume production, he said, but that “over the remainder of 2019” we will start to see the availability of robust PAM4 devices that can successfully inter-operate and offer a “price-attractive roadmap to volume deployment by 2021.” IT’S TESTING On the test and measurement side, Charles Seifert, senior manager for NTS product management at Ixia Solutions Group, which is a Keysight business, acknowledges some of the issues around PAM4. He says, “The main challenges are testing of BER for the line (Layer 1), the PCS per lane, and the FEC per symbol, per lane and per port, to be sure these operations meet or exceed IEEE tolerances for all network devices. Once these are overcome, then we move onto

testing will become a little easier as the fundamental PAM4-based technology matures




| ISSUE 17 | Q2 2019

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