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Marvell Doubles PAM4 PHY Density

March 27, 2018

Author: Bob Wheeler

Marvell is sampling its first 50Gbps PAM4 PHY, which doubles the port density of shipping Broadcom and Inphi chips. Targeting systems rather than optical modules, the 88X7120 arrives at the same time as the first switch chips to support the new 50Gbps, 200Gbps, and 400Gbps Ethernet standards. It handles 16x50GbE ports or 2x400GbE ports as well as intermediate speeds. The design is also versatile, offering retiming as well as multiple gearbox and reverse-gearbox modes for both PAM4 and NRZ interfaces.

The 88X7120 implements 16 bidirectional channels on both the host and line sides. Like Marvell’s 25Gbps retimers, it provides long-reach serdes on the host side in addition to the line side. These serdes comply with the 802.3cd 50GBase-CR/KR specifications for PAM4 operation, and they support an NRZ mode compliant with 100GBase-CR4/KR4. The company claims the 7120 meets the 802.3cd standard for single-lane 50Gbps Ethernet. The chip also handles 802.3bs specifications for 100Gbps, 200Gbps, and 400Gbps Ethernet.

Between the host- and line-side interfaces, the 7120 includes a pair of PCS+FEC (physical coding sublayer + forward error correction) blocks for gearboxing between PAM4- and NRZ-based standards. It offers both 2:1 host-to-line gearboxing and 1:2 reverse gearboxing as well as 16:16 retiming, which handles 2x400GbE, 4x200GbE, 8x100GbE, and 16x50GbE ports. It provides a repeater mode that bypasses the PCS and FEC functions to minimize latency. The chip includes an MDIO control interface, eye monitoring for all serdes channels, and clock recovery for SyncE.

Although Marvell trails Broadcom and Inphi in bringing PAM4 PHYs to market, the first-generation designs were inside optical modules rather than on line-card and switch PCBs. The 88X7120 is sampling at the same time as the first switch chips integrating PAM4 serdes, which will create both retiming and reverse-gearbox opportunities. Given its excellent density, OEMs should evaluate this new alternative.

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