PAM4: The New Modulation Standard for High-Speed Ethernet Serdes

When work began on 200 Gbps and 400Gbps Ethernet in the IEEE’s 802.3bs study group five  years ago, multi-level modulation was viewed as a potential substitute at high speeds for the widely used and well-characterized Non-Return to Zero (NRZ) modulation, common at both 10Gbps and 25Gbps. By 2015, however, the IEEE had mandated the use of four-level Pulse Amplitude Modulation (PAM4) for all single-lane serdes channels of 56Gbps and higher . These lanes are used for both 50 Gigabit Ethernet in its native format, and for lane aggregation in 100 Gigabit Ethernet, 200 Gigabit Ethernet, and 400 Gigabit Ethernet.