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PAM4 Drives Serdes to 100Gbps

December 11, 2018

Author: Bob Wheeler

Data centers are on the cusp of 400G Ethernet adoption enabled by 100Gbps-per-lambda technology. Arista Networks and Cisco Systems recently announced the first networking equipment using 12.8Tbps switch silicon, enabling high-density 400G Ethernet (400GbE) for the first time. These data-center systems connect with optical modules using 8x50Gbps PAM4 electrical I/O, but optics are quickly pushing to 100Gbps per wavelength (or “lambda”). Optical modules for 100Gbps-per-lambda connections require PHYs that integrate sophisticated signal processing, and six vendors are now sampling such chips. They’re pursuing a mix of 4x100Gbps PHYs for 400G Ethernet and 1x100Gbps PHYs for single-lambda 100G Ethernet (100GbE).

Some vendors are already shipping 50Gbps PAM4 PHYs, whereas others skipped the first generation to focus on 100Gbps designs. In the latter camp, Macom acquired AppliedMicro (a pioneer in 100Gbps PAM4 DSPs), and its first customers are qualifying modules for production. Vendors of 50Gbps PAM4 PHYs now sampling 100Gbps chips include Broadcom, Credo Semiconductor, and Inphi. Among the new entrants are MaxLinear and MultiPhy, which are sampling their first PAM4 designs. Semtech had worked with and planned to acquire MultiPhy, but it abandoned that plan and instead announced unique PHYs for active optical cables (AOCs).

As 100Gbps-per-lambda optical modules reach production, standards bodies are working on 100Gbps PAM4 electrical interfaces. Moving module-electrical I/O from 50Gbps to 100Gbps per lane has a number of benefits, such as eliminating 2:1 multiplexing and enabling 800Gbps modules. As switch chips implement 100Gbps-per-lane ports, they can double density to 25.6Tbps in the same footprint as a 12.8Tbps chip. Hyperscale-data-center operators are hungry for greater network bandwidth, but they also demand the lowest cost per gigabit as well as high-volume manufacturing. For chip vendors, satisfying these customers is far different from satisfying traditional telecommunications customers, which pay high prices for leading-edge speeds and also ramp slowly.

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